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
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Published:2022-09-10
Issue:5
Volume:78
Page:789-797
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ISSN:2052-5206
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Container-title:Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials
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language:
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Short-container-title:Acta Crystallogr Sect B
Author:
Matsuura Mimiko,
Okudera HirokiORCID
Abstract
Single crystal specimens of apatite-type compounds Ca5(VO4)3Cl and Ca4.78 (1)Na0.22(PO4)3Cl0.78 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. Ca4.78 (1)Na0.22(PO4)3Cl0.78 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.
Publisher
International Union of Crystallography (IUCr)
Subject
Materials Chemistry,Metals and Alloys,Atomic and Molecular Physics, and Optics,Electronic, Optical and Magnetic Materials