Phase transformation studies of Na3PO4 by thermo-Raman and conductivity measurements

In this work, phase transformations of anhydrous Na3PO4 prepared by two methods have been studied by thermo-Raman spectroscopy (TRS). For dehydrated Na3PO4, four characteristic spectra were found corresponding to four phases and the transformation temperatures were 229, 262 and 356 °C, respectively. Differential scanning calorimeter (DSC) showed endotherms at around 228, 247 and 358 °C, whereas differential thermal analyzer (DTA) showed them at around 274, 339 and 438 °C, respectively. Probably, these phase transformations observed during heating process might be unreported and are tentatively assigned as phases IV, III, II and I, respectively. During cooling of the same sample, only one phase transformation was detected in the temperature range from 342 to 320 °C from phase I to phase II. For Na3PO4 preheated to 600 °C, only one phase transformation was observed at 339 °C by thermo-Raman spectroscopy during heating process. This should be the phase transformation from α phase (tetragonal) to γ phase (cubic). Reversed transformation from γ to α phase during cooling of the same sample was observed at 313 °C. The band width for the band at 934 cm−1 also showed that the phase transformation was of order–disorder type. DSC and DTA for preheated Na3PO4 during heating showed an endotherm at 320 and 316 °C, respectively. The conductivity study supported the phase transformation to be in the range from 316 to 360 °C and was clearly evident from the increase in conductivity during the phase transformation. Further coupling of LCR meter to thermo-Raman spectrometer ensured the transformation and the temperature range measured by both conductivity and thermo-Raman spectroscopy simultaneously and its nature to be of order–disorder type.