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-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” -  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 .
5.1 Apatite-type lanthanium silicates
During recent decades, oxyapatite --type structure with the general formula: REE9.33+xSi6O26+3x/2 (where REE is rare-earth element) ,, REE9.33□0.67(SiO4)6O2  or REE10−x(SiO4)6O2+y  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 ,,,,.
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 by
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 by
|Apatite-type phase||Lattice parameters [Å]||M||V||Density||E||σ0|
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 P6
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 .
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))  - 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 ,,,.
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 .
Conventional oxide ion conductors are designed on the basis of the oxygen vacancy model by the introduction of aliovalent -  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 .
Apparent exchange of O(1), O(2) and O(3) oxide ions bonded to Si was observed by 17O NMR measurement on La9.33Si6O26 by
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 by
|Apatite-type lanthanium silicate||Lattice parameters [Å]||V||E (600 – 800°C)||E (400 – 550°C)|
All synthesized samples have hexagonal lattice structure with the space group of P6
The oxygen ionic and electronic transport in apatite ceramics with the composition of La10Si6−xFexO27−x/2 (x = 1 – 2)  and La10−xSi6−yAlyO27−3x/2−y/2 (x = 0 – 33; y = 0.5 – 1.5) , was investigated by
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 ,.
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 - ,,, 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 .
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 .
The ionic conduction in cation-deficient apatite La9.33−2x/3MxSi6O26, where M = Mg, Ca and Sr was investigated by
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 :
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  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 P6
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 :
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 - , concentrations, and the cation vacancy concentrations appear to be the dominant factor in energetics .
The schematic diagram of the sol-gel process used by
The La10Si6O27 nanopowders with apatite structure were synthesized by the
La2O3 and TEOS in stoichiometric amount were used by
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 by
Vanadium-doped oxyapatite phases of the composition of La10−xVx(SiO4)6O3+x were prepared by
The phase La5Si2BO13 ,, crystallizes with apatite-related structure (Fig. 4) with the space group P6
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 .
The preparation of La-Si-O apatite-type thin films was described by
The formation of ternary compound with apatite structure in the system La2O3-Ga2O3-SiO2 (Fig. 5) was first reported by
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 .
The CaO-La2O3-SiO2-P2O5 phase diagram was investigated by
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 ,:
Sr2+xLa8−x(SiO4)6−x(PO4)xO2 with 0 ≤ x ≤ 6  -;
Sr3+xLa6−x(SiO4)6−x(PO4)x with 0 ≤ x ≤ 1.5;
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 - of strontium-doped apatite-type lanthanum silicates (SrxLa10−xSi6O27−x/2) under isothermal conditions (1250 – 1550°C),
The ternary phase diagram Al2O3-SiO2-La2O3 at 1300°C (Fig. 7) was investigated by
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 .
The preparation of single crystal of apatite-type lanthanium germanate of the composition of La9.33Ge6O26 was reported by
Apatite-type lanthanium germanate possesses hexagonal structure with the space group P6
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 . 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 ,,.
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 , ,.
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 by
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 .
The hydrothermal synthesis of apatite-type compound NaRE9(GeO4)6O2 (RE = Nd, Pr) with the hexagonal structure of the space group of P6
The high-temperature flux method for the preparation of single crystal of hexagonal NaLa9Ge6O26 apatite-type germanate (space group P6
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 .
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 by
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 .
The structure and optical properties of noncentrosymmetric borate RbSr4(BO3)3 (RSBO) was described by
Europium borate fluoride, Eu5(BO3)3F with an apatite-type structure, was synthesized by
The structure of single crystal of strontium phosphate orthoborate metaborate (Sr10[(PO4)5.5(BO4)0.5](BO2)) that was grown from the melt by
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  (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) .
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 by
5.4 Other apatite-type REE silicates
Hexagonal apatite-type phase of the composition of Pr9K(SiO4)6O2 (space group P6
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 .
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 by
The M(2) sites are almost exclusively occupied by praseodymium. The complete series of apatite-like compounds REE9.33□0.67[SiO4]6O2, LiREE9[SiO4]6O2 and NaREE9[SiO4]6O2 were synthesized by
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 by
Na0.50Nd4.50(SiO4)3O: a = 9.5400 Å and c = 7.033 Å;
Na0.63Nd4.37(SiO4)3O0.74F0.26: a = 9.5533 Å and c = 7.0510 Å;
Na4.74Nd4.26(O0.52F0.48)[SiO4]4: a = 12.1255 Å and c = 5.4656 Å.
Double REE silicate Gd4.33Ho4.33(SiO4)6(OH)2 with the hydroxylapatite structure was synthesized by
The structure analysis reveals that the hexagonal compound crystallizes in usual apatite space group P6
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 by
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 by
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) .
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 by
Different compositions of apatite-type La10Si6−xWxO27+δ ceramics were prepared successfully by
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 ), which is stable between 1650 and 1950°C, was reported by
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 . This phase was not reported either by other studies of Y2O3-SiO2 system ,,, 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 ) 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 Y4.67□0.33(SiO4)3O (7:9) prepared via the oxidation of nitrogen apatite Y5(SiO4)3N was reported by other authors , . The apatite-like phase Y4.67(SiO4)3O possesses hexagonal structure with the space group P6
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 P2
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) by
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 .
A new phase of yttrium magnesium silicate having the apatite structure was prepared by
The phase diagram of Al2O3-SiO2-La2O3 system (Fig. 20) can be compared with the Y2O3-Al2O3-SiO2 ternary diagram examined by
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 by
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 . The apatite-type phase of the composition of Sr2Y8Si6O26 (Sr2Y8(SiO4)6O2) was prepared by
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 by
Lithium yttrium orthosilicate (LiY9(SiO4)6O2, lithium nonayttrium hexakis(silicate) dioxide) crystallizes in centrosymmetric space group P6
The preparation, the properties and the effect of sintering additives of hexagonal (P6
The course of synthesis can be expressed by the following reaction formula :
This reaction 12 is too general to describe formed intermediates (SrSiO3, Sr2SiO4, SrY2O4, … -) 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.
|Effect on sinter-crystallization process||Decreasing intensity or temperature of effect →|
|Expansion before sinter-crystallization||Pure >> 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-crystallization||Pure ≈ Na2CO3 ≈ Talc > Li2B4O7 > K2CO3 ≈ NaF > LiBO2 > Li2CO3|
|Maximum rate of sinter-crystallization||Li2CO3 > Pure > Li2B4O7 ≈ Talc ≈ NaF ≈ K2CO3 ≈ Na2CO3 > LiBO2|
|Length of interval of sinter-crystallization||LiBO2 >> Li2B4O7 ≈ NaF > Li2CO3 > pure > Talc > Na2CO3 > K2CO3|
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 .
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.
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.
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).
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.
The main secondary phases in Jänecke prism -  for Si3N4-Al2O3-SiO2-Y2O3-YN-AlN system - are shown in Fig. 28. The formal exchange of oxygen by nitrogen leads to the compounds of N-apatite (Y10(SiO4)6N2, H-phase -), N-melilite -  (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) ,,,,,.
Y10(SiO4)6N2 (N-apatite, H-phase, (Y,Si,□)10[Si(O,N)4]6(O,N,□)2 , silicon-yttrium oxynitride) was first identified by
The ternary phase diagrams of the Si3N4-Y2O3-SiO2  and Si3N4-La2O3-SiO2 systems  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 .
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 ,,,,,. The absorption bands observed in infrared spectrum of lanthanum oxynitrides are introduced in Table 4.
|Wavenumber [cm−1]||Mode||Frequency [cm−1]||[cm−1]|
|225||δ (Si-O) (A1)||730||ν (Si-N)|
|327||δ (Si-O) (B2)||840||Si-N|
|337||Si-O-Si (A1)||872||SiO4 (ν1)|
|380||SiO4 (ν4)||909||ν (Si-N) vs|
|376 – 385||δ (Si-N) (sh)||930||Si-O-Nx|
|396||SiO4 (ν4)||940||ν Si-N vs|
|432||δ (Si-N) (sh)||960|
|448||Si-N s or O-Si-O bend.||905|
|462||SiO4 (ν4)||934||SiO4 (s) (ν3)|
|490||ν Si-O (A1)||980||ν Si-N (s)|
|542||ν Si-N (s)||1060||ν Si-O (ν3)|
|600||ν Si-O||1090||ν Si-N (sh)|
|648||ν Si-O (w)||1130||ν Si-O (B1)|
|679||ν Si-O m (B2)||—||—|
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 - and β-Si3N4 or a glass. The reactions occurring during heating were determined as follows :
The results of
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 . 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 ,.
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 by
5.7. Apatite-type yttrium phosphates
The following compositions having the apatite structure were prepared by
Me(II)2+xMe(III)8−x(SiO4)6−x(PO4)xO2, where 0 ≤ x ≤ 6;
Me(II)3+xMeIII6−x(SiO4)6−x(PO4)x, where 0 ≤ x ≤ 1.5;
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 by
A ceramic proton conductor was obtained in the solid solutions of yttrium-substituted oxyhydroxyapatite (Ca10−xYx)(PO4)6((OH)2−x−2yOx+y□y) . 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 .
- 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  described below.
- The equation of Svante August Arrhenius ,, 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 .
- 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 .
- 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 , 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.% ) 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 .
- The p-type carriers possess typically higher mobility .
- The technique is based on the Mössbauer effect of recoil-free nuclear resonance fluorescence , 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 . The 57Fe Mössbauer isotope is the most frequently used . 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 .
- 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) . 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) .
- Interstitial sites are sites between normal (equilibrium) atomic positions of ideal lattice atoms . 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 .
- Wanmaker et al  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 :
- 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) , . Numerous Kagomé compounds built from stacked Kagomé layers were found in Alunite (Jarosite, KFe3+3(SO4)2(OH)6, (e)) family of minerals .
- Detailed description of formed intermediates can be found in work .
- 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 .
- Yttria is often used additive to improve the sintering behavior of Si3N4 .
- 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 .
- 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 . 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 .