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

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)(AsO₄), (2) CdCu(OH)(AsO₄), (3) SrCo(OH)(AsO₄), (4) SrZn(OH)(AsO₄), (5) SrCu(OH)(VO₄), (6) CdCo(OH)(VO₄), and (7) CdCu(OH)(VO₄) (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)(AsO₄), 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-(P2₁2₁2₁) type compounds adopt the same topology: the atomic arrangement is characterized by M2O₆ octahedrons (M2 = Mg²⁺, Al³⁺, Mn^2+,3+, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺) edge-linked into chains. These chains are interconnected by XO₄ tetrahedrons (X = Si⁴⁺, P⁵⁺, V⁵⁺, As⁵⁺, Mo⁶⁺) into a three-dimensional framework. Cavities host M1 atoms (M1 = Na⁺, Ca²⁺, Cd²⁺, Hg²⁺, Pb²⁺); 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_M1 + r_X)/2.