In Situ Raman Spectroscopy and DFT Studies of the Li₂GeO₃ Melt Structure

Knowledge of the molecular-level structure of the Li₂GeO₃ melt is essential to understand its basic physicochemical properties. In this work, in situ Raman spectroscopy, factor group analysis, and density functional theory (DFT) calculations were applied to investigate the Li₂GeO₃ crystal Raman spectrum and its transformation during the crystal melting process. Finally, the Li₂GeO₃ melt structure was determined. The Li₂GeO₃ lattice phonons were fully analyzed by the factor group. The DFT calculations confirmed the analysis results and assigned all of the experimental Raman bands. There are two characteristic Raman bands in the experimental spectrum. The 495 cm^-1 band (mid-frequency band) is attributed to the symmetric bending vibration of the Ge-O-Ge bond, and the 814 cm^-1 band (high-frequency band) arises from the symmetric stretching vibration of the O-Ge-O bond. The mid-frequency band anomalously shifted to a higher frequency and the high-frequency band normally shifted to a lower frequency when the crystal melted. The DFT method was employed to investigate two possible Li₂GeO₃ melt structures, one consisting of the [GeO₂Ø₂] _n (Ø = bridging oxygen) chain and the other consisting of the [Ge₃O₉] ring. The chain-type structure was demonstrated to provide a better description of the Li₂GeO₃ melt than the ring-type structure. The anomalous shift of the mid-frequency band is related to the shrinkage of the [GeO₂Ø₂] _n chain. On the basis of the chain-type structure, the high viscosity of the Li₂GeO₃ melt and the growth phenomena of the Li₂GeO₃ crystal were explained.