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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.

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The crystal structure of the paraelastic phase of KFe(MoO4)2 at 360 K was reinvestigated and high pressure Raman scattering experiments were performed on this material. The studies indicated that this molybdate crystallizes in the structure above 312 K. At room temperature the structure is monoclinic and it transforms under pressure into , and low symmetry phases at 0.25, 1.3 and 1.6 GPa, respectively. The phase transitions observed at 0.25 and 1.6 GPa are irreversible whereas the 1.3 GPa transition is reversible. The lattice dynamics calculations performed for the phase allowed us to obtain an assignment of observed modes and helped us to obtain insight into the mechanism driving the structural changes occurring in this material. The x-ray study of the highest pressure phase, recovered during the decompression experiment, shows that the crystal structure of this phase is monoclinic or triclinic. When this phase is subjected to heat treatment at 673 K, it either transforms into another phase or decomposes.

MoO 4 undergoes a sequence of three phase transitions, which were observed at 783-803, 823-913 and 943-950 K. In both crystals, a strong first-order phase transition occurs as indicated by the behavior of the Raman modes: the Raman bands split and new bands appear below 100 cm −1 . These transitions are connected with tilting and/or rotations of the WO 4 and MoO 4 tetrahedra, which leads to a disorder at the WO 4 and MoO 4 sites.

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