Date for paper“Structural investigations on two typical lithium germanate melts by in situ Raman spectroscopy and density functional theory calculations."

Deeply understanding the physical and chemical properties of a melt requires an accurate description of its structure. In this paper, we investigated the structures of Li4GeO4 and Li6Ge2O7 melts which are of interest for growing germanate functional crystals. The structural changes of polycrystalline Li4GeO4 and Li6Ge2O7 from room temperature to their melting temperatures were monitored by Raman spectroscopy and analyzed by density functional theory (DFT) and factor group theory (FGT) calculations. A striking spectral change was observed for polycrystalline Li4GeO4, which is due to the orthorhombic-to-monoclinic phase transition and the Li+ disorder induced by the phase transition. Spectral similarity between monoclinic Li4GeO4 and its corresponding melt indicates that the anion motif of the Li4GeO4 melt is the GeO(4)(4-)tetrahedron. The Li6Ge2O7 crystal has 180 lattice phonons (45A(g) + 45A(u) + 45B(g) + 45B(u)) at the centre of the Brillouin zone, including 90 Raman-active phonons (45A(g) + 45B(g)). All the Raman-active bands were assigned by DFT calculations. The Raman spectrum of polycrystalline Li6Ge2O7 varies smoothly and continuously in the heating/melting process, indicating that the Ge2O76- anion groups in the crystal are preserved in the melt. The disproportionation reaction that often occurs in silicate melts was not found in the Li6Ge2O7 melt. A nucleophilic substitution reaction is proposed.