Investigation on the Structure of a LiB₃O₅-Li₂Mo₃O₁₀ High-Temperature Solution for Understanding the Li₂Mo₃O₁₀ Flux...

LiB₃O₅ is the most widely used nonlinear optical crystal. Li₂Mo₃O₁₀ (a nominal composition) is a typical flux used to produce large-sized and high-quality LiB₃O₅ crystals. The structure of the LiB₃O₅-Li₂Mo₃O₁₀ high-temperature solution is essential to understanding the flux behavior of Li₂Mo₃O₁₀ but still remains unclear. In this work, high-temperature Raman spectroscopy combined with density functional theory (DFT) was applied to study the LiB₃O₅-Li₂Mo₃O₁₀ solution structure. Raman spectra of a LiB₃O₅-Li₄Mo₅O₁₇-Li₂Mo₄O₁₃ polycrystalline mixture were recorded at different temperatures until the mixture melted completely. The solution structure was deduced from the spectral changes and verified by DFT calculations. When the mixture began to melt, its molybdate component first changed into the Li₂Mo₃O₁₀ melt; meanwhile, the complicated molybdate groups existing in the crystalline state transformed into Mo₃O₁₀^2- groups, which are formed by three corner-sharing MoO₃Ø^-/MoO₂Ø₂ (Ø = bridging oxygen atom) tetrahedra. When LiB₃O₅ dissolved in the Li₂Mo₃O₁₀ melt, the crystal structure collapsed into polymeric chains of [B₃O₄Ø₂^-]_n. Its basic structural unit, the B₃O₄Ø₂^- ring, coordinated with the Mo₃O₁₀^2- group to form a MoO₃·B₃O₄Ø₂^- complex and a Mo₂O₇^2- group. On the basis of the LiB₃O₅-Li₂Mo₃O₁₀ solution structure, we discuss the LiB₃O₅ crystal growth mechanism and the compositional dependence of the solution viscosity.