Thermo-Raman Studies on NaH2PO4·2H2O for Dehydration, Condensation, and Phase Transformation

Thermo-Raman spectroscopy was applied to study the thermal properties of NaH(2)PO(4)-2H(2)O from room temperature up to 600 degrees C. Raman spectra recorded at every degree of the temperature interval by monitoring the internal modes gave the microscopic picture of dehydration of NaH(2)PO(4)-2H(2)O, the condensation of NaH(2)PO(4), and the phase transformation of NaPO(3). The spectral variation observed could distinguish two steps of dehydration, resulting in the formation of NaH(2)PO(4)-H(2)O and NaH(2)PO(4) as compositional species from NaH(2)PO(4)-2H(2)O. The thermo-Raman intensity (TRI) and differential thermo-Raman intensity (DTRI) thermograms also showed two steps of dehydration in the temperature range from 42 to 52 degrees C and from 60 to 72 degrees C with a maximum rate at 48 and 68 degrees C, respectively. Furthermore, condensation resulted in sodium dihydrogen diphosphate (Na(2)H(2)P(2)O(7)) and sodium metaphosphate (NaPO(3)) in two steps at 212-224 and 260-360 degrees C, respectively. Clear evidence in the spectral variation representing two phase transformations of NaPO(3) at the temperature of 345 and 515 degrees C with three different polymorphs (phase III, phase II, and phase I of NaPO(3) in increasing order of temperature) was also observed. The thermal methods thermogravimetry, differential thermogravimetry, differential thermal analysis, and differential scanning calorimetry also supported the results but could not give a clear idea due to the lack of direct structural information.