Cadmium phosphates Cd₂(PO₄)OH and Cd₅(PO₄)₂(OH)₄ crystallizing in mineral structures

Single crystals of two basic cadmium phosphates, dicadmium orthophosphate hydroxide, Cd₂(PO₄)OH, and penta-cadmium bis-(orthophosphate) tetra-kis-(hydroxide), Cd₅(PO₄)₂(OH)₄, were obtained under hydro-thermal conditions. Cd₂(PO₄)OH adopts the triplite [(Mn,Fe)₂(PO₄)F] structure type. Its asymmetric unit comprises two Cd, one P and five O sites, all situated at the general Wyckoff position 8 f of space group I2/a; two of the O atoms are positionally disordered over two sites, and the H atom could not be localized. Disregarding the disorder, distorted [CdO₆] polyhedra form a tri-periodic network by edge-sharing with neighbouring [CdO₆] units and by vertex-sharing with [PO₄] units. The site associated with the OH group is coordinated by four Cd atoms in a distorted tetra-hedral manner forming ¹ _∞[(OH)Cd_4/2] chains parallel to [001]. The oxygen environment around the OH site suggests multiple acceptor atoms for possible O-H⋯O hydrogen-bonding inter-actions and is the putative reason for the disorder. Cd₅(PO₄)₂(OH)₄ adopts the arsenoclasite [Mn₅(AsO₄)₂(OH)₄] structure type. Its asymmetric unit comprises five Cd, two P, and twelve O sites all located at the general Wyckoff position 4 a of space group P2₁2₁2₁; the H atoms could not be localized. The crystal structure of Cd₅(PO₄)₂(OH)₄ can be subdivided into two main sub-units. One consists of three edge-sharing [CdO₆] octa-hedra, and the other of two edge- and vertex-sharing [CdO₆] octa-hedra. Each sub-unit forms corrugated ribbons extending parallel to [100]. The two types of ribbons are linked into the tri-periodic arrangement through vertex-sharing and through common [PO₄] tetra-hedra. Qu-anti-tative structure comparisons are made with isotypic M ₅(XO₄)₂(OH)₄ crystal structures (M = Cd, Mn, Co; X = P, As, V).