Novel Method for Synthesis of Well-Crystallized Vanadyl(2+) Hydrogen Phosphate Hemihydrate: Crystallization in Organic Media Containing a Small Amount of Water

Vanadyl(2+) hydrogen phosphate hemihydrate (VOHPO₄0.5H₂O, VHP) that had high crystallinity and showed a strong (001) peak in its X-ray diffractometry (XRD) pattern was synthesized via thermal treatment of a mixture of vanadyl(2+) acetylacetonate and triethyl phosphate at 200°C in organic solvents that contained a small amount of water. Scanning electron microscopy observations revealed that the product consisted of thin plates (2-4 µm wide and <0.5 µm thick). When the reaction was conducted at 250°C, a VHP sample was formed that exhibited stronger XRD peaks and had a plate width of >20 µm. The present VHP sample was topotactically transformed to divanadyl(2+) pyrophosphate, which showed a strong (020) XRD peak, via thermal treatment in a nitrogen atmosphere at 500°C.