Structures of Ca5(VO4)3Cl and Ca4.78(1)Na0.22(PO4)3Cl0.78: positions of channel anions and repulsion on the anion in apatite-type compounds

Single crystal specimens of apatite-type compounds Ca 5 (VO 4 ) 3 Cl and Ca 4.78 (1) Na 0.22 (PO 4 ) 3 Cl 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 Ca 5 (VO 4 ) 3 Cl 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 Ca 2+ 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 (PO 4 ) 3 Cl 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. B 28 , 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 O 2− and Cl − and a demand for keeping Ca 2+ and Cl − apart are balanced at the positions where bond-valence sums for Cl − are around 1.2.