In-situ studies on the micro-structure evolution of AWO (A=Li, Na, K) during melting by high temperature Raman spectroscopy and density functional theory.

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 AWO (A=Li, Na, K). The [WO] octahedra were found to be connected to each other by corner and edge sharing in the crystalline LiWO and KWO compounds. In the crystal lattice of NaWO, on the other hand, the [WO] tetrahedra and [WO] octahedra were found to coexist and paired by corner sharing. Although the structural diversity has clearly led to distinct Raman spectra of the crystalline AWO compounds, the spectra of their melts tended to be analogous, showing the typical vibration modes of (WO) dimer. A mechanism was then proposed to explain the structure evolution occurring during the melting process of AWO. The effect of A cation on the Raman bands of (WO) dimer in molten AWO 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 (non-bridging oxygen) in (WO) 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. In addition, the relative intensity of this band was also influenced by the cation and it was increased in the order of LiWO, NaWO and KWO, which has been explained by the charge transfer process and confirmed by Mulliken overlap population analysis.