Raman and Density Functional Theory Studies of Li₂Mo₄O₁₃ Structures in Crystalline and Molten States

The Li₂Mo₄O₁₃ melt structure and its Raman spectral characteristics are the key for establishing the composition-structure relationship of lithium molybdate melts. In this work, Raman spectroscopy, factor group analysis, and density functional theory (DFT) were applied to investigate the structural and spectral details of the H-Li₂Mo₄O₁₃ crystal and a Li₂Mo₄O₁₃ melt. Factor group analysis shows that the crystal has 171 vibrational modes (84A_g + 87A_u), including three acoustic modes (3A_u), six librational modes (2A_g + 4A_u), 21 translational modes (7A_g + 14A_u), and 141 internal modes (75A_g + 66A_u). All of the A_g modes are Raman-active and were assigned by the DFT method. The Li₂Mo₄O₁₃ melt structure was deduced from the H-Li₂Mo₄O₁₃ crystal structure and demonstrated by the DFT method. The results show that the Li₂Mo₄O₁₃ melt is made up of Li⁺ ions and Mo₄O₁₃^2- groups, each of which is formed by four corner-sharing MoO₃Ø/MoO₂Ø₂ tetrahedra (Ø = bridging oxygen). The melt has three acoustic modes (3A) and 54 optical modes (54A). All of the optical modes are Raman-active and were accurately assigned by the DFT method.