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

Abstract

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 WOnb (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 WOnb. 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.