Thermally induced phase transformations of 12-tungstophosphoric acid 29-hydrate: synthesis and characterization of PW8O26-type bronzes
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The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150°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°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°C. The “D” phase is transformed by solid-solid recrystallization at about 600°C in a new monophosphate bronze type compound PW8O26. Unit cell dimensions were calculated from XRPD data (ao=0.7515 nm). With the temperature increasing up to 1150°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.
KeywordsPhase Transition Differential Scanning Calorimetry Tungsten Recrystallization Phase Transformation
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