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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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