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

Abstract

Single crystals of two basic cadmium phosphates, dicadmium orthophosphate hydroxide, Cd 2 (PO 4 )OH, and pentacadmium bis(orthophosphate) tetrakis(hydroxide), Cd 5 (PO 4 ) 2 (OH) 4 , were obtained under hydrothermal conditions. Cd 2 (PO 4 )OH adopts the triplite [(Mn,Fe) 2 (PO 4 )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 I 2/ a ; two of the O atoms are positionally disordered over two sites, and the H atom could not be localized. Disregarding the disorder, distorted [CdO 6 ] polyhedra form a tri-periodic network by edge-sharing with neighbouring [CdO 6 ] units and by vertex-sharing with [PO 4 ] units. The site associated with the OH group is coordinated by four Cd atoms in a distorted tetrahedral 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 interactions and is the putative reason for the disorder. Cd 5 (PO 4 ) 2 (OH) 4 adopts the arsenoclasite [Mn 5 (AsO 4 ) 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 P 2 1 2 1 2 1 ; the H atoms could not be localized. The crystal structure of Cd 5 (PO 4 ) 2 (OH) 4 can be subdivided into two main sub-units. One consists of three edge-sharing [CdO 6 ] octahedra, and the other of two edge- and vertex-sharing [CdO 6 ] octahedra. 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 [PO 4 ] tetrahedra. Quantitative structure comparisons are made with isotypic M 5 ( X O 4 ) 2 (OH) 4 crystal structures ( M = Cd, Mn, Co; X = P, As, V).