Rare-earth Element-bearing Apatites and Oxyapatites

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 descrip‐ tion of synthetic compounds of apatite structure. Attention will be directed to descrip‐ tion 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.

In low atomic number rare-earth silicate systems, an apatite phase occurs with a range of stability extending from Ln 4.67 (SiO 4 ) 3 O to Ln 4 (SiO 4 ) 3 . The stability decreases as the rare-earth atomic number increases, with a mixture of Ln 2 SiO 5 and Ln 2 Si 2 O 7 replacing apatite as the preferred phase assemblage [7], [8], [9], [10], [11]. chemical synthesis of apatite-type lanthanum silicates from the mixture of La 2 O 3 and amorphous silica without post-milling thermal treatment was described by FUENTES et al [4].
Rare-earth element-doped apatite-type lanthanum silicates of the composition of La 9 MSi 6 O 27 , where M = Nd, Sm, Gd and Yb, were synthesized by the high-temperature solid-state reaction process by XIANG et al [3]. All rare-earth oxide powders (La 2 O 3 , Nd 2 O 3 , Sm 2 O 3 , Gd 2 O 3 and Yb 2 O 3 ) 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.  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 P6 3 /M. The temperature dependence of total electrical conductivity for different compositions is determined using the Arrhenius equation 4 [13], [14], [15] in the following form [3], [5], [16], [17]: where σ is the total electrical conductivity, σ 0 is the pre-exponential factor related to the effective number of mobile oxide ions, E is the activation energy for the electrical conduction process, k B is the Boltzmann constant and T is absolute temperature. ΔH m and ΔH a 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 La 10 Si 6 O 27 to La 9 GdSi 6 O 27 . 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 La 9 NdSi 6 O 27 . The total electrical conductivity is also a function of partial pressure of oxygen [3]. 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 Ln 9.33 (SiO 4 ) 6 O 2 , 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. SiO 4 tetrahedra are isolated mutually, and Ln ions (REE ions, in general) at 6h sites (sevenfold coordinated site (x,y,¼)) [18] 5 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 apatitetype 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 6 [21] cations. In Ln 9.33 (SiO 4 ) 6 O 2 , 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]. 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 La 10 Si 6−x Fe x O 27−x/2 (x = 1 -2) [25] and La 10−x Si 6−y Al y O 27−3x/2−y/2 (x = 0 -33; y = 0.5 -1.5) [26], [27] was investigated by SHAULA et al In both cases, the essential role of oxygen content on the ionic conductivity of apatite phase was recognized. The ion transference number 8 [28] increases with decreasing partial pressure of oxygen. Such behavior indicates that the conduction under oxidizing condition is predominantly of p-type 9 (with respect to n-type of conductivity). Similar to the foundation of SHI and ZHANG [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, La 10 Si 4 Fe 2 O 26 ). 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 La 9.33+x/3 Si 6−x Al x O 26 , 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 La 9.83−x Pr x Si 4.5 Fe 1.5 O 26+δ (x = 0 -6) decreases the unit cell volume, suppresses the Fe 4+ formation according to Mössbauer spectroscopy 10 [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 La 10 [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 La 2 O 3 into La 9.33 (SiO 4 ) 6 O 2 to form La 10 (SiO 4 ) 6 O 3 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 La 2 O 3 into La 9.33 (SiO 4 ) 6 O 2 can therefore be expressed as [10]: 8 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]. 9 The p-type carriers possess typically higher mobility [25]. 10 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 57 Fe 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 substitution of La 2 O 3 by SrO, taking into account the charge balance and the oxygen content, can be represented as follows (KRÖGER-VINK notation 11 [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 Sr 2+ doping, with no cation vacancies or interstitial oxygen atoms, is the most stable composition. The energetics of lanthanum silicate apatite materials (La 9.33+x (SiO 4 ) 6 O 2+3x/2 and La 10−x Sr x (SiO 4 ) 6 O 3−0.5x ) depends on lanthanum deficiency and oxygen interstitial 12 [42], [43] concentrations, and the cation vacancy concentrations appear to be the dominant factor in energetics [39]. 11 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: M Y 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, Al i ••• is Al 3+ ion at interstitial site (i), V Al ʹʹʹ is Al 3+ vacancy, V O •• is O 2− vacancy, Sr La ʹ (Eq. 8) means Sr 2+ ion replacing La 3+ at lattice site, Ti Al • means Ti 4+ replacing Al 3+ 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]. 12 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].  The schematic diagram of the sol-gel process used by TAO and IRVINE [44] for the preparation of apatite-type lanthanium silicates is shown in Fig. 3. The room-temperature structure is hexagonal, the space group is P6 3 or P6 3 /M, with a = 9.722 and c = 7.182 Å for La 10 (SiO 4 ) 6 O 3 and a = 9.717 and c = 7.177 Å for La 9.33 (SiO 4 ) 6 O 2 , i.e. the cell volume of La 10 (SiO 4 ) 6 O 3 is a little greater than that of La 9.33 (SiO 4 ) 6 O 2 . 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 La 10 (SiO 4 ) 6 O 3 is higher than that of La 9.33 (SiO 4 ) 6 O 2 and this indicates that oxygen interstitials may be introduced into the apatite lattice of La 10 (SiO 4 ) 6 O 3 , which may benefit the oxygen ion transportation [44].
The La 10 Si 6 O 27 nanopowders with apatite structure were synthesized by the LI et al [45] coprecipitation method. After the calcination at 900°C and then removing of La 2 O 3 by acid washing, the pure stoichiometric La 10 Si 6 O 27 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. La 2 O 3 and TEOS in stoichiometric amount were used by MASUBUCHI et al [46] as the starting materials for the preparation of both powder and film of apatite-type La 9.33 (SiO 4 ) 6 O 2 via the alkoxide hydrolysis. Lanthanum oxide was dissolved in HNO 3 (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) 6 O 26+z , where Ti substituted at the Si/Ge site, were reported by SANSOM et 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 La 10−x V x (SiO 4 ) 6 O 3+x were prepared by YUAN et 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 La 9 V(SiO 4 ) 6 O 4 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 V 5+ doped for La 3+ does lead to the formation of hexagonal apatites even with high oxygen contents.
The phase La 5 Si 2 BO 13 [49], [50], [51] crystallizes with apatite-related structure ( Fig. 4) with the space group P6 3 /M and 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: The preparation of La-Si-O apatite-type thin films was described by VIEIRA et 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 La 2 Si 2 O 7 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. The formation of ternary compound with apatite structure in the system La 2 O 3 -Ga 2 O 3 -SiO 2 ( Fig. 5) was first reported by WANG and UDA [53]. The apatite phase, which precipitates from the melt of the composition around that of stoichiometric La 3 Ga 5 SiO 14 (LGS), can be described by the formula: La 14 Ga x Si 9−x O 39−x/2 , where 0 ≤ x ≤ 3.5. Since there is a large field for the formation of solid solutions with the range extending from La 14 Si 9 O 39 to Ga 2 O 3 , some Si 4+ sites are probably substituted by Ga 3+ .
The liquidus surface of LS(G) was determined to be the field on the Ga 2 O 3 -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 Ga 2 O 3 . E and F are eutectic points, where LGS + LaGaO 3 + Ga 2 O 3 + liquid and LGS + Ga 2 O 3 + La 2 Si 2 O 7 + liquid were found, respectively [53].
The CaO-La 2 O 3 -SiO 2 -P 2 O 5 phase diagram was investigated by EL OUENZERFI et al [54] in order to determine a domain inside which all points correspond to pure apatitic oxyphosphosilicates with the general formula: Ca x La y (SiO 4 ) 6−u (PO 4 ) u O t . 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. 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]: Inside this domain, the solid solution continuously varies between pure phosphate apatites Ca x La y (PO 4 ) 6 O t and pure silicate apatites Ca x La y (SiO 4 ) 6 O t and also between oxyapatites Ca x La y (SiO 4 ) 6−u (PO 4 ) u O t and nonoxyapatites Ca x La y (SiO 4 ) 6−u (PO 4 ) u .
During the investigation of the kinetics of solid-state sintering 14 of strontium-doped apatitetype lanthanum silicates (Sr x La 10−x Si 6 O 27−x/2 ) under isothermal conditions (1250 -1550°C), BONHOMME et al [56] recognized that the densification mechanism of the apatite ceramics was 13 WANMAKER et al [55] reported the synthesis of apatite-type compounds of the composition: where M(II) = Ca, Sr, Ba, Mg, Zn or Cd and M(III) Y or La. The paper also contains structural data for several other newly prepared oxy-britholites, 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 Al 2 O 3 -SiO 2 -La 2 O 3 at 1300°C (Fig. 7) was investigated by MAZZA and RONCHETTI [57]. La 14 Si 9 O 39 was described by KUZ'MIN and BELOV [58] as an apatite-like structure of hexagonal symmetry (space group P6 3 /M). Isomorphous compounds were also reported for Nd [59], Ce [60] and Sm [58]. The La 14 Si 9 O 39 compound extends its stability range in the interior of the phase diagram, forming the solid solution of the type La 14+1x/3 Si 9−x Al x O 39 , 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]. Ternary phase diagram at 1300°C in air [57]. 14 The densification rate is considered as the function of temperature (T) and mean grain size (D m ). Constant A depends on the surface energy (γ sg ) 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 coefficient of diffusion D is thermally activated: where D 0 is the pre-exponential coefficient of diffusion, R is the universal gas constant and E a is the apparent activation energy of diffusion of the rate limiting process. Exponent n in Eq. (a) depends on the mechanism of transport of the limiting species governing the kinetics of densification.

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].  (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 La 9.33 Ge 6 O 26 single crystal showed little anisotropy in conductivity, the conductivity of La 9.33 Si 6 O 26 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]. The selective doping of La 9.33+x (GeO 4 ) 6 O 2+3x/2 with Y leads to the stabilization of hexagonal lattice, even at high oxygen contents. Furthermore, this has the effect of enhancing the lowtemperature 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 La 8 Y 2 (GeO 4 ) 6−x (GaO 4 ) x O 3−x/2 can be prepared for 0 ≤ x ≤ 2 with all samples showing the hexagonal symmetry, compared to the series without Y co-doping, La 10 (GeO 4 ) 6−x (GaO 4 ) x O 3−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, La 7.33+y/3 Y 2 (GeO 4 ) 6−y (GaO 4 ) y O 2 (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 extent of, and the structural changes within, the apatite domain in the LaO 1.5 -GeO 2 -SrO ternary system at 1100°C was studied and the single-phase samples were obtained for La 9.33+x −2y/3 Sr y (GeO 4 ) 6 O 2+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 NaRE 9 (GeO 4 ) 6 O 2 (RE = Nd, Pr) with the hexagonal structure of the space group of P6 3 /M was described by EMIRDAG-EANES et al [65]. The structure is composed of REO 7 and REO 9 polyhedra as well as GeO 4 tetrahedra (Fig. 10). The high-temperature flux method for the preparation of single crystal of hexagonal Na-La 9 Ge 6 O 26 apatite-type germanate (space group P6 3 /M, a = 9.883, c = 7.267 Å and Z = 1) was used by TAKAHASHI et al [66]. The crystal structure (Fig. 11) was found to be similar to that of silicate oxyapatite NaY 9 Si 6 O 26 . 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: A x Ln 10−x B 6 O 24 O 3−x . GeO 4 La(1) / Na La (2) O(4) Fig. 11. The structure of sodium lanthanum germanate NaLa 9 Ge 6 O 26 [66].     (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].

Apatite-type borates
The structure and optical properties of noncentrosymmetric borate RbSr 4 (BO 3 ) 3 (RSBO) was described by XIA and LI [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 "A 5 (TO n ) 3 X". When the structures of all apatite-like crystals are presented in orthorhombic unit cell, the arrangements of planar anionic BO 3 groups are all similar to one-third of the BO 3 groups aligned perfectly parallel at corner-and face-centered locations, whereas the other twothirds of BO 3 groups are distributed differently.   The structure of single crystal of strontium phosphate orthoborate metaborate (Sr 10 [(PO 4 ) 5.5 (BO 4 ) 0.5 ](BO 2 )) that was grown from the melt by CHEN et al [70] is shown in Fig. 14(a) The comparison of the nearest neighbors around [BO 2 ] − and F − located within the channels is shown in Fig. 14(b,c). The structure is constituted of AlO 6 or GaO 6 octahedral chains interconnected by BO 3 groups in the ab plane to form a Kagomé-type lattice 15 [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 Y 3+ and M 3+ sites, respectively, and the preference of rare-earth ions to be located in the trigonal tunnel is correlated to the sizes of the M 3+ ions.

Other apatite-type REE silicates
Hexagonal apatite-type phase of the composition of Pr 9 K(SiO 4 ) 6 O 2 (space group P6 3 /M, a = 9.6466 Å and c = 1136 Å, V = 573.28 Å 3 , ρ calc = 5.48 g·cm −3 and Z = 1) was synthesized by WERNER 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. 15 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 cornersharing 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, KFe 3+ 3 (SO 4 ) 2 (OH) 6 , (e)) family of minerals [74]. The preparation and the structure of single crystal of strontium tetrapraseodymium tris(silicate) oxide (SrPr 4 (SiO 4 ) 3 O), which was grown by the self-flux method using SrCl 2 , was described by SAKAKURA et al [77].     (4) in the apatite channel is located on the 6 3 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 KNd 6 (SiO 4 ) 6 The raw meal was prepared via mixing SrCO 3 , La 2 O 3 , SiO 2 , SrF 2 and (NH 4 ) 2 HPO 4 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 (Sr 10−x La x (PO 4 ) 6−x (SiO 4 ) x F 2 ) and oxygen (Sr 10−x La x (PO 4 ) 6−x (SiO 4 ) x O). 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 SiO 2 ) for 12 h. The samples were heated and cooled with the rate of 10°C·min −1 . The incorporation of La 3+ and SiO 4 4− ions into the apatite structures, i.e. the substitution of the pair La 3+ and SiO 4 4− for Sr 2+ and PO 4 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 Yb y Ce 9.33−y (SiO 4 ) 6 O 2 via the solid-state synthesis was described by MAŁECKA and KĘPIŃSKI [85]. The phase was identified as an intermediate formed during the synthesis of Ce-Yb silicates.

Yttrium silicates
The formation of the phase with the composition (Y 4 Si 3 O 12 , Y 4 (SiO 4 ) 3 or 2Y 2 O 3 ·3SiO 2 [88]), which is stable between 1650 and 1950°C, was reported by TOROPOV and BONDAR [89] in the binary system of Y 2 O 3 -SiO 2 and by TOPOROV and FEDOROV [90] in the ternary system of CaO-Y 2 O 3 -SiO 2 ( Fig. 19(a)). This was the first reported occurrence of the phase with the composition between yttrium orthosilicate (Y 2 SiO 5 , oxyorthodsilicate, YSO, Y 2 O 3 :SiO 2 = 1:1 [91]) and yttrium disilicate Y 2 Si 2 O 7 (yttrium pyrosilicate, YPS, 1:3). The structure of this phase was described as the garnet type [88].  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 Y 2 SiO 5 and Y 2 Si 2 O 7 [93]. This phase was not reported either by other studies of Y 2 O 3 -SiO 2 system [92], [94], [95], which contains two compounds. Y 2 SiO 5 andY 2 Si 2 O 7 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)).
Since the structure of YSO containing two different types of anions includes the [SiO 4 ] 4− complex ion and an additional non-silicon-bonded oxygen ion (NBO), it could be written as Y 2 (SiO 4 )O. This compound also displays two structure types of monoclinic symmetry with different linking of O-Y 4 tetrahedra. Low-temperature X 1 phase and high-temperature X 2 phase belong to the space groups of P2 1 /C (Z = 2) and C2/C (Z = 8), respectively [91].
The samples of the composition of Y 4 (SiO 4 ) 3 , and similar ones containing small amount of iron oxide, corresponding to an overall composition of Fe 0.2 Y 4 (SiO 4 ) 3 O 0.2 , were produced by the mixed powder method and by the sol-gel route using yttrium nitrate (Y(NO 3 ) 3 ·5H 2 O), TEOS (tetraethylorthosilicate) and iron nitrate (Fe(NO 3 ) 3 ·9H 2 O) by PARMENTIER et 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 by SUWA et al [100] [105], does.
Since the formation of Y 4 Si 3 O 12 phase was not confirmed, it may be stabilized by impurities [57], [92].
Lithium yttrium orthosilicate (LiY 9 (SiO 4 ) 6 O 2 , lithium nonayttrium hexakis(silicate) dioxide) crystallizes in centrosymmetric space group P6 3 /M. The structure closely resembles those of fluorine apatite. There are two different crystallographic sites for Y 3+ 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 (P6 3 /M) strontium-yttrate-silicate oxyapatite (oxybritholite2) with the composition of SrY 4 (SiO 4 ) 3 O 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-Y 2 O 3 -SiO 2 system, was described by PTÁČEK et 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 1.04·10 45 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]: Since the formation of SrY 4 (SiO 4 ) 3 O 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.   Table 3. The influence of sintering additive on the behavior during thermal treatment [116].
Apatites and their Synthetic Analogues -Synthesis, Structure, Properties and Applications 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 SrY 4 (SiO 4 ) 3 O 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.  While the synthesis of CaY 4 (SiO 4 ) 3 O leads to well-developed hexagonal crystals (Fig. 25), the attempts for the preparation of BaY 4 (SiO 4 ) 3 O phase were not successful. This synthesis leads to well-developed crystals of yttrium orthosilicate (Y 2 SiO 5 ) surrounded by BaO-Y 2 O 3 -SiO 2 glassy phase (Fig. 26).

N-apatite
The main secondary phases in Jänecke prism 17 [117] for Si 3 N 4 -Al 2 O 3 -SiO 2 -Y 2 O 3 -YN-AlN system 18 are shown in Fig. 28. The formal exchange of oxygen by nitrogen leads to the compounds of N-apatite (Y 10 (SiO 4 ) 6 N 2 , H-phase 19 ), N-melilite 20 [118] (Y 2 Si 3 O 3 N 4 , M-phase), N-wollastonite (YSiON 2 , K-phase) and N-woehlerite (Y 4 Si 2 O 7 N 2 , J-phase). The latter one forms a complete solid solution with Y 4 Al 2 O 9 (YAM) of the composition of Y 4 Si 2−x Al x O 7+x N 2−x (Jss-phase) [119], [120], [121], [122], [123], [124].  [125], silicon-yttrium oxynitride) was first identified by RAE et al [126] as a compound with the compositional mixture of 10Y 2 O 3 ·9SiO 2 ·Si 3 N 4 that was stable up to 1750°C. There were other suggested compositions, such as Y 10 Si 7 O 23 N 4 [127]. Later work by GAUCKLER et al [128] established N-apatite as a stoichiometric compound with the formula unit of Y 10 [SiO 4 ] 6 N 2 and the apatite structure (space group P6 3 /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 Y 10 (SiO 4 ) 6 N 2 was studied by CHING et 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 cdirection and planar N-Y bonding in the x-y plane.
The ternary phase diagrams of the Si 3 N 4 -Y 2 O 3 -SiO 2 [123] and Si 3 N 4 -La 2 O 3 -SiO 2 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]. The hexagonal lanthanum N-apatite phase of the composition of La 5 (SiO 4 ) 3 N (isostructural with apatite) can be prepared from the mixture of La 2 O 3 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 La 5 (SiO 4 ) 3 N. Prolonged heating of this compound yields La 4.67 (SiO 4 ) 3 O [129], [131], [132], [133], [134], [135]. The absorption bands observed in infrared spectrum of lanthanum oxynitrides are introduced in Table 4  MITOMO et al [129] used the mixture of Si 3 N 4 and La 2 O 3 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 results of SAKAI et 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 Si 3 N 4 leads to the formation of protective SiO 2 layer on the surface: SiO MeO MeSiO + + ® (16) but in the case of MgO, the formed layer did not act as protection [136]. Y 4.67 (SiO 4 ) 3 O apatite (britholite phase 2 ) is formed as the oxidation product of silicon yttrium oxynitride (H-phase) in the temperature range from 700 to 1400°C [137], [138].

REE vanadocalcic apatite
The synthesis and physicochemical study of rare-earth-containing vanadocalcic oxyapatites where the pair Ca 2+ and □ was substituted by Ln 3+ and 1/2O 2− was described by BENMOUSSA et al [139]. This substitution leads to lanthanum and praseodymium dioxyapatites Ca 8 Ln 2 (VO 4 ) 6 O 2 , where Ln = La and Pr. Regarding rare earths such as neodymium, samarium, europium, gadolinium and terbium, the Ln 3+ content limit varies from one REE to another. It decreases when the REE ionic radius declines.

Apatite-type yttrium phosphates
The following compositions having the apatite structure were prepared by WANMAKER et al [140]: a. Me(II) 2+x Me(III) 8 (PO 4 ). 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 OWADA et 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 [Ca 9.0 Y 1.0 ] (PO 4 ) 6 [O 1.5 □ 0.5 ] 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 (Ca 10−x Y x )(PO 4 ) 6 ((OH) 2−x−2y O x+y □ y ) [142]. Using the hydrogen concentration cells, it was confirmed that the specimens with the composition of x ≤ 0.65 have the protonic transference number (t i ) is equal to one, while the values of t i 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 −4 S·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.