Raman spectroscopy study of Na₂MoO₄·2H₂O and Na₂MoO₄ under hydrostatic pressure

Abstract

Raman spectroscopy measurements of polycrystalline Na 2 MoO 4 ·2H 2 O (NMHO) and Na 2 MoO 4 (NM) under hydrostatic pressure (from 0 to 10 GPa) were performed. This study allowed us to monitor the stretching and bending vibrations of MoO 4 ions as well as the translational modes as a function of pressure. The pressure dependence of the wavenumbers of the modes indicates that the Na 2 MoO 4 ·2H 2 O undergoes two phase transitions at about ∼3 and ∼4 GPa. When releasing pressure, we have observed that the original spectrum is recovered, thereby pointing to a reversible process. The Na 2 MoO 4 (NM) starting phase was found to be stable up to 10 GPa. The pressure-dependent Raman data for NM did not reveal any structural modification. The influence of the pressure-transmitting medium on the phase transitions is also discussed.

Powellite‐Rich Glass‐Ceramics: A Spectroscopic Study by EPR and Raman Spectroscopy

“…Its relative intensity compared with the main peak of powellite increases with the incorporation rate and its FWHM is quite small (around 15 cm −1 ). According to literature, CaMoO 4 main peak is usually found between 877 and 879 cm −1 and Na 2 MoO 4 main peak is found between 891 and 897 cm −1 . This peak cannot be attributed to simple molybdates such as CaMoO 4 or Na 2 MoO 4 as its position is 907 cm −1 .…”

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

“…Particularly the large linewidths of these bands (in the region where it is expected to be observed), associated with the stretching vibrations of WO 4 tetrahedra, indicate disorder of the aforementioned units after the NWHO undergoes the phase transition. As a matter of fact, we also observed disorder with other members of the tungstates and the molybdates crystals, when submitted to high pressure condition [1,11,21].…”

“…The vibrational properties of the above stated materials have been investigated by different authors through the observation of Raman and infrared (IR) active modes, which have been previously reported [9][10][11]. Recently, we have also reported results of pressure-dependence Raman spectra in the Na 2 MoO 4 Á2H 2 O.…”

“…Recently, we have also reported results of pressure-dependence Raman spectra in the Na 2 MoO 4 Á2H 2 O. The vibrational study of polycrystalline orthorhombic molybdate allowed us to propose the assignment of vibrational modes [11]. The research work herein: (i) reports a conformational change in the Na 2-MoO 4 Á2H 2 O and in the Na 2 WO 4 Á2H 2 O compounds using Raman spectroscopy techniques to access the associated vibrational changes of both materials at low temperatures and (ii) shows that the Na 2 WO 4 Á2H 2 O undergoes a phase transition in high pressure conditions.…”

“…These results suggest that water Table 1 List of observed Raman wavenumbers for the Na 2 MoO 4 Á2H 2 O and the Na 2 WO 4 Á2H 2 O system and the assignment according to Refs. [10][11][12]. x obs (cm À1 ) stands for observed modes; a (cm À1 ) and b (cm À1 ) stand for the parameters of Eq.…”