**4.5.3 Other chlorapatites**

The structure of apatite phase of the composition Ba5(OsO5)3Cl (P63CM, *a* = 10.928 Å, *c* = 7.824 Å, *V* =809.2 Å3 , Z = 2 and *ρ* = 6.29 g·cm−3) where PO4 tetrahedra are replaced by pyramidal OsO5 groups was reported by PLAISIER et al [145] as isomorphous with Ba5(ReO5)3Cl (BESSE et al [146]) and Ba5(ReO5)3I (BAUD et al [147]). The structure (**Fig. 25**) consists of columns of Ba(1) atoms parallel to the *c*-axis and chains of ClBa6 octahedra with common faces along the *c*-axis. Among these are isolated pyramidal OsO5 groups. Ba(1) atoms lie on the threefold-axis and are surrounded by nine oxygen atoms. Ba(1)-O distances vary between 2.74 and 2.76 Å. Atom of Ba(2) is surrounded by seven oxygen atoms and two atoms of chlorine.

**Fig. 25.** Projection of the structure of Ba5(OsO5)3Cl along the c-axis [145].

SUZUKY and KIBE [148] used the NaCl flux method to prepare barium (Ba5(PO4)3Cl) and strontium chlorapatite (Sr5(PO4)3Cl) crystals and modified.47 Wilhelmy method [149],[150], [151]48 for the determination of surface free energy (~26 mN·m−1 for both apatite crystals49). The determination of specific surface free energies (surface tension) for single crystal of Sr5(PO4)3Cl [152] (aspect ratio is 3.2) via the measurement of contact angles of water and formamide (CH3NO) shows that ideal flat surface without a step should have uniform specific surface free energy, estimated to ≤ 26 and ≤ 50 mN·m−1 for (101̅0) and (101̅1) faces,<sup>50</sup> respec‐ tively. Experimentally obtained specific surface free energies roughly satisfy the Wulff's relationship [153],[154]:

$$\frac{\mathcal{N}\_i}{\mathbf{h}\_i} = \text{const},\tag{30}$$

where *γ*<sup>i</sup> is the specific surface free energy of the *i*-th face of the crystal and *h*<sup>i</sup> is the distance of face from the Wulff's (central) point of crystal.
