In-situ high-temperature Raman spectroscopic studies of the vibrational characteristics and microstructure evolution of sodium tungstate dihydrate crystal during heating and melting

In-situ studies of the vibrational characteristics and microstructure evolution of the Na(2)WO(4)2H(2)O (sodium tungstate dihydrate) crystal during the temperature-induced solid-state phase transformation and melting process were carried out using high-temperature Raman spectroscopic technique. Results showed that the thermal decomposition process of the Na(2)WO(4)2H(2)O crystal takes place mainly within the temperature range of 348-383K, along with the structure transforming from the orthorhombic to cubic symmetry. As the sample temperature increased further, another solid-state phase transformation from the cubic to orthorhombic structure was observed approximately at 893K before melting occurred at 1023K. Although the isolated [WO4](2-) tetrahedron was preserved within the entire temperature range from room temperature to 1023K, subtle changes were observed with the mean bond length of W-O bonds in the tetrahedron unit. Furthermore, Raman active vibrational modes of Na(2)WO(4)2H(2)O, two Na2WO4 crystal phases, and corresponding melt were assigned based on the density functional theory simulation and compared with the literature data. Finally, four-molecule cluster arranged as T-d symmetry is considered to be the most likely configuration in the molten state according to density functional theory simulation based on the different multimolecular clusters proposed.