**4.2.2 Other compounds of apatite type**

**4.2.1 Fluorapatite**

material [87].

Single crystals of fluorapatite up to 5 cm long and of 1 cm maximum diameter were first prepared via the Kyropoulos method (pulling the crystal from the melt) by JOHNSON [114]. The Czochralsky method was used by MAZELSKI et al [114],[115] to grow the crystals up to 30 cm long. The ratio of CaF2 to Ca3(PO4)2 as determined by chemical analysis of crystals depends upon the value of the same ration in the melt. Even if the melt had correct stoichiometric

**Fig. 17.** Classification of various crystal growth processes using capillary forces for maintaining or shaping the molten

202 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

Single crystals of apatite-type Nd9.33(SiO4)6O2, Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 were pre‐ pared by HIGUCHI et al [102],[118],[119] from the stoichiometric mixture of Nd2O3, Pr6O11 and Sm2O3 with SiO2 (9.33 : 6) via the floating zone method. The crystal growth using the optical floating zone technique (a) was extensively used to grow a variety of bulk crystals, particu‐ larly single crystals of metal oxides [120],[121].

The pseudobinary phase diagram for the Nd2O3–SiO2 system around the apatite phase is shown in **Fig. 19** [118],[122]. With the except of the end-member Nd2O3 and SiO2, the apatite phase (Nd2O3 : SiO2 = 7:9), Nd2SiO5 and Nd2Si2O7 are observed. Both, Nd2SiO5 and Nd2Si2O7 melt incongruently, while the apatite phase melts congruently.

YOSHIKAWA et al [123] prepared <0001> oriented Ca8La2(PO4)6O2 (CLPA) single crystals with the apatite structure, which were grown by the Czochralsky method. This material can be used as substrate for the growth of <0001> GaN epitaxial layers.

**Fig. 18.** Schematic diagram of the furnace with double ellipsoidal mirrors (a) and single grown crystals of Pr9.33(SiO4)6O2 (b), Nd9.33(SiO4)6O2 (c) and Sm9.33(SiO4)6O2 (d) [118].

**Fig. 19.** Reconstructed pseudobinary phase diagram around the apatite phase Nd9.33(SiO4)6O2 in the Nd2O3-SiO system [118].

The growth of single crystal of synthetic analogue of vanadinite (lead vanado-chlorapatite, Pb5(VO4)3Cl) using the CsCl flux method was performed by MASAOKA and KYONO [124]. No impurity phases were formed from this crystal growth method. Crystals obtained via this method exhibit well-developed hexagonal prismatic form of the size of several millimeters along the [0001] direction. The largest crystals were approximately 6×1×1 mm.

The first hydrothermal growth of single crystals of chlorapatite was reported by ROUFOSSE et al [125]. Crystals grown from the system chlorapatite-HCl-H2O at 50 000 psi and pH = 1 with the growth zone at 465°C and dissolution zone at 360°C were found to be of high stoichiometry.
