Thermal transformations of α-H6P2Mo18O62·nH2O heteropolyacid

According to the 31P NMR spectroscopy, heteropolyacid (HPA) H6P2Mo18O62·nH2O (P2Mo18), α-isomer of the Dawson structure, transforms upon heating above 80 °C partially (up to 30%) to γ-isomer, in which both polar groups Mo3O13 of the heteropolyanion are turned by 60° around the N3 axis, and partially to β-isomer in which only one group is turned. The β- and γ-isomers of P2Mo18 have been found for the first time. Their transformation into the α-isomer occurs upon rehydration in one week in air and in 1 h in an aqueous solution. HPA P2Mo18 decomposes on heating up to 350 °C to HPA H3PMo12O40 (PMo12) and a previously unknown phase of the HPMo6I21 composition, which in its turn decomposes at ∼375 °C to molybdenyl phosphates and IiI3. The PMo12 decomposition occurs via two routes to form the same products at temperatures of ∼400 and 450 °C with corresponding exotherms of IiI3 crystallization.