Structures of Ca₅(VO₄)₃Cl and Ca_4.78(1)Na_0.22(PO₄)₃Cl_0.78: positions of channel anions and repulsion on the anion in apatite-type compounds

Single crystal specimens of apatite-type compounds Ca₅(VO₄)₃Cl and Ca_4.78 (1)Na_0.22(PO₄)₃Cl_0.78 were prepared with a flux-growth technique and their structures were examined with single-crystal X-ray diffraction. The anion channel is defined by a face-sharing array of nearly regular Ca octahedra, which run along c, together with flat O3 trigonal antiprisms that are concentric with the Ca octahedra and highly oblate in [001]. The position of the channel anion in Ca₅(VO₄)₃Cl is split into two at [0, 0, ±0.1691 (6)] with half occupancies. Dynamic disorder among these two positions is suggested from a saddle-shaped electron density distribution through the Ca regular triangle, i.e. the shared face of the Ca octahedra at z = ¼. The position of Cl⁻ was too close to Ca²⁺ under bond-valence consideration. This is due to repulsion on Cl⁻ from the flat O3 trigonal antiprism which is located at z = 0, namely, in between pairs of split Cl site positions. Ca_4.78 (1)Na_0.22(PO₄)₃Cl_0.78 crystallizes as a disordered hexagonal structure in which the crystallographic pattern is not an intermediate state but a projection of two distinct halves of the monoclinic pattern [doubled in b; Mackie, Elliot & Young (1972). Acta Cryst. B28, 1840–1848] in one hexagonal cell. In spite of quite different environments, the bond-valence sums for Cl⁻ in these structures are large and similar to each other. The repulsion between O²⁻ and Cl⁻ and a demand for keeping Ca²⁺ and Cl⁻ apart are balanced at the positions where bond-valence sums for Cl⁻ are around 1.2.