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

摘要

Temperature-dependent vibrational (Raman and infrared) spectroscopy and X-ray diffraction studies have been performed on polycrystalline Na _2WO_4s2H_2O and Na_2MoO _4s2H_2O crystals. Raman data at low temperature 13-300 K for sodium tungstate Na_2WO_4s2H_2O was compared with the sodium molybdate Na_2MoO_4s2H_2O. 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 NaO_6 modes and the libration modes of water molecules at about 100-120 K. Additionally, Raman spectroscopy measurements of Na_2WO_4s2H_2O 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 WO4-2 units.