Keggin’s ion structural modification and expansion of dodecatungstophosphoric acid hexahydrate induced by temperature treatment: In situ X-ray powder diffraction and raman investigations

The changes in lattice and molecular structure of the dodecatungstophosphoric acid hexahydrate (WPA-6) induced by the temperature treatment between 244 and 373 K, were studied in situ by X-ray powder diffraction and Raman spectroscopy. The increase in temperature was followed with the slight modifications of Keggin’s anion, expansion of the unit cell and an abrupt change in the crystallite microstructure of WPA-6 what was monitored by the Rietveld and Warren–Averbach methods. Those two discontinuities in the thermal expansion of 4 0 0 interplanar spacing, observed at about 320 and 340 K, are result of the non-convergent reversible phase transition, produced by partial reduction and oxidation of the tungsten, known as an ion with mixed valence. The extraordinary compression of P–Oa bond length [1.61(1) Å at 298 K and 1.44(2) Å at 373 K], broadening and splitting of ν1(P–Oa) bond with the increase in temperature, the abrupt change of crystallite dimension and the strain at 338 K, indicate a modification in the charge distribution followed with the partial reduction of W6+ into W5+ ions (i.e. ‘heteropoly blue’ formation). The partial reduction is closely related to the change in a dynamic equilibrium between different proton species and hydrated proton entities, such as: oxonium H3O+, dioxonium H5O2+, triaquaoxonium H7O3+, etc.