Vibrational and structural properties in the dihydrate sodium tungstate and in the dihydrate sodium molybdate crystals

Temperature-dependent vibrational (Raman and infrared) spectroscopy and X-ray diffraction studies have been performed on polycrystalline Na2WO4⋅2H2O and Na2MoO4⋅2H2O crystals. Raman data at low temperature 13–300 K for sodium tungstate Na2WO4⋅2H2O was compared with the sodium molybdate Na2MoO4⋅2H2O. The infrared and the X-ray diffraction obtained in the 80–300 K suggest that both crystals have undergone conformational changes connected with an increase in the Raman intensity of NaO6 modes and the libration modes of water molecules at about 100–120 K. Additionally, Raman spectroscopy measurements of Na2WO4⋅2H2O under hydrostatic pressure (from 0 to 5 GPa) were performed. The pressure-dependent studies indicate the starting orthorhombic structure is stable in the 0.0–3.2 GPa pressure range, and undergoes a phase transition at about 3.9 GPa, associated with rotation of WO 4 - 2 units.