Reduction by hydrogen of vanadium in vanadate apatite solid solutions

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.