Temperature‐dependent Raman scattering studies of Na₂MoO₄

Abstract

The mode assignment of the cubic phase of anhydrous Na 2 MoO 4 was carried out on the basis of lattice dynamic calculation using the classical rigid-ion model. Temperature-dependent studies indicate that this crystal remains in the cubic structure in the 15-773 K range and undergoes a phase transition at around 783 K. The behavior of the Raman modes indicates that this transition is strongly first-order in nature and the phase above 773 K may have an orthorhombic symmetry. This transition is connected with tilting and/or rotations of the MoO 4 tetrahedra, which lead to a disorder at the MoO 4 sites. Our results give also evidence that the Mo-O bond lengths decrease in the high-temperature phase.

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

“…The mode assignment was made based on lattice dynamics calculations for the cubic NM system. [11] This assignment is shown in Table 2. Figure 2(a) shows the Raman spectra of NMHO in the low wavenumber region recorded in the 0.0-7.1 GPa pressure range during compression.…”

MoO3 incorporation in magnesium aluminosilicate glasses

“…The 320 cm -1 band is a convolution of the symmetric and asymmetric bending vibration modes ( 2 and 4) in MoO4 2-tetrahedra while the 965 cm -1 band is a convolution of the symmetric and asymmetric stretching vibration modes ( 1 and 3) in MoO4 2-tetrahedra. The vibration frequencies of MoO4 2-tetrahedra are given in [32] and [33] for alkali and alkaline earth molybdate crystals. In amorphous materials such as glass, the variable local environments means that only two broad bands are attained.…”

Temperature‐induced phase transformations in Na2WO4 and Na2MoO4 crystals

“…[11] However, in that work the temperature-dependent Raman spectra covered only the 15-803 K range. Here, that study was extended further and two new phase transitions are observed at 823 and 943 K (Fig.…”

“…The first phase transition in Na 2 MoO 4 crystal and the splitting of the υ 3 mode into three components were pointed out as being consistent with the supposition of the decrease of symmetry from cubic to orthorhombic, since in this case the triply degenerate F 2g mode should split into three components. [11] These three components are present in phase III of Na 2 MoO 4 and in phase II of Na 2 WO 4 , but now these modes are related to the B 1g + B 2g + B 3g modes of the symmetry Fddd (Table 2).…”

“…The highest temperature phase is hexagonal, P6 3 /mmc, [9,10] while the phase stable in the 853-910 K range is orthorhombic, Fddd. [9,10] Temperature-dependent Raman spectra of the Na 2 WO 4 system have not yet been obtained and those of Na 2 MoO 4 have been obtained only up to 803 K. [11] The Raman scattering studies showed that the second phase of Na 2 MoO 4 crystal could not be compared with the orthorhombic symmetry (Pnam) proposed by Pistorius at 773 K, but with another phase of Na 2 MoO 4 which is stable in the 853-910 K temperature range. This phase is most likely orthorhombic, Fddd.…”