Raman scattering study of molten alkali-metal molybdates and tungstates rich in basic oxides

Structural transformations of alkali-metal (Li, K, and Na) mono- and dimolybdates and tungstates have been studied by Raman scattering spectroscopy from room temperature to above their melting point. The structure of the Mo-O and W-O complexes in the form of isolated [MoO₄] and [WO₄] tetrahedra in crystalline monomolybdates and monotungstates has been shown to persist in the molten state. At the same time, the mixed anion motif made up of tetrahedra and octahedra in dimolybdate and tungstate crystals transforms on melting into complexes made up of two corner-sharing tetrahedra. We have identified the internal vibrational modes of such complexes in molten alkali-metal dimolybdates and ditungstates. Using rapid quenching of K₂O · 2MoO₃ and K₂O · 2WO₃ melts, we have obtained transparent glasses whose Raman spectra suggest that they have the same anion motif as the corresponding melts.