Reduction by hydrogen of vanadium in vanadate apatite solid solutions

Abstract

Solid solutions of phosphate and vanadate calcium apatites, Ca₁₀(PO₄)_6−x(VO₄)_x(OH)₂, were treated with hydrogen at high temperatures and studied by chemical analysis, XRD, FTIR, EPR and ENDOR spectroscopies. Only one reduced oxidation state, V⁴⁺, was detected. The extent of the reduction depended on the vanadium content as well as the treatment time. For solid solutions with x < 1.5, a fast reduction of V⁵⁺ to V⁴⁺ up to the limit V⁴⁺/(V + P) = 1/3 is achieved and the long-range order of the apatite lattice is preserved. Previous dehydroxylation of apatite with formation of an oxyapatite enhances the reduction process. For solid solutions with x > 1.5, the reaction results in apatite decomposition and the formation of a perovskite, CaVO₃. EPR and ENDOR spectroscopy reveal that the V⁵⁺ → V⁴⁺ reaction induces strong changes in the vanadium site structure with the formation of a vanadyl bond and the loss of the nearest OH⁻ group. Atomistic simulations were made to estimate local distortions around vanadium due to the reduction mechanism. Density functional calculations were performed to characterize the chemical environment induced by the V⁵⁺ → V⁴⁺ reaction at VO₄ sites through Mulliken atomic-orbital population analysis as well as charge and spin density maps. A mechanism for the overall reaction is proposed.