A single-crystal X-ray and Raman spectroscopic study of hydrothermally synthesized arsenates and vanadates with the descloizite and adelite structure types

Abstract

Studying the reason for the formation of two structural sub-types, seven arsenate and vanadate compounds of descloizite and adelite groups [specifically ( 1 ) CdCo(OH)(AsO 4 ), ( 2 ) CdCu(OH)(AsO 4 ), ( 3 ) SrCo(OH)(AsO 4 ), ( 4 ) SrZn(OH)(AsO 4 ), ( 5 ) SrCu(OH)(VO 4 ), ( 6 ) CdCo(OH)(VO 4 ), and ( 7 ) CdCu(OH)(VO 4 ) (bold numbers throughout paper refer to these compounds)] were synthesized under low-temperature hydrothermal conditions. 1 – 2 and 6 – 7 are isostructural with descloizite, and 3 – 5 are with adelite-group minerals and several synthetic compounds. Together with a sample of conichalcite, (8) CaCu(OH)(AsO 4 ), they were investigated using single-crystal X-ray diffraction [ R ( F ) = 0.0153–0.0283 for 1 – 5 and 8 ; for 6 and 7 , R ( F ) = 0.0603 and 0.0444, respectively] and Raman spectroscopy. Although crystallizing in different orthorhombic space groups, the atomic arrangements of descloizite-( Pnam ) and adelite-( P 2 1 2 1 2 1 ) type compounds adopt the same topology: the atomic arrangement is characterized by M 2O 6 octahedrons ( M 2 = Mg 2+ , Al 3+ , Mn 2+,3+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) edge-linked into chains. These chains are interconnected by X O 4 tetrahedrons ( X = Si 4+ , P 5+ , V 5+ , As 5+ , Mo 6+ ) into a three-dimensional framework. Cavities host M 1 atoms ( M 1 = Na + , Ca 2+ , Cd 2+ , Hg 2+ , Pb 2+ ); their coordination varies from 7 for descloizite-type representatives to 8 for adelite-type structures. The OH stretching frequencies in the Raman spectra are in good agreement with the observed O–H···O donor-acceptor distances. A detailed discussion of the crystal chemistry of these compounds and their influence on the space-group symmetry indicate a distinct dependence of the structural changes on the average ionic radii ( r M 1 + r X )/2.