Thermal transformations of α-H6P2Mo18O62 • nH2O heteropolyacid

According to the P-31 NMR spectroscopy, heteropolvacid (HPA) H6P2Mo18O62.nH(2)O (P2Mo18), alpha-isomer of the Dawson structure, transforms upon heating above 80 degreesC partially (up to 30%) to gamma-isomer, in which both polar groups Mo3O13 of the heteropolyanion are turned by 601 around the C-3 axis, and partially to beta-isomer in which only one group is turned. The beta- and gamma-isomers of P2Mo18 have been found for the first time. Their transformation into the alpha-isomer occurs upon rehydration in one week in air and in I h in an aqueous solution. HPA P2Mo18 decomposes on heating up to 350 degreesC to HPA H3PMo12O40 (PMo12) and a previously unknown phase of the HPMo6O21 composition, which in its turn decomposes at similar to375 degreesC to molybdenyl phosphates and MoO3. The PMo12 decomposition occurs via two routes to form the same products at temperatures of similar to400 and 450 degreesC with corresponding exotherms Of MoO3 crystallization.