THERMALLY-INDUCED PHASE-TRANSFORMATIONS OF 12-TUNGSTOPHOSPHORIC ACID 29-HYDRATE - SYNTHESIS AND CHARACTERIZATION OF PW8O26-TYPE BRONZES

The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150-degrees-C were investigated and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), infrared (i.r.) and Raman spectroscopies. From room temperature to 550-degrees-C, 29-WPA passes through a dehydration process, which characterizes the formation of different crystallohydrates, in anhydrous form as well as ''denuded'' Keggin's anions, the D-phase (PW12O38). During these processes, Keggin's anions are not disturbed too much and they are preserved up to about 550-degrees-C. The ''D'' phase is transformed by solid- solid recrystallization at about 600-degrees-C in a new monophosphate bronze type compound PW8O26. Unit cell dimensions were calculated from XRPD data (a0 = 0.7515 nm). With the temperature increasing up to 1150-degrees-C, novel synthesized cubic bronze passed through three polymorphous phase transitions. According to a general formula for monophosphate tungsten bronzes (WO3)2m (PO4)4 all four polymorphs have m = 16.