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 Na₂WO₄ṡ2H₂O and Na₂MoO₄ṡ2H₂O crystals. Raman data at low temperature 13-300 K for sodium tungstate Na₂WO₄ṡ2H₂O was compared with the sodium molybdate Na₂MoO₄ṡ2H₂O. 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 Na₂WO₄ṡ2H₂O 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.

 

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