In-situ studies on the micro-structure evolution of A₂W₂O₇ (A=Li, Na, K) during melting by high temperature Raman...

In-situ high temperature Raman spectroscopic (HTRS) technique in combination with density functional theory (DFT) analysis has been adopted to investigate the micro-structure of solid and molten A₂W₂O₇ (A=Li, Na, K). The [WO₆] octahedra were found to be connected to each other by corner and edge sharing in the crystalline Li₂W₂O₇ and K₂W₂O₇ compounds. In the crystal lattice of Na₂W₂O₇, on the other hand, the [WO₄] tetrahedra and [WO₆] octahedra were found to coexist and paired by corner sharing. Although the structural diversity has clearly led to distinct Raman spectra of the crystalline A₂W₂O₇ compounds, the spectra of their melts tended to be analogous, showing the typical vibration modes of (W₂O₇)^2- dimer. A mechanism was then proposed to explain the structure evolution occurring during the melting process of A₂W₂O₇. The effect of A⁺ cation on the Raman bands of (W₂O₇)^2- dimer in molten A₂W₂O₇ has also been investigated. Both the wavenumber and full width at half-height (FWHH) of the characteristic band assigned to the symmetrical stretching vibration mode of WO_nb (non-bridging oxygen) in (W₂O₇)^2- were found to decrease in the sequence of Li⁺, Na⁺ and K⁺, indicating the cation effect on the mean bond length and its distribution range of WO_nb. In addition, the relative intensity of this band was also influenced by the cation and it was increased in the order of Li₂W₂O₇, Na₂W₂O₇ and K₂W₂O₇, which has been explained by the charge transfer process and confirmed by Mulliken overlap population analysis.