Solid-State Crystal-to-Crystal Phase Transitions and Reversible Structure-Temperature Behavior of Phosphovanadomolybdic Acid, H sub(5)PV sub(2)Mo sub(10)O sub(40)

The crystal packing and secondary structure of H sub(5)PV sub(2)Mo sub(12)O sub(40) was followed by careful X-ray diffraction studies that revealed four unique structures and three solid phase transitions at temperatures between 25 and 55 [degrees]C, with loss of solvated water and concomitant contraction of the volume and increase of the packing density. Above 60 [degrees]C H sub(5)PV sub(2)Mo sub(12)O sub(40) becomes amorphous and then anhydrous although the polyoxometalate cluster is stable indefinitely up to 300 [degrees]C. Above this temperature, combined IR, Raman, XRD, and XPS measurements show the decomposition of H sub(5)PV sub(2)Mo sub(12)O sub(40) to crystalline MoO sub(3) and probably amorphous vanadium oxide and vanadylphosphate, the latter appearing to cover the surface of MoO sub(3). Importantly, H sub(5)PV sub(2)Mo sub(12)O sub(40) can be easily recovered by dissolution in water at 80 [degrees]C.