Open access peer-reviewed chapter

Rare-earth Element-bearing Apatites and Oxyapatites

By Petr Ptáček

Reviewed: January 8th 2016Published: April 13th 2016

DOI: 10.5772/62209

Downloaded: 1550


A number of prepared alkaline-earth-rare-earth silicates and germanates also have the structure of apatite type. The fifth chapter of this book then continues with description of synthetic compounds of apatite structure. Attention will be directed to description of rare-earth element bearing apatites and oxyapatites. The structure, properties and preparation of apatite-type silicates, germanates and borates were described. This chapter gives also description of oxygen-rich apatites, which are promising material for electrolytes in solid oxide fuel cells and sensors and explain the basic concepts between structure and conductivity of these compounds. The additional information about application of apatites is given in the last chapter of this book. Furthermore, N-apatite, REE vanadocalcic apatite and apatite type yttrium phosphates were described.


  • Apatite
  • Oxyapatite
  • Conductivity
  • Silicates
  • Germanates
  • Borates
  • N-apatite
  • Apatite-type yttrium phosphates

In this chapter, the preparation and the properties of synthetic phases of apatite are given; the geological role is described in Section 7.3. The ideal general formula of an apatite-type oxide may be written as M10(XO4)6O2 (M = alkaline-earth and/or rare-earth element, X = Si, Ge, P, V, …). The structure (Fig. 1) can be described in terms of a “microporous”[1] - [1] framework (A(1)4(XO4)6) composed of face sharing M(1)O6 trigonal meta-prismatic columns, which are corner connected to MO4 tetrahedra. This framework allows some flexibility to accommodate remaining M(2)6O2 units [2].

Fig. 1.

Illustration of “microporous” M(1)(XO4)6 framework of the apatite (M10(XO4)6O2) structure (a): tetrahedra MO4, M(1) cation at the center of trigonal meta-prism. Remaining M(2)6O2 units occupy the cavities within this framework (b): large spheres are M(2) cations and small spheres are O anions [2].

5.1 Apatite-type lanthanium silicates

During recent decades, oxyapatite[1] --type structure with the general formula: REE9.33+xSi6O26+3x/2 (where REE is rare-earth element) [3],[4], REE9.330.67(SiO4)6O2 [5] or REE10−x(SiO4)6O2+y [6] has attracted considerable attention as oxide ion conductors. Apatite-type oxides have attracted much attention as the material for electrolytes in solid oxide fuel cell and sensors (Chapter 10).

In low atomic number rare-earth silicate systems, an apatite phase occurs with a range of stability extending from Ln4.67(SiO4)3O to Ln4(SiO4)3. The stability decreases as the rare-earth atomic number increases, with a mixture of Ln2SiO5 and Ln2Si2O7 replacing apatite as the preferred phase assemblage [7],[8],[9],[10],[11].

Apatite-type rare-earth element (REE) silicates of the composition of REE10−x(SiO4)6O2+y, where REE = La, Nd, Gd and Dy, were prepared byMartínez-Gonzálezet al [6] via the mechanochemical synthesis (stabilized zirconia planetary ball mill: ball-to-powder ratio ~10:1, 350 rpm for maximum time of 9 h) starting from the stoichiometric mixtures of constituent oxides, REE2O3 and SiO2 (molar ratio = 4:5), followed by post-milling thermal treatment (1500°C for 3 h). The ionic conductivity increases with the increasing size of REE cations.[1] - The mechanochemical synthesis of apatite-type lanthanum silicates from the mixture of La2O3 and amorphous silica without post-milling thermal treatment was described byFuenteset al [4].

Rare-earth element-doped apatite-type lanthanum silicates of the composition of La9MSi6O27, where M = Nd, Sm, Gd and Yb, were synthesized by the high-temperature solid-state reaction process byXianget al [3]. All rare-earth oxide powders (La2O3, Nd2O3, Sm2O3, Gd2O3 and Yb2O3) were firstly pre-calcined at 900°C for 2 h in order to achieve complete decarbonation and dehydroxylation before weighing. The stoichiometric mixtures were mechanically mixed in absolute ethanol for 24 h using zirconia milling media at the speed of 400 rpm and dried at 100°C in air. The powder mixture was calcined at 1350°C for 10 h and then ground by hand with an agate mortar and pestle to reduce the particle size. After that, the powders were uniaxially pressed at 20 MPa and then statically cold pressed at 200 MPa for 5 min. The compacts were pressureless sintered at 1650 K for 10 h in air.

Apatite-type phaseLattice parameters [Å]MVDensityEσ0

Table 1.

The properties of apatite-type lanthanium silicates [3],[8].

The lattice parameters and the properties of prepared apatite-type lanthanium silicates are listed in Table 1. All prepared compounds possess hexagonal apatite structure with the space group P63/m. The temperature dependence of total electrical conductivity for different compositions is determined using the Arrhenius equation[1] - [13],[14],[15] in the following form [3],[5],[16],[17]:


where σ is the total electrical conductivity, σ0 is the pre-exponential factor related to the effective number of mobile oxide ions, E is the activation energy for the electrical conduction process, kB is the Boltzmann constant and T is absolute temperature. ΔHm and ΔHa denote the migration enthalpy of oxygen ion and the association enthalpy of defects, respectively. The determined activation energy and pre-exponential factor are listed in Table 1. It can be seen that the activation energy gradually increases from La10Si6O27 to La9GdSi6O27. Total electrical conductivity can be calculated from the following equation:


where h is the thickness of the specimen, S is the electrode area of the specimen surface and R is the total resistance including grain and grain boundary resistance. Lanthanum silicates doped with Nd or Yb cations exhibit higher total electrical conductivity than undoped lanthanum silicates. The highest total conductivity value obtained at 500°C is 4.31·10−4 S·cm−1 for La9NdSi6O27. The total electrical conductivity is also a function of partial pressure of oxygen [3].

Fig. 2.

Crystal structure of apatite-type rare-earth element silicate viewed along the c-axis [12].

The measurements using single crystals revealed definite anisotropy of the electrical conductivity of Ln9.33(SiO4)6O2, that is, the conductivity parallel to the c-axis is larger by one order of magnitude than that perpendicular to the c-axis. This fact clearly indicates that the channel oxide ions not bonded to silicon are the principal charge carriers in apatite-type lanthanum silicates. The structure of apatite-type rare-earth silicate is shown in Fig. 2. SiO4 tetrahedra are isolated mutually, and Ln ions (REE ions, in general) at 6h sites (sevenfold coordinated site (x,y,¼)) [18][1] - form channels, in which oxide ions at 2a sites are located (possess the threefold coordination with rare-earth ions at the 6h sites in the same plane), along the c-axis.

These mobile ions at these sites have much larger anisotropic displacement parameters in the direction of the c-axis than those in the direction of the a-axis, even at room temperature, which reflects high oxide ion conduction along the c-axis. The ninefold coordinate position (4f site (1/3,2/3,z)) is the second site for the accommodation of REE cations in the structure of apatite-type REE silicate [6],[12],[19],[20].

Since the interstitial space provided by these rare-earth ions is the smallest throughout the channel along the c-axis of the apatite structure, the migration of oxide ions through the channel will not be affected significantly even if the sizes of rare-earth ions are varied. It is therefore reasonable that the electrical conductivities of apatite-type rare-earth silicates are independent on the kind of rare-earth elements [12].

Conventional oxide ion conductors are designed on the basis of the oxygen vacancy model by the introduction of aliovalent[1] - [21] cations. In Ln9.33(SiO4)6O2, however, cation vacancies are present rather than oxygen vacancies. Therefore, the introduction of cation vacancies into the structure of an oxide material may induce high oxide ion conductivity if the structure has a channel or a plane that can be a path for the migration of oxide ions [12].

Apparent exchange of O(1), O(2) and O(3) oxide ions bonded to Si was observed by 17O NMR measurement on La9.33Si6O26 byKiyinoet al [22], while it was not observed for oxide ion on the isolated site O(4). The results indicate that oxide ions bonded to Si at the position O(1), O(2) and O(3) are the main diffusion species in the oxide ion conductivity.

Trivalent and divalent dopants[1] - [23] have been introduced into the La9.33(SiO4)6O2 structure according to the following nominal mechanisms [24]:


where M = Al, Ga, B, Co, Fe, Mn, … and AEE denotes the alkaline-earth elements (Ca, Sr and Ba). Doping with Al, Ga and B according to the formula: La9.33+x/3(SiO4)6−x(MO4)xO2, via the mechanism in Eq. 3, causes that bulk conductivity increases in up to two orders of magnitude in the case of Al for x = 1 – 1.5. If, however, the sample is stoichiometric on both cation and anion sites, as for La8Sr2(SiO4)6O2, the AEE doping reduces the conductivity and increases the activation energy for the conduction compared to La9.33(SiO4)6O2.

The effect of Fe doping on the electrical properties of lanthanum silicates of the composition of La10Si6−xFexO27−x/2 (where x = 0.2, 0.4, 0.6, 0.8 and 1.0) was performed byShiandZhang[16] via the sol-gel process. Tetraethyl orthosilicate (TEOS), La(NO3)3·6H2O and Fe(NO3)3·9H2O were used as starting materials. Stoichiometric amounts of Fe(NO3)3·9H2O and La(NO3)3·6H2O were dissolved in the mixture of ethanol, acetic acid and distilled water. The appropriate amount of TEOS was added to the solution while continuous stirring. The solution became gradually a purple clear sol. After refluxing at 80°C for 1 – 2 h, the sol transferred to a clear gel. Then, the wet gel was dried at 100°C for 20 h. The gel was heated at 600°C for 4 h to remove water and organic components and to decompose nitrates. In order to get the desired phase, obtained precursor was then calcined at 1000°C for 4 h.

Apatite-type lanthanium silicateLattice parameters [Å]VE (600 – 800°C)E (400 – 550°C)

Table 2.

Lattice parameters of Fe-doped apatite-type lanthanium silicates [16].

All synthesized samples have hexagonal lattice structure with the space group of P63/m. The lattice parameters of prepared Fe-doped apatite-type lanthanium silicates and the activation energy of conductivities (Eq. 1) for different Fe contents are listed in Table 2. When x = 0.6, La10Si5.4Fe0.6O26.7 exhibits the lowest activation energy. The lattice parameters of La10Si6O27 (Table 1) and doped specimen (Table 2) show that the values of a, c and V increase with the content of iron. The conductivity of La10Si6−xFexO27−x/2 is independent of oxygen partial pressure in the range from 0 to 100 kPa, which indicates that the conductivity of all samples is mainly ionic [16].

The oxygen ionic and electronic transport in apatite ceramics with the composition of La10Si6−xFexO27−x/2 (x = 1 – 2) [25] and La10−xSi6−yAlyO27−3x/2−y/2 (x = 0 – 33; y = 0.5 – 1.5) [26],[27] was investigated byShaulaet al In both cases, the essential role of oxygen content on the ionic conductivity of apatite phase was recognized. The ion transference number[1] - [28] increases with decreasing partial pressure of oxygen. Such behavior indicates that the conduction under oxidizing condition is predominantly of p-type[1] - (with respect to n-type of conductivity). Similar to the foundation ofShiandZhang[16], the conductivity of these phases is predominantly ionic and almost independent on partial pressure of oxygen. The ion transference numbers are higher than 0.99, while the p-type electronic contribution to total conductivity is about 3% (700 – 950°C, La10Si4Fe2O26). The oxygen ionic conductivity should increase with decreasing iron content due to higher concentration of oxygen interstitials.

Another important factor influencing the oxygen diffusion is M-site deficiency, which affects the unit cell volume and may cause the O(5) ion displacement into interstitial sites, thus creating the vacancies in the O(5) sites at fixed total oxygen content. In particular, an enhanced ionic conduction was found in the system La9.33+x/3Si6−xAlxO26, where Al doping is compensated by the A-site vacancy concentration without oxygen content variations [29],[30].

The incorporation of praseodymium in the apatite-type lattice of La9.83−xPrxSi4.5Fe1.5O26+δ (x = 0 – 6) decreases the unit cell volume, suppresses the Fe4+ formation according to Mössbauer

spectroscopy[1] - [31],[32],[33],[34] and increases p- and n-type electronic contributions to total conductivity under oxidizing conditions, while the level of oxygen ionic transport at temperatures above 1000 K remains unaffected [35].

Since the size of the conduction channel increases with the Mg doping, the enhancement of the ionic conductivity of lanthanum silicate-based apatites can be reached by optimizing the La content and the Mg doping level at the same time. The ionic conductivities of La10Si5.8Mg0.2O26.8 and La9.8Si5.7Mg0.3O26.4 at 800°C are 88 and 74 mS·cm−1 with the activation energy of 0.43 and 0.42 eV, respectively [36].

The ionic conduction in cation-deficient apatite La9.33−2x/3MxSi6O26, where M = Mg, Ca and Sr was investigated byYuanet al [37]. The nature of dopant and the extent of substitution have a significant effect on the conductivity. The greatest decrease in conductivity is observed for Mg doping followed by Ca- and Sr-doped apatites. The effect is ultimately attributed to the amount of oxygen interstitials, which is affected by the crystal lattice distortion arising from the cation vacancies.

The incorporation of additional La2O3 into La9.33(SiO4)6O2 to form La10(SiO4)6O3 or intermediate compositions can most obviously be achieved by filling empty interstitial sites with oxygen. The only alternative scenario would involve the creation of cation vacancies on the Si sublattice, which is unlikely as Si is present as a complex anion. The incorporation of excess of La2O3 into La9.33(SiO4)6O2 can therefore be expressed as [10]:


Thus, in the ideal pure La10(SiO4)6O3, the 4f and 6h sites are fully occupied by La3+ ions, while an extra oxygen interstitial is introduced into the lattice to maintain the electroneutrality. The oxygen interstitial may benefit the oxide ion transportation if it is located nearby the [001] direction c-axis of the conventional unit cell. From the space-filling consideration, the most appropriate sites for the oxygen occupation are in this position; however, some distortion of the O 2a sites would be required to accommodate extra oxygen atoms. This could be achieved by decreasing the symmetry from P63/mto P63allowing oxygen to move from 0,0,1/4 to 0,0,x. Recent studies suggest that a range of partially occupied (0,0,x) sites may accommodate this extra interstitial oxygen. From this point of view, La10(SiO4)6O3 should exhibit higher conductivity than La9.33(SiO4)6O2 [10],[38].

Introducing Sr2+ cations to the La3+ atomic positions, as in the La10(SiO4)6O3 phase, leads to complete elimination of vacancies according to the substitution [39]:


The substitution of La2O3 by SrO, taking into account the charge balance and the oxygen content, can be represented as follows (Kröger-Vinknotation[1] - [40],[41]):


Lanthanum oxyapatite phases are substantially stable with respect to their binary oxides. The general trend in the formation enthalpies as a function of (La + Sr)/(La + Sr + Si) shows that the apatite phase becomes more energetically stable as the cation vacancy and oxygen excess concentrations decrease. The stoichiometric sample achieved by Sr2+ doping, with no cation vacancies or interstitial oxygen atoms, is the most stable composition. The energetics of lanthanum silicate apatite materials (La9.33+x(SiO4)6O2+3x/2 and La10−xSrx(SiO4)6O3−0.5x) depends on lanthanum deficiency and oxygen interstitial[1] - [42],[43] concentrations, and the cation vacancy concentrations appear to be the dominant factor in energetics [39].

Fig. 3.

Schematic diagram of preparation of lanthanum silicate by a sol-gel process [44].

The schematic diagram of the sol-gel process used byTaoandIrvine[44] for the preparation of apatite-type lanthanium silicates is shown in Fig. 3. The room-temperature structure is hexagonal, the space group is P63or P63/m, with a = 9.722 and c = 7.182 Å for La10(SiO4)6O3 and a = 9.717 and c = 7.177 Å for La9.33(SiO4)6O2, i.e. the cell volume of La10(SiO4)6O3 is a little greater than that of La9.33(SiO4)6O2. Both compositions exhibit high ionic conductivity, although the grain boundary resistance is the dominant feature in the impedance spectrum of both. In general, the conductivity of La10(SiO4)6O3 is higher than that of La9.33(SiO4)6O2 and this indicates that oxygen interstitials may be introduced into the apatite lattice of La10(SiO4)6O3, which may benefit the oxygen ion transportation [44].

The La10Si6O27 nanopowders with apatite structure were synthesized by theLiet al [45] co-precipitation method. After the calcination at 900°C and then removing of La2O3 by acid washing, the pure stoichiometric La10Si6O27 nanopowders are obtained. The oxyapatite ceramics with the density higher than 95% can be obtained at rather low sintering temperature of 1300°C, and it has comparable total conductivity with the samples sintered at 1650°C from the powders prepared by solid-state reaction.

La2O3 and TEOS in stoichiometric amount were used byMasubuchiet al [46] as the starting materials for the preparation of both powder and film of apatite-type La9.33(SiO4)6O2 via the alkoxide hydrolysis. Lanthanum oxide was dissolved in HNO3 (6 mol·dm−3) and mixed with ethanol. Then, stoichiometric amount of TEOS in ethanol was added (La:Si = 9.33:6) to this solution. The precursor solution was obtained by refluxing for one night. This solution was heated to gelating on the hot plate followed by calcination and annealing in powder preparation. Either quartz glass or Pt foil substrate was dipped to the gelatinous solution and dried for the film preparation. It was calcined at 500°C for 1 h to remove the organic contents and then fired at 1000°C for 10 h. This preparation steps were repeated to increase the film thickness. The film showed preferred orientation of the apatite crystal in thinner film. The conductivity of sintered body was lower in about one order of magnitude than the value of single crystal perpendicular to c-axis [46].

The synthesis and the conductivities of Ti-doped apatite-type phases of the composition of (La/Ba)10−x(Si/Ge)6O26+z, where Ti substituted at the Si/Ge site, were reported bySansomet al [47]. The conductivities were shown to be the highest for the samples containing either cation vacancies or oxygen excess, which is consistent with previous studies of apatite-type oxide ion conductors. However, the Ti doping was shown to generally decrease the conductivity in comparison with equivalent samples containing only Si/Ge at the tetrahedral sites, with the greatest decrease for Si-containing samples.

Vanadium-doped oxyapatite phases of the composition of La10−xVx(SiO4)6O3+x were prepared byYuanet al [48] via the sol-gel method. The apatite phase begins to form at 800°C, which is much lower than in the case of conventional solid-state synthesis method. The best conductivity of La9V(SiO4)6O4 is 1.67·10−2 S·cm−1, which is significantly higher than that for lanthanum silicate oxides (1.19·10−2 S·cm−1). The valence ion V5+ doped for La3+ does lead to the formation of hexagonal apatites even with high oxygen contents.

The phase La5Si2BO13 [49],[50],[51] crystallizes with apatite-related structure (Fig. 4) with the space group P63/mand the cell parameters a = 9.5587 Å, c =7.2173 Å and Z = 2. The composition of these apatite-like compounds can be also expressed via the general formula: La9.33+xSi6−2yB3yO6, where 0 ≤ x ≤ 0.67. At limiting compositions x = 0.67, La(2) site is fully occupied, and the formula referred to the unit cell is La10Si4B2O26 or, more simply, La5Si2BO13.

Fig. 4.

The crystal structure of La5Si2BO13 (a) [51] and the upper section of the ternary phase diagram La2O3-SiO2-B2O3 at 1100°C (b) [49].

The comparison with other apatite-like structures shows lower distortion in the M(1) polyhedron and unusually short bond length from La in the M(2) site and O(4) oxygen in the column site (2.303 Å). These results can be explained in view of the presence of trivalent La and divalent O, respectively, in the M(1) and M(2) sites and in the column anion site, whereas, in apatites, these sites are occupied by divalent and monovalent ions, respectively [49].

The preparation of La-Si-O apatite-type thin films was described byVieiraet al [52] with Si/(La + Si) atomic ratios ranging from 0.36 to 0.43 being produced via the magnetron sputtering in reactive Ar/O discharge gas. The apatite-type lanthanum silicate phase was formed in all as-deposited films upon the annealing at 900°C for 1 h. The lanthanum silicate films obtained by annealing the as-deposited films with lower Si/(La + Si) atomic ratios have a preferential orientation with the c-axis perpendicular to the substrate, while low-intensity diffraction peaks ascribed to La2Si2O7 phase were detected in the films deposited with higher Si content. Preferentially oriented films have higher activation energy and lower ionic conductivity, as the ionic conductivity measurements were performed in the direction perpendicular to the c-axis. The highest ionic conductivity was obtained for the film deposited with a Si/(La + Si) atomic ratio of 0.42, with a value of 1.2 × 10−2 S·cm−1 at 750°C. By the incorporation of oxygen in the as-deposited films, the silicon segregation upon annealing was avoided.

Fig. 5.

Primary phase diagram of ternary system La2O3-Ga2O3-SiO2 around LGS [53].

The formation of ternary compound with apatite structure in the system La2O3-Ga2O3-SiO2 (Fig. 5) was first reported byWangandUda[53]. The apatite phase, which precipitates from the melt of the composition around that of stoichiometric La3Ga5SiO14 (LGS), can be described by the formula: La14GaxSi9−xO39−x/2, where 0 ≤ x ≤ 3.5. Since there is a large field for the formation of solid solutions with the range extending from La14Si9O39 to Ga2O3, some Si4+ sites are probably substituted by Ga3+.

The liquidus surface of LS(G) was determined to be the field on the Ga2O3-poor side of boundary curve ABCD. The liquidus surface of LS(G) covers the stoichiometric composition of LGS. In this field, the crystallization of LS(G) aciculae was observed in all samples that were heated to temperatures above 1500°C. The liquidus volume of LGS is denoted by the field BCEF. It seems to be a narrow field in the composition between the liquidus surfaces of LS(G) and Ga2O3. E and F are eutectic points, where LGS + LaGaO3 + Ga2O3 + liquid and LGS + Ga2O3 + La2Si2O7 + liquid were found, respectively [53].

The CaO-La2O3-SiO2-P2O5 phase diagram was investigated byEl Ouenzerfiet al [54] in order to determine a domain inside which all points correspond to pure apatitic oxyphosphosilicates with the general formula: CaxLay(SiO4)6−u(PO4)uOt. The defined domain (Fig. 6) is only a part of the whole existence domain of the apatitic structure, but it allows to prepare pure apatitic samples with well-controlled composition.

Fig. 6.

Britholite stability domain in the CaO-SiO2-La2O3-P2O5 quaternary system [54].

For these samples, the continuous change of the stoichiometry of each element proves that it exists as a solid solution including the oxygen content. This observation completes the literature data where britholites are presented as limited to three series corresponding to the stoichiometry [54],[55]:

  1. Sr2+xLa8−x(SiO4)6−x(PO4)xO2 with 0 ≤ x ≤ 6 [1] -;

  2. Sr3+xLa6−x(SiO4)6−x(PO4)x with 0 ≤ x ≤ 1.5;

  3. Sr4+xLa6−x(SiO4)6−x(PO4)xO with 0 ≤ x ≤ 6.

Inside this domain, the solid solution continuously varies between pure phosphate apatites CaxLay(PO4)6Ot and pure silicate apatites CaxLay(SiO4)6Ot and also between oxyapatites CaxLay(SiO4)6−u(PO4)uOt and nonoxyapatites CaxLay(SiO4)6−u(PO4)u.

During the investigation of the kinetics of solid-state sintering[1] - of strontium-doped apatite-type lanthanum silicates (SrxLa10−xSi6O27−x/2) under isothermal conditions (1250 – 1550°C),Bonhommeet al [56] recognized that the densification mechanism of the apatite ceramics was controlled by the diffusion of rare-earth element (La) at the grain boundaries. This process showed the activation energy of 470 kJ·mol−1.

The ternary phase diagram Al2O3-SiO2-La2O3 at 1300°C (Fig. 7) was investigated byMazzaandRonchetti[57]. La14Si9O39 was described byKuz’minandBelov[58] as an apatite-like structure of hexagonal symmetry (space group P63/m). Isomorphous compounds were also reported for Nd [59], Ce [60] and Sm [58]. The La14Si9O39 compound extends its stability range in the interior of the phase diagram, forming the solid solution of the type La14+1x/3Si9−xAlxO39, which is stable from x = 0 to x = 1.5. This substitution stoichiometry (Al + 1/3La ↔ Si) can be described as a tetrahedral Al for Si substitution on the 6h position and contemporary occupation of vacant La sites [57].

Fig. 7.

Ternary phase diagram at 1300°C in air [57].

5.2 Apatite-type lanthanium germanates

Lanthanum germanate and silicate apatite-based materials, both undoped and with partial substitution of, for example, Al, B instead of Si and Sr in place of La, are promising oxide ion conductors with potential applications as high-temperature solid electrolytes. Considerable uncertainties remain over the stoichiometry, the defect structure and the conductivity variations within various apatite systems, partly caused by the fact that the La:(Ge, Si) ratio is variable, giving rise to the solid solutions in the undoped systems as well as to the solid solutions formed by partial replacement of La and/or (Si, Ge) together with, depending on the solid solution mechanism, variations in oxygen content [24].

The preparation of single crystal of apatite-type lanthanium germanate of the composition of La9.33Ge6O26 was reported byNakayamaandSakamoto[61]. The mixtures of La2O3 and GeO2 were well mixed in ethanol under an atomic ratio of La:Ge = 9.33:6 using a ball mill. The mixture was dried and then calcined in air at 1000°C for 2 h. The resulting La9.33Ge6O26 powders were further ball milled into finer powders. After the pre-sintering, the closely packed La9.33Ge6O26 powders were heated at 1300°C for 2 h in air, and the surface of the specimen was mirror polished. The polished surface of polycrystalline La9.33Ge6O26 ceramics was then bonded to a <001> face of La9.33Ge6O26 seed crystal prepared by theCzochralsky(Section 4.2) method. On heating of the bonded sample at 1525 – 1550°C, continuous grain growth of polycrystalline La9.33Ge6O26 occurred and the single crystal was gradually grown from the seed crystal into the polycrystalline region [61].

Apatite-type lanthanium germanate possesses hexagonal structure with the space group P63/mand the lattice parameters: a = 9.9256 and c = 7.2900 Å, V = 621.97 Å3 and Z = 2. The calculated density of the phase is 2.148 g·cm−3. Similar to apatite-type lanthanium silicate (La9.33Si6O26) described above, the structure of La9.33Ge6O26 (Fig. 8) contains two different sites for atoms of La. The La(1) and La(2) sites are located at 4f and 6h, respectively. While the La9.33Ge6O26 single crystal showed little anisotropy in conductivity, the conductivity of La9.33Si6O26 single crystal gave 100 times higher value parallel to the c-axis than that perpendicular to the c-axis at each temperature (Section 5.1) [61].

Fig. 8.

Hexagonal structure proposed for the apatite-type phase of La9.33X6O26, where X = Si and Ge [61].

The selective doping of La9.33+x(GeO4)6O2+3x/2 with Y leads to the stabilization of hexagonal lattice, even at high oxygen contents. Furthermore, this has the effect of enhancing the low-temperature conductivities [62]. Depending on the composition, the cell can be either hexagonal or triclinic, with the evidence of reduced low-temperature conductivities for the latter, attributed to increased defect trapping in this lower symmetry cell. In summary, it was shown that the series La8Y2(GeO4)6−x(GaO4)xO3−x/2 can be prepared for 0 ≤ x ≤ 2 with all samples showing the hexagonal symmetry, compared to the series without Y co-doping, La10(GeO4)6−x(GaO4)xO3−x/2, for which all compositions display the triclinic symmetry [24],[62],[63].

The effect of Ga doping of the oxygen stoichiometric series containing the cation vacancies, La7.33+y/3Y2(GeO4)6−y(GaO4)yO2 (0 ≤ y ≤ 2), single-phase samples was obtained for y ≥ 1.0, with small impurities observed at lower Ga contents. The conductivities were shown to increase with increasing cation vacancy content, reaching the values of ≈0.02 S·cm−1 at 800°C, which are similar to the oxygen excess series. These results are in agreement with previous reports on the apatite systems, which showed that the oxide ion conductivity was maximized in samples containing the oxygen excess and/or the cation vacancies [24], [62],[63].

The series of apatite-type silicates/germanates of the composition of La8+xSr2−xSi6O26+x/2 (0 ≤ x ≤ 1) and La8+xSr2−xGe6O26+x/2 (0 ≤ x ≤ 2) were prepared from high-purity La2O3, SrCO3, SiO2 and GeO2 byOreraet al [64] via the thermal treatment of these components mixed in the stoichiometric ratio.

The extent of, and the structural changes within, the apatite domain in the LaO1.5-GeO2-SrO ternary system at 1100°C was studied and the single-phase samples were obtained for La9.33+x−2y/3Sry(GeO4)6O2+1.5x with x = 0.17 and 0.34. The hexagonal to triclinic transition is clearly associated with increasing oxygen content rather than with filling the La sites by the addition/substitution of Sr into the structure. The limits of undoped solid solution are ~0.17 ≤ x ≤0.5 at 1100°C [24].

The hydrothermal synthesis of apatite-type compound NaRE9(GeO4)6O2 (RE = Nd, Pr) with the hexagonal structure of the space group of P63/mwas described byEmirdag-Eaneset al [65]. The structure is composed of REO7 and REO9 polyhedra as well as GeO4 tetrahedra (Fig. 10). The unit cell dimensions are: a = 9.782(1) Å, c = 7.083(1)Å (T = 293 K) and V = 587.0(2)Å3 for REE = Nd and a = 9.802(1) Å, c = 7.116(1)Å (T = 293 K) and V = 592.1(2)Å3 for REE = Pr.

Fig. 9.

Apatite solid solutions in the LaO1.5-GeO2-SrO ternary system [24]: pure phases are indicated by filled circles and the presence of secondary phases is shown by half-filled circles.

Fig. 10.

The unit cell view with GeO4 and NdO7 (a) and GeO4 and NdO9 down the c-axis. NdO7 and NdO9 are dotted polyhedra, and GeO4 are lined polyhedra [65].

The high-temperature flux method for the preparation of single crystal of hexagonal NaLa9Ge6O26 apatite-type germanate (space group P63/m, a = 9.883, c = 7.267 Å and Z = 1) was used byTakahashiet al [66]. The crystal structure (Fig. 11) was found to be similar to that of silicate oxyapatite NaY9Si6O26. The 4f cation sites are occupied disorderedly by La and Na. On the other hand, the 6h cation sites are occupied by La only. This compound constitutes a new member of the oxyapatite-type structure family with the composition given by general formula: Ax Ln10−xB6O24O3−x.

Fig. 11.

The structure of sodium lanthanum germanate NaLa9Ge6O26 [66].

Fig. 12.

Coordination environment of La atoms [66].

The coordination environments of La atoms by O atoms are shown in Fig. 12. La(1) atom on the 4f site is coordinated by nine O atoms. It is linked to three O(1) atoms in a distance of about 0.2479 nm, to three O(2) atoms in a distance of about 0.2561 nm and to three O(3) atoms in a distance of about 0.2924 nm. Because the distance between La(1) and O(3) is relatively large, La(1) atom can be also regarded to be in sixfold coordination, the environment of which is fairly distorted from an ideal octahedron. On the other hand, La(2) atom, which occupies the 6h position, is coordinated by seven O atoms, that is, O(1), O(2), O(4) and O(3). The distances of those two types of bonds between La(2) and O(3) atoms are 0.2618 nm×2 and 0.2431 nm×2, respectively. Those between La(2) and O(1), O(2) and O(4) are 0.2732, 0.2521 and 0.23281 nm, respectively [66].

5.3 Apatite-type borates

Two high terbium content apatites Tb5Si2BO13 (a = 9.2569 Å, c = 6.8297 Å, V = 506.83 Å3 and Z = 2) and Tb4.66Si3O13 (a = 9.493 Å, c = 6.852 Å, V = 534.70 Å3 and Z = 2) were prepared byChenandLi[67] via spontaneous crystallization and synthesized with high purities and excellent crystallinities by the sol-gel process. Both compounds are isostructural with P63/mspace group and exhibit paramagnetic behavior down to 2 K. Owing to high Tb3+ ion concentrations and good transmittance in the range from 500 to 1500 nm, Tb5Si2BO13 and Tb4.66Si3O13 may be promising magneto-optical materials in the visible-near-IR range.

Both Tb5Si2BO13 and Tb4.66Si3O13 contain two distinct sites for Tb3+ cations, which are depicted in Fig. 13. Tb(2) is at the 4f site, which is on a threefold axis and coordinated by nine oxygen ions. However, for Tb4.66Si3O13, the Tb(2) site is not fully occupied but leaves one of six Tb(2) positions randomly vacant while fully occupied in Tb5Si2BO13. In contrast, Tb(1) at the 6h site is fully occupied and sevenfold coordinated in both Tb5Si2BO13 and Tb4.66Si3O13. Moreover, in Tb5Si2BO13, one third of Si is disorderly occupied by B, which gives rise to extra 1/3 Tb3+ ion for the charge balance. The Tb(2)O9 polyhedron consists of Tb(2) and nine oxygens along the c-axis. Six Tb(1) comprise a sixfold channel parallel to the c-axis. It is worth to note that the channel is considered to play an extremely important role in oxide ion conductivity [67].

Fig. 13.

Unit cell of Tb5Si2BO13 (a) and coordination environment of kinds of Tb cations (b) [67].

The structure and optical properties of noncentrosymmetric borate RbSr4(BO3)3 (RSBO) was described byXiaandLi[68]. RSBO can be viewed as a derivative of the apatite-like structure. Based on the anionic group approximation, the optical properties of the compound are compared to those of the structure-related apatite-like compounds with the formula “A5(TOn)3X”. When the structures of all apatite-like crystals are presented in orthorhombic unit cell, the arrangements of planar anionic BO3 groups are all similar to one-third of the BO3 groups aligned perfectly parallel at corner- and face-centered locations, whereas the other two-thirds of BO3 groups are distributed differently.

Europium borate fluoride, Eu5(BO3)3F with an apatite-type structure, was synthesized byKazmierczakandHöppe[69] as single-phase crystalline powder starting from europium oxide, europium fluoride and boron oxide at 1370 K. Eu5(BO3)3F crystallizes in the space groupPnma.

Fig. 14.

The crystal structure of Sr10[(PO4)5.5(BO4)0.5](BO2) (1): (a) the projection along [001] showing the channels formed by Sr3 (gray triangles) and the positions of the XO4 tetrahedra (gray; Z = 11/12 P + 1/12B) and (b) the side view with emphasized [BO2]− groups and the coordinating trigonal antiprism formed by Sr3. The comparison of the arrangement of [BO2]− and F- ions within the Sr channels of Sr10(PO4)5.5(BO4)0.5(BO2) (c(1)) and Sr10(PO4)6F2 (c(2)) [70].

The structure of single crystal of strontium phosphate orthoborate metaborate (Sr10[(PO4)5.5(BO4)0.5](BO2)) that was grown from the melt byChenet al [70] is shown in Fig. 14(a). The phase crystallizes in the space group P3 with the cell parameters a = 9.7973 Å, c = 7.3056 Å, V = 607.19 Å3 and Z = 1. The crystal structure is closely related to apatite and contains linear metaborate groups, [BO2] (point group D∞h, B-O = 1.284 Å), taking positions within the channels running along the threefold inversion axis. Strontium sites are found to be fully occupied, while [PO4]3− tetrahedra are partially replaced by[BO 4]5− groups.

The comparison of the nearest neighbors around [BO2] and F located within the channels is shown in Fig. 14(b,c). F ions (0,0,1/4) are situated on the mirror plane in the center of Sr triangle. As a result, constant F…F distances of 3.64 Å (a/2) are observed along [001] (b). In Sr10[(PO4)5.5(BO4)0.5](BO2), the incorporation of boron atoms between two O atoms draws these atoms closer (d(O-B-O) = 2.57 Å) and at the same time increases the gaps between two neighboring [BO2] units (d(O…O) = 4.73 Å), which results in alternating O…O distances along the c-axis (a) [70].

By replacing Mn in YCa3(MnO)3(BO3)4 with trivalent Al and Ga, two new borates with the compositions of YCa3(MO)3(BO3)4 (M = Al, Ga) were prepared via the solid-state reaction byYuet al [71]. The phases are isostructural to gaudefroyite with the hexagonal space group P63/m. The cell parameters of a = 10.38775 Å, c = 5.69198Å for the Al-containing compound and a = 10:5167 Å, c = 5:8146 Å for the Ga analogue were obtained from the refinements. The structure is constituted of AlO6 or GaO6 octahedral chains interconnected by BO3 groups in the ab plane to form a Kagomé-type lattice[1] - [72],[73],[74], leaving trigonal and apatite-like tunnels. It was found that most rare-earth and Cr, Mn ions can be substituted into the Y3+ and M3+ sites, respectively, and the preference of rare-earth ions to be located in the trigonal tunnel is correlated to the sizes of the M3+ ions.

5.4 Other apatite-type REE silicates

Hexagonal apatite-type phase of the composition of Pr9K(SiO4)6O2 (space group P63/m, a = 9.6466 Å and c = 1136 Å, V = 573.28 Å3, ρcalc = 5.48 g·cm−3 and Z = 1) was synthesized byWerner and Kubel[75] in a potassium fluoride flux. Potassium fills one (4f) of two metal positions present in the structure (Fig. 15) with the occupancy factor of 25%. The remaining positions of this site (Pr2/K2) are occupied by praseodymium.

Fig. 15.

The perspective view of Pr9K(SiO4)6O2 along the c-axis: regular planar coordination of the channel oxygen O(4) and the SiO4 tetrahedra are marked in gray, and the unit cell is outlined [75].

Oxygen from the silicate groups forms a coordination polyhedron (ninefold) in the shape of a distorted threefold capped trigonal prism. These face sharing [(Pr2/K2)O9]-polyhedra build up chains, which are interconnected via the SiO4 groups. The resulting channel framework accommodates sevenfold oxygen-coordinated praseodymium (Pr1), attached to the inside of the tubes that are aligned parallel to the c-axis. Oxide ions O4, located on the longitudinal axis of the channels, exhibit anomalously high atomic displacement parameters along the c-direction [75].

Single crystals of apatite-type Nd9.33(SiO4)6O2, Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 were described in Section 4.2.2. The structure of samarium orthosilicate oxyapatite (Sm5(SiO4)3O, Fig. 16) was resolved byMorganet al [76]. The phase crystallizes in hexagonal system with the space group P63/mand the cell parameters: a = 9.4959 Å, c = 7.0361 Å, c:a = 0.7410 and V = 549.46 Å3. The preparation and the structure of single crystal of strontium tetrapraseodymium tris(silicate) oxide (SrPr4(SiO4)3O), which was grown by the self-flux method using SrCl2, was described bySakakuraet al [77].

Fig. 16.

The structure of Sm5(SiO4)3O viewed along the c-axis [76].

The M(2) sites are almost exclusively occupied by praseodymium. The complete series of apatite-like compounds REE9.330.67[SiO4]6O2, LiREE9[SiO4]6O2 and NaREE9[SiO4]6O2 were synthesized byFelsche[78] with REE: La → Lu. Apatite-type neodymium silicates doped with various cations at the Si site, Nd10Si5BO27−δ (B = Mg, Al, Fe, Si), were synthesized byXianget al [79] via the solid-state reaction.

The crystal growth and the structure of three new neodymium-containing silicates, Na0.50Nd4.50(SiO4)3O, Na0.63Nd4.37(SiO4)3O0.74F0.26 and Na4.74Nd4.26(O0.52F0.48)[SiO4]4, prepared using the eutectic mixture of KF/NaF were investigated byLatshawet al [80]. Na0.50Nd4.50(SiO4)3O and Na0.63Nd4.37(SiO4)3O0.74F0.26 adopt the apatite structure and crystallize in hexagonal space group P63/m, while Na4.74Nd4.26(O0.52F0.48)[SiO4]4 crystallizes in tetragonal space group I-4 and exhibits rare-earth mixing on the sodium site. The unit cell parameters of the crystals are:

  1. Na0.50Nd4.50(SiO4)3O: a = 9.5400 Å and c = 7.033 Å;

  2. Na0.63Nd4.37(SiO4)3O0.74F0.26: a = 9.5533 Å and c = 7.0510 Å;

  3. Na4.74Nd4.26(O0.52F0.48)[SiO4]4: a = 12.1255 Å and c = 5.4656 Å.

Fig. 17.

Bond distances (Å) and anisotropic displacement in Gd4.33Ho4.33(SiO4)6(OH)2: note that the exaggerated anisotropic displacement of O(3) is attributable to the high proportion of vacancies in REE(1) and near-equatorial distribution of strong bonds to Si and REE(2) [81].

Double REE silicate Gd4.33Ho4.33(SiO4)6(OH)2 with the hydroxylapatite structure was synthesized byWanget al [81] using the piston-cylinder high-pressure apparatus at the pressure of 2.0 GPa and the temperature of 1450°C. Since they have nearly identical chemical character, two REE cations (Ho and Gd) are distributed randomly among the M(1) and M(2) sites, and the charge balance is maintained by the cation vacancies in M(1). The presence of two different REE cations in the same compound might promote better understanding of the cooperative effects of ions under the solid-state conditions (Fig. 17).

The structure analysis reveals that the hexagonal compound crystallizes in usual apatite space group P63/mwith lattice parameters a = 9.3142 Å and c = 6.7010 Å. REE(1) atoms are connected to nine oxygen atoms with the REE(1)-O bond distances ranging from 2.317 to 2.751 Å (mean 2.478 Å) and REE(2) atoms are connected to seven oxygen atoms with the RE(2)-O bond distances ranging from 2.223 to 2.690 Å (mean 2.393 Å). Oxygen anion O(4) in the apatite channel is located on the 63 axis and coordinated with three REE(2) cations arranged in a tricluster perpendicular to the c-axis. An isotropic displacement parameter was used for O(4), and H atom was assumed to ride on it. OH anions are stacked in regular column in the apatite channel, and in locally ordered structure, their polar direction is flipped in neighboring channel [81].

The formation of apatite-type phases of the composition of KNd6(SiO4)6O2 from the glass precursor (4K2O-Nd2O3-17SiO2) during the hydrothermal experiments (500°C and 825 bar) carried out at KOH molarities of 6 or greater was reported byHaileet al [82]. High temperatures, high pressures and long times tended to favor the synthesis of this apatite-type phase over the K8Nd3Si12O32OH phase. In comparison with the potassium system, the concentration of NaOH required for the synthesis of NaNd6(SiO4)6O2 phase (system 4Na2O-M2O3-17SiO2, where M = Nd and Y) is very low. The formation of apatite-type phases in the 4Na2O-Y2O3-17SiO2 system was not observed [83].

Two series of strontium-lanthanum apatites, Sr10−xLax(PO4)6−x(SiO4)xF2 and Sr10−xLax(PO4)6−x(SiO4)xO with 0 ≤ x ≤ 6, were synthesized byBoughzalaet al [84] via the solid-state reaction in the temperature range of 1200 – 1400°C:




where x = 0, 1, 2, 4 and 6. Sr2P2O7 was synthesized by the following reaction at 900°C:


The raw meal was prepared via mixing SrCO3, La2O3, SiO2, SrF2 and (NH4)2HPO4 in required stoichiometric amounts (0 ≤ x ≤ 6). The mixture was ground in an agate mortar, pressed to pellets and calcined at the temperature of 900°C for 12 h under the flow of argon (Sr10−xLax(PO4)6−x(SiO4)xF2) and oxygen (Sr10−xLax(PO4)6−x(SiO4)xO). The product was ground and pressed again in order to improve its homogeneity. Next, thermal treatment was performed at the temperature of 1200 and 1400°C (depending on the content of SiO2) for 12 h. The samples were heated and cooled with the rate of 10°C·min−1. The incorporation of La3+ and SiO4 4− ions into the apatite structures, i.e. the substitution of the pair La3+ and SiO4 4− for Sr2+ and PO4 3−, induced an increase of parameter a and decrease of parameter c (Fig. 18) [84].

The formation of nanocrystalline Ce-Yb mixed silicate-type oxyapatite of the composition of YbyCe9.33−y(SiO4)6O2 via the solid-state synthesis was described byMałeckaandKępiński[85]. The phase was identified as an intermediate formed during the synthesis of Ce-Yb silicates.

Fig. 18.

The variation of a and c parameters of Sr10−xLax(PO4)6−x(SiO4)xF2 and Sr10−xLax(PO4)6−x(SiO4)xO phases with the value of x [84].

Different compositions of apatite-type La10Si6−xWxO27+δ ceramics were prepared successfully byXianget al [86] via the high-temperature solid-state reaction route. Doping with W6+ is beneficial to the removal of La2SiO5 impurity phase. When the doping content of W6+ is more than 0.1, the rod-like grains of La10Si6−xWxO27+δ ceramics are replaced gradually by equiaxed apatite-type grains, and randomly shaped convex La6W2O15 particles appear at the grain boundaries. While doping with Nb5+ leads to the hexagonal-phase La9.5Ge5.5Nb0.5O26.5, the addition of MO6+ leads to the compound La9.5Ge5.5Mo0.5O26.75 with triclinic symmetry [87].

5.5 Apatite-type yttrium silicates

5.5.1 Yttrium silicates

The formation of the phase with the composition (Y4Si3O12, Y4(SiO4)3 or 2Y2O3·3SiO2 [88]), which is stable between 1650 and 1950°C, was reported byToropovandBondar[89] in the binary system of Y2O3-SiO2 and byToporovandFedorov[90] in the ternary system of CaO-Y2O3-SiO2 (Fig. 19(a)). This was the first reported occurrence of the phase with the composition between yttrium orthosilicate (Y2SiO5, oxyorthodsilicate, YSO, Y2O3:SiO2 = 1:1 [91]) and yttrium disilicate Y2Si2O7 (yttrium pyrosilicate, YPS, 1:3). The structure of this phase was described as the garnet type [88].

Fig. 19.

Phase equilibrium in the system Ca2SiO4-Y4(SiO4)3 according to Toporov and Fedorov [90] and Y2O3-SiO2 phase diagram [92].

Since then, authors have disagreed about the existence of such a phase because the attempts to make it starting with yttria and silica powders resulted in the formation of only Y2SiO5 and Y2Si2O7 [93]. This phase was not reported either by other studies of Y2O3-SiO2 system [92],[94],[95], which contains two compounds. Y2SiO5 andY2Si2O7 were found, with two (A and B) and five (y, α, β, γ and δ, also called y, B, C, D and E [96]) polymorphs, respectively. The first has a congruent melting, whereas the second has an incongruent one (Fig. 19(b)).

Nevertheless, the formation of oxyapatite phase of the composition of Y4.670.33(SiO4)3O (7:9) prepared via the oxidation of nitrogen apatite Y5(SiO4)3N was reported by other authors [7], [97]. The apatite-like phase Y4.67(SiO4)3O possesses hexagonal structure with the space group P63/m, a = 9.368 and c = 6.735 Å [78],[98]. The specific gravity of the phase is 4.39 g·cm−3 and the hardness on the Mohs scale is 5 – 7 [99].

Since the structure of YSO containing two different types of anions includes the [SiO4]4− complex ion and an additional non-silicon-bonded oxygen ion (NBO), it could be written as Y2(SiO4)O. This compound also displays two structure types of monoclinic symmetry with different linking of O-Y4 tetrahedra. Low-temperature X1 phase and high-temperature X2 phase belong to the space groups of P21/c(Z = 2) and C2/c(Z = 8), respectively [91].

The samples of the composition of Y4(SiO4)3, and similar ones containing small amount of iron oxide, corresponding to an overall composition of Fe0.2Y4(SiO4)3O0.2, were produced by the mixed powder method and by the sol-gel route using yttrium nitrate (Y(NO3)3·5H2O), TEOS (tetraethylorthosilicate) and iron nitrate (Fe(NO3)3·9H2O) byParmentieret al [7]. Nitrate was dissolved in ethanol/water mixture (volume ratio 7:3), the amount of the latter being controlled to give final Si concentration. Iron nitrate was added at this stage in calculated amounts corresponding to the final iron-doped apatite composition. The solution was stirred for a few hours and TEOS was added to give the appropriate silicon content and then the solution was placed in an oven at 60°C until the gelation occurred. The gel was dried at 80°C and calcined at 600°C for 1 h.

Powders prepared by the two routes were uniaxially pressed into pellets and treated to temperatures up to 1650°C in air in a Pt crucible, or for the heat treatments at 1700°C, carbon element furnace was used, and the samples were heated in a BN-lined crucible in nitrogen atmosphere. Iron appears to have two roles depending on the temperature; it stabilizes the apatite phase at high temperatures when produced by the sol-gel route and catalyzes the decomposition of sol-gel-derived apatite at low temperatures [7].

A new phase of yttrium magnesium silicate having the apatite structure was prepared bySuwaet al [100] at 1500°C in air. Its chemical composition can vary from (Y4Mg)Si3O13 to (Y4.33Mg1.13)Si3.34O13. The hexagonal unit cell dimensions a0 and c0 of (Y4Mg)Si3O13 are 9.298 ± 0.002A and 6.635 ± 0.001A, respectively, and its axial ratio c/a is 0.714. It is optically uniaxial negative with ε = 1.810 ± 0.005, ω = 1.820 ± 0.005 and ωε = 0.010. The cleavages parallel and perpendicular to the c-axis were recognized. The formation of the apatite-type phase of the composition of NaY9Si6O26 in the ternary system Na2O-Y2O3-SiO2 was also reported byLeeet al [101].

Fig. 20.

Calculated liquidus surface of the Y2O3-Al2O3-SiO2 system: three-phase equilibria with liquid phase (thick lines), liquidus surfaces for various solids (labeled area) and isothermal section (dotted lines, temperature in hundreds °C) [92].

The phase diagram of Al2O3-SiO2-La2O3 system (Fig. 20) can be compared with the Y2O3-Al2O3-SiO2 ternary diagram examined byBondarandGalakhovin 1964 [102]. The latter represents the only other example of REE2O3-Al2O3-SiO2 phase diagram found so far in the literature. The authors identified the liquidus surface of the whole ternary field, but they failed to elucidate the subsolidus phase relationships among different binary compounds. Due to the much smaller ionic size of Y3+ ion with respect to La3+ ion (1.18 and 1.015 Å for the eightfold coordination, respectively [103]) and lower bond-valence parameter (2.019 and 2.172 Å [104]), the stability of the binary compositions is substantially altered [57].

The β-alumina-like phase LaAl11O18 is no longer stable, while the garnet-like phase Y3Al5O18 and Y4Al2O9 monoclinic compound exist. The lacunar apatite-like phase Y14Si9O39 reported byWillset al [105] does not appear in the Y2O3-Al2O3-SiO2 ternary diagram; however, a compound with similar Y/Si atomic ratio, namely Y4Si3O12, also reported byWillset al [105], does. Since the formation of Y4Si3O12 phase was not confirmed, it may be stabilized by impurities [57],[92].

5.5.2 AM and AEE-yttrium orthosilicate oxyapatites

Alkaline metals (AM) and alkaline-earth element oxyapatites (oxybritholites) are described in this chapter. Phosphate minerals of the apatite supergroup possess strong affinity for strontium [106]. The apatite-type phase of the composition of Sr2Y8Si6O26 (Sr2Y8(SiO4)6O2) was prepared byZuevet al [107] via the two-stage calcination of mixture of SrCO3, Y2O3, Eu2O3 and SiO2 in air in order to investigate the spectral characteristics of Sr2Y8(SiO4)6O2:Eu polycrystals. The structure of apatite phase is shown in Fig. 21. The sol-gel synthesis and the characterization of Sr2Y8(1−x)Eu8xSi6O26 solid solution doped by Eu, where x = 0.01 – 0.4, was described byKarpovandZuev[108]. The formation of calcium analogue (Ca2Y8(SiO4)6O2, calcium-yttrium-silicate oxyapatite) was observed during the crystallization of SiO2-Al2O3-CaO-Na2O-K2O-F-Y2O3 glass ceramics [109] and during the degradation of advanced environmental barrier coatings [110]. The phase Ca4Y6(SiO4)6O2 was often prepared in order to investigate its luminescent properties [111],[112].

Fig. 21.

The structure of Sr2Y8(SiO4)6O2 [107].

Fig. 22.

The structure of NaY9(SiO4)6O2 oxyapatite (the perspective view along the c-axis) [114].

The precipitation of NaY9(SiO4)6O2 apatite-type compound (sodium nonayttrium hexakis(silicate) dioxide) in the SiO2-B2O3-Al2O3-Y2O3-CaO-Na2O-K2O-F glass-ceramics system (Section 10.3.8) was described byvan’t Hoenet al [113]. The hexagonal structure of NaY9(SiO4)6O2 oxyapatite (Fig. 22) was resolved byGunawardaneet al [114]. The phase crystallizes in P63/mspace group with the cell parameters a = 9.334, c = 6.759 Å, c:a = 0.7241, V = 509.97 Å and Z = 1).

Lithium yttrium orthosilicate (LiY9(SiO4)6O2, lithium nonayttrium hexakis(silicate) dioxide) crystallizes in centrosymmetric space group P63/m. The structure closely resembles those of fluorine apatite. There are two different crystallographic sites for Y3+ ion, which are coordinated by seven and nine O atoms. One-fourth of the nine-coordinated site is occupied by Li atoms, thus maintaining the charge balance. Si atom occupies the tetrahedral site [115].

The preparation, the properties and the effect of sintering additives of hexagonal (P63/m) strontium-yttrate-silicate oxyapatite (oxybritholite2) with the composition of SrY4(SiO4)3O as the main product of sinter-crystallization process, in which the non-equilibrium melt was formed in the temperature interval from 1300 to 1550°C in the SrO-Y2O3-SiO2 system, was described byPtáčeket al [116]. The formation of non-equilibrium melt is facilitated by borate fluxes, alkaline fluxes and talc. The apparent activation energy and the frequency factor of the sinter-crystallization process were determined to be 1525 kJ mol−1 and 1.04·1045 s-1, respectively. The material shows low value of linear thermal expansion coefficient of (1.1 ± 0.1)·10−6°C−1 in the temperature range from 25 to 850°C.

The course of synthesis can be expressed by the following reaction formula [116]:


This reaction 12 is too general to describe formed intermediates (SrSiO3, Sr2SiO4, SrY2O4, …[1] -) and the process of sinter-crystallization of apatite:


Since the formation of SrY4(SiO4)3O proceeds thorough non-equilibrium melt phase, the effect of sintering additives such as borate fluxes, fluorides and carbonates of alkaline metals as well as talc was investigated. Sintering additives facilitate the formation of melt phase and increase the length of sinter-crystallization interval. The expansion after the thermal decomposition of strontium carbonate is reduced as well. Calcinate, treated to the temperature lower than the temperatures of sinter-crystallization interval, has hydraulic activity. Therefore, it can be applied in special composite cements as an activator for latent hydraulic and pozzolanic materials.

Fig. 23.

SEM picture and WDX analysis of hexagonal crystal of CaY4(SiO4)3O apatite phase.

Effect on sinter-crystallization processDecreasing intensity or temperature of effect →
Expansion before sinter-crystallizationPure >> NaF >> Talc ≈ Li2CO3 ≈ Li2B4O7 >> Na2CO3 > LiBO2 > K2CO3
Firing shrinkage (sample treated to 1600°C)LiBO2 > Li2B4O7 ≈ Pure > NaF > K2CO3 ≈ Na2CO3 > Talc > Li2CO3
Initial temperature of sinter-crystallizationPure ≈ Na2CO3 ≈ Talc > Li2B4O7 > K2CO3 ≈ NaF > LiBO2 > Li2CO3
Maximum rate of sinter-crystallizationLi2CO3 > Pure > Li2B4O7 ≈ Talc ≈ NaF ≈ K2CO3 ≈ Na2CO3 > LiBO2
Length of interval of sinter-crystallizationLiBO2 >> Li2B4O7 ≈ NaF > Li2CO3 > pure > Talc > Na2CO3 > K2CO3

Table 3.

The influence of sintering additive on the behavior during thermal treatment [116].

After the process of sinter-crystallization, the reactivity of glassy phase with water drops. A significant benefit of talc is the fact that the glassy phase surrounding the crystals of apatite phase becomes resistant against the influence of water with this sintering additive. Furthermore, magnesium is not being incorporated into the structure of apatite phase during the crystallization of SrY4(SiO4)3O from non-equilibrium melt. The influence of sintering additives on the behavior during the thermal treatment is summarized in Table 3 [116].

The important feature of this compound is the formation of colored center after the exposition to X-ray radiation (Fig. 24); hence, the prepared material is an important candidate for optical applications, sensors and dosimeters.

Fig. 24.

Semitransparent disc of SrY4(SiO3)4O sintered specimen (a) and formation of colored center on X-ray irradiated area (Cu(Ka), 40 kV and 30 mA) [116].

On the other hand, this reaction also indicates that the synthesis of individual apatite analogues (AEEY4(SiO4)3O, where AEE = Ca, Sr and Ba) and their solid solutions proceeds via similar ical intermediates formed in the temperature range, which is affected by the thermal stability of AEE carbonates that increases in the order: CaCO3, SrCO3 and BaCO3.

Fig. 25.

SEM picture of hexagonal crystal of CaY4[SiO4]3O apatite phase.

While the synthesis of CaY4(SiO4)3O leads to well-developed hexagonal crystals (Fig. 25), the attempts for the preparation of BaY4(SiO4)3O phase were not successful. This synthesis leads to well-developed crystals of yttrium orthosilicate (Y2SiO5) surrounded by BaO-Y2O3-SiO2 glassy phase (Fig. 26).

Fig. 26.

SEM of Y2SiO5 crystal surrounded by barium-containing glassy phase.

The investigation of this system leads to the conclusion that non-limited Ca2+ ↔ Sr2+ substitution can be performed in the binary system of (Ca-Sr)Y4[SiO4]3O. On the contrary, the BaY4[SiO4]3O analogue of CaY4[SiO4]3O and SrY4[SiO4]3O apatite cannot be prepared; therefore, the extent of Ca2+ ↔ Ba2+ and Sr2+ ↔ Ba2+ substitutions is limited to 28 ± 4 and 38 ± 4%, respectively. The field of ternary solid solutions in the AEEY4[SiO4]3O system, where AEE = Ca, Sr and Ba, is shown in Fig. 27.

Fig. 27.

The miscibility in the AEE Y4(SiO4)3O system.

5.5.3. N-apatite

The main secondary phases in Jänecke prism[1] - [117] for Si3N4-Al2O3-SiO2-Y2O3-YN-AlN system[1] - are shown in Fig. 28. The formal exchange of oxygen by nitrogen leads to the compounds of N-apatite (Y10(SiO4)6N2, H-phase[1] -), N-melilite[1] - [118] (Y2Si3O3N4, M-phase), N-wollastonite (YSiON2, K-phase) and N-woehlerite (Y4Si2O7N2, J-phase). The latter one forms a complete solid solution with Y4Al2O9 (YAM) of the composition of Y4Si2−xAlxO7+xN2−x (Jss-phase) [119],[120],[121],[122],[123],[124].

Fig. 28.

Representation of the Si3N4-Al2O3-SiO2-Y2O3-YN-AlN phase equilibrium.

Y10(SiO4)6N2 (N-apatite, H-phase, (Y,Si,□)10[Si(O,N)4]6(O,N,□)2 [125], silicon-yttrium oxynitride) was first identified byRaeet al [126] as a compound with the compositional mixture of 10Y2O3·9SiO2·Si3N4 that was stable up to 1750°C. There were other suggested compositions, such as Y10Si7O23N4 [127]. Later work byGauckleret al [128] established N-apatite as a stoichiometric compound with the formula unit of Y10[SiO4]6N2 and the apatite structure (space group P63/m). The lattice constants of the hexagonal cell were reported to be a = 9.638 Å and c = 6.355 Å. The electronic structure and bonding of the complex ceramic crystal Y10(SiO4)6N2 was studied byChinget al [121]. This crystal is an insulator with direct band gap of 1.3 eV. It has some unique properties related to one-dimensional chain structure in the c-direction and planar N-Y bonding in the x-y plane.

The ternary phase diagrams of the Si3N4-Y2O3-SiO2 [123] and Si3N4-La2O3-SiO2 systems [129] are shown in Fig. 29(a) and (b). The apatite phase is able to form various solid solutions that may influence the development of strength in silicon nitride densified by yttria [130].

Fig. 29.

Phase relationships in the systems Si3N4-SiO2-Y2O3 [123] (a) and Si3N4-SiO2-La2O3 at 1700°C and 1550°C (dashed lines) [129] (b).

The hexagonal lanthanum N-apatite phase of the composition of La5(SiO4)3N (isostructural with apatite) can be prepared from the mixture of La2O3 and Si powder sintered at temperatures in the range from 900 to 1200°C under the flow of nitrogen. The melting temperature of this phase was determined to be ~1600°C. It was observed that continuous heating and addition of Pd into the reaction mixture favored the formation of La5(SiO4)3N. Prolonged heating of this compound yields La4.67(SiO4)3O [129],[131],[132],[133],[134],[135]. The absorption bands observed in infrared spectrum of lanthanum oxynitrides are introduced in Table 4.

Wavenumber [cm−1]ModeFrequency [cm−1][cm−1]
225δ (Si-O) (A1)730ν (Si-N)
327δ (Si-O) (B2)840Si-N
337Si-O-Si (A1)872SiO4 (ν1)
380SiO4 (ν4)909ν (Si-N) vs
376 – 385δ (Si-N) (sh)930Si-O-Nx
396SiO4 (ν4)940ν Si-N vs
432δ (Si-N) (sh)960
448Si-N s or O-Si-O bend.905
462SiO4 (ν4)934SiO4 (s) (ν3)
490ν Si-O (A1)980ν Si-N (s)
542ν Si-N (s)1060ν Si-O (ν3)
600ν Si-O1090ν Si-N (sh)
648ν Si-O (w)1130ν Si-O (B1)
679ν Si-O m (B2)

Table 4.

The absorption bands observed in infrared spectrum of lanthanum oxynitrides [131].

Mitomoet al [129] used the mixture of Si3N4 and La2O3 powder heated in a 10 mm diameter carbon die using a hot-pressing apparatus. In some compositions and at temperatures above 1600°C, liquid phases were squeezed out of the die by applied pressure resulting in a change in the overall composition. The compacted powder was therefore heated at the pressure of 20 MPa up to 1400°C and the temperature was then raised to 1700°C without the pressure. The specimen was kept at 1700°C for 1 h and then allowed to cool.

The sintering temperature of Si3N4 with La2O3 additions is 1700 to 1800°C. Heating of powder mixture of various Si3N4/L2O3 ratios at 1700°C results in the formation of 2Si3N4·La2O3, La5(SiO4)3N[1] - and β-Si3N4 or a glass. The reactions occurring during heating were determined as follows [129]:


The results ofSakaiet al [136] indicate that N-apatite and N-diopside containing grain boundary phase may improve the oxidation resistance of silicon nitride. Since the oxidation of Si3N4 leads to the formation of protective SiO2 layer on the surface:


the silicon nitride shows excellent oxidation resistance. Formed SiO2 then reacts with the grain boundary constituents to form silicates:


but in the case of MgO, the formed layer did not act as protection [136]. Y4.67(SiO4)3O apatite (britholite phase2) is formed as the oxidation product of silicon yttrium oxynitride (H-phase) in the temperature range from 700 to 1400°C [137],[138].

5.6. REE vanadocalcic apatite

The synthesis and physicochemical study of rare-earth-containing vanadocalcic oxyapatites where the pair Ca2+ and □ was substituted by Ln3+ and 1/2O2− was described byBenmoussaet al [139]. This substitution leads to lanthanum and praseodymium dioxyapatites Ca8Ln2(VO4)6O2, where Ln = La and Pr. Regarding rare earths such as neodymium, samarium, europium, gadolinium and terbium, the Ln3+ content limit varies from one REE to another. It decreases when the REE ionic radius declines.

5.7. Apatite-type yttrium phosphates

The following compositions having the apatite structure were prepared byWanmakeret al [140]:

  1. Me(II)2+xMe(III)8−x(SiO4)6−x(PO4)xO2, where 0 ≤ x ≤ 6;

  2. Me(II)3+xMeIII6−x(SiO4)6−x(PO4)x, where 0 ≤ x ≤ 1.5;

  3. Me(II)4+xMe(III)6−x(SiO4)6−x(PO4)xO, where 0≤ x ≤ 6.

with Me(II) = Ca, Sr, Ba, Mg, Zn or Cd and Me(III) = Y or La. Among these, there are several new compounds, e.g. Zn2La8(SiO4)6O2, BaMgY8(SiO4)6O2, Zn2Y8(SiO4)6O2, Cd2Y8(SiO4)6O2, Ca4La5(SiO4)5(PO4) and Ba4La5(SiO4)5(PO4). The crystallographic parameters were determined and their luminescence was studied. The most efficient activator proved to be trivalent antimony, especially in the compositions of type I. At 77°K, an emission band at about 400 nm was observed in many of these apatites.

The humidity-sensitivity of yttrium-substituted calcium oxyhydroxyapatites was studied byOwadaet al [141]. The logarithm of the electrical resistance of present sensors decreased linearly with increasing relative humidity (RH) from 30 to 65%. The resistance of [Ca9.0Y1.0](PO4)6[O1.50.5] with the largest OH vacancy content was about one order of magnitude lower than that of calcium hydroxyapatite. It was found that the larger the ratio of surface hydroxyl groups per unit surface area in the sample, the lower the resistance and the higher the amount of OH vacancies.

A ceramic proton conductor was obtained in the solid solutions of yttrium-substituted oxyhydroxyapatite (Ca10−xYx)(PO4)6((OH)2−x−2yOx+yy) [142]. Using the hydrogen concentration cells, it was confirmed that the specimens with the composition of x ≤ 0.65 have the protonic transference number (ti) is equal to one, while the values of ti of specimens with 0.65 < x < 1 were smaller than one. The conduction properties were also dependent on the composition of apatites. At x = 0.65, the conductivity (σ) showed the maximum value (5·10−4S·cm−1 at 800°C) in the relationship between σ and x, while the activation energy was the lowest (about 1.0 eV) at corresponding x. The applicability of proton conductive apatite for a fuel cell was discussed in Section 10.4.


  • Microporous material is defined as containing pores with the diameters >2 nm. The materials with the pore diameter in the range from 2 to 50 nm and higher than 50 nm are termed as mesoporous and macroporous, respectively. In combination with nanotechnology, the term nanoporous material is often used. Despite the fact that there is not clear definition, usually the pores with the size from 0.1 to 100 nm are considered. In other words, nanoporous covers the range from microporous to macroporous [1].
  • Since the prefix “oxy-“ can be explained as containing oxygen or additional oxygen, and the prefix “oxo-“ is used for the functional group or substituent oxygen atom connected to another atom by a double bond (=O), the names oxyapatite, oxy-apatite, oxoapatite and oxo-apatite can be considered as synonyms. In the published literature, the name oxyapatite is the most frequently used (~90%), and the term oxy-apatite is the second (~8%). The names oxoapatite and oxo-apatite are used much rarely (only about ~2%).
  • This conclusion is in discrepancy with the findings of Higuchi et al [12] described below.
  • The equation of Svante August Arrhenius [13],[14], which predicts that the rate constant k depends on the temperature: k = A exp (-Ea/RT), where A is the frequency (pre-exponential factor), Ea is the activation energy, R is universal gas constant (8.314 J·K−1·mol−1) and T is the thermodynamic temperature [13].
  • According to the Wyckoff notation: the specification of actual coordinates of atoms within the unit or primitive cell, which can be generated by the point-group operations or may be found by reference in the International Tables for Crystallography [18].
  • Cation with different valence. Apatite structure shows large flexibility upon the substitution of other aliovalent cations at the ‘Ca’ sites, pentavalent and tetravalent ions such as V5+, As5+ and Si4+ at the ‘P’ site and halide, oxide ions at the ‘OH’ site [21], as was described.
  • Dopants are also termed as doping agents. It can be defined as an impurity element added to the material structure in low concentration (usually <1 wt.% [23]) in order to alter its properties.
  • The fraction of total current that is transferred by a given ion is affected by its mobility. The sum of transport numbers for all ions in electrolyte is equal to one [28].
  • The p-type carriers possess typically higher mobility [25].
  • The technique is based on the Mössbauer effect of recoil-free nuclear resonance fluorescence [31], i.e. the phenomenon of emission or absorption of X-ray photon without the loss of energy. The Mössbauer effect has been detected in a total of 88 X-ray transitions in 72 isotopes of 42 different elements [32]. The 57Fe Mössbauer isotope is the most frequently used [33]. The Mössbauer spectroscopy can be used to determine the oxidation states of iron in minerals and to identify the presence of some mineral species in samples of unknown composition [31].
  • The Kröger-Vink notation indicates the lattice position for the point defect species in the crystal and its effective electric charge relative to the perfect lattice: MY Z is the atomic species M (or vacancy V) that occupies the lattice site Y and possesses the effective charge Z, where the symbols ●, ʹ and × are used for the effective charge +1, -1 and neutral particle, respectively) [40]. For example, Ali ●●● is Al3+ ion at interstitial site (i), VAlʹʹʹ is Al3+ vacancy, VO ●● is O2− vacancy, SrLaʹ (Eq. 8) means Sr2+ ion replacing La3+ at lattice site, TiAl ● means Ti4+ replacing Al3+ at lattice site, eʹ is electron and h● is the hole. The equation must fulfill the following three rules: mass balance (1), electroneutrality or charge balance (2) and site ratio conservation balance (3) [41].
  • Interstitial sites are sites between normal (equilibrium) atomic positions of ideal lattice atoms [42]. Interstitial atoms and vacancies (lattice site where atom is absent) are the simplest types of point defects in a crystal. A vacancy and interstitial atoms positioned close together are referred to as the Frenkel pair. Apart from the point defects, the line crystal defects (dislocation and disclination) are recognized [43].
  • Wanmaker et al [55] reported the synthesis of apatite-type compounds of the composition:
  • The densification rate is considered as the function of temperature (T) and mean grain size (Dm). Constant A depends on the surface energy (γsg) of grains, on the apatite molar volume (Ω) and on average coefficient of diffusion of limiting species D. This relationship can be written as follows [56]:
  • The Kagomé lattice (d) is one of the most interesting lattices in 2D, especially in materials in which the Kagome lattice is built from magnetic ions. Each of its vertices touches a triangle, hexagon, triangle and hexagon (the planes of corner-sharing equilateral triangles). The vertices correspond to the edges of the hexagonal (honeycomb) lattice (c), which in turn is the dual of triangular lattice (can be derived from triangular lattice by periodical removal of ¼ sites) (b). Since it has the same coordination number (z = 4), the Kagomé lattice is also related to the square lattice (a) [72], [73]. Numerous Kagomé compounds built from stacked Kagomé layers were found in Alunite (Jarosite, KFe3+3(SO4)2(OH)6, (e)) family of minerals [74].
  • Detailed description of formed intermediates can be found in work [116].
  • Jänecke prism is used to visualize the phase relationships among α-sialon, β-sialon and other phases in the M-Si-Al-O-N system. α- and β-sialons are isostructural with α- and β-Si3N4, respectively. The substitution of Al-O for Si-N in β-Si3N4 yields β-sialon with general formula: Si6−xAlzOzN8−z (0 < z < 4.2). The structure is built up by Si and Al tetrahedra coordinated with oxygen and nitrogen. The unit cell contains two Si3N4 units. α-sialons are solid solutions based on the α-Si3N4 structure, with the general formula: Mp+xSi12−(m+n)Al(m+n)OnN16−n, where M is metal ion such as Li, Ca, Ba, Y and RE with a valence of p+ and index m = px [117].
  • Yttria is often used additive to improve the sintering behavior of Si3N4 [124].
  • In dependence on the system composition, the general composition of H-phase (N-apatite) can be written as (M,REE)10(SiO4)6N2. The specification of cations then leads to the names such as Mg-Nd-N-apatite [123].
  • The melilite-type structure (tetragonal mineral melilite ((Ca,Na)2(Al,Mg,Fe2+)(Si,Al)2O7)) is sorosilicate from the group of melilite, first described (Capo di Bove, near Rome, Italy) in 1976 and named from the Greek words meli “honey” and lithos “stone”). Y2Si3O3N4 was described by Fang et al [118]. N atoms fully occupy the bridging site (2c) and O atoms fully occupy the terminal site (4e) with 2 O and 6 N atoms at the bridging 8f site. The preferential distribution of O and N atoms at the 8f site results in two different local coordinations of Y and three different types of Si atoms.
  • The presence of La5(SiO4)3N is inevitable in the production of high-density materials by liquid-phase sintering; therefore, the amount of La5(SiO4)3N and glassy phase must be minimized to obtain materials with good high-temperature strengths [129].

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Petr Ptáček (April 13th 2016). Rare-earth Element-bearing Apatites and Oxyapatites, Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications, Petr Ptacek, IntechOpen, DOI: 10.5772/62209. Available from:

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