The investigation of thermal decomposition mechanism of ammonium phosphotungstate hydrate in inert gas atmosphere

In this study, TGA–DTA–MS and TGT techniques were used to investigate the thermal decomposition of ammonium phosphotungstate hydrate ((NH4)3(PW12O40)·xH2O, APhT) in inert gas (Ar) atmosphere. The thermal decomposition products of APhT obtained in Ar atmosphere at various temperatures were characterized by using XRD, FT-IR and SEM analyses.APhT losses its crystal water in the first step and forms anhydrous ammonium phosphotungstate ((NH4)3(PW12O40), anhydrous APhT). Anhydrous APhT is stable up to 640 K and it is transformed into cubic structured, dark blue colored phosphotungstate bronze ((PO2)2(WO3)24, PhTB) which is stable up to 1250 K. PhTB is then decomposed gradually into light black colored WO3 and (PO)2(WO3)16 mixture. This mixture does not vaporize until 1423 K. After this temperature, sublimation takes place followed by melting at 1545 K and weight loss is observed due to evaporation. At 1573 K the samples are transformed into light black colored tungsten suboxide (WO3−y).

► Thermal decomposition of (NH4)3(PW12O40)·xH2O occurred in two steps.
► The intermediate decomposition product was bronze ((PO2)2(WO3)24).
► The end decomposition product was a mixture of WO3 and WO3−y.