Solid-state crystal-to-crystal phase transitions and reversible structure-temperature behavior of phosphovanadomolybdic acid, H₅PV₂Mo₁₀O₄₀

The crystal packing and secondary structure of H₅PV₂Mo₁₂O₄₀ was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 °C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 °C H₅PV₂Mo₁₂O₄₀ becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 °C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H₅PV₂Mo₁₂O₄₀ to crystalline MoO₃ and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO₃. Importantly, H₅PV₂Mo₁₂O₄₀ can be easily recovered by dissolution in water at 80 °C.