Investigation on the Structure of a LiB 3 O 5 -Li 2 Mo 3 O 10 High-Temperature Solution for Understanding the Li 2 Mo 3 O 10 Flux Behavior.

LiB3 O5 is the most widely used nonlinear optical crystal. Li2 Mo3 O10 (a nominal composition) is a typical flux used to produce large-sized and high-quality LiB3 O5 crystals. The structure of the LiB3 O5 -Li2 Mo3 O10 high-temperature solution is essential to understanding the flux behavior of Li2 Mo3 O10 but still remains unclear. In this work, high-temperature Raman spectroscopy combined with density functional theory (DFT) was applied to study the LiB3 O5 -Li2 Mo3 O10 solution structure. Raman spectra of a LiB3 O5 -Li4 Mo5 O17 -Li2 Mo4 O13 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 Li2 Mo3 O10 melt; meanwhile, the complicated molybdate groups existing in the crystalline state transformed into Mo3 O10 2- groups, which are formed by three corner-sharing MoO3 Ø- /MoO2 Ø2 (Ø = bridging oxygen atom) tetrahedra. When LiB3 O5 dissolved in the Li2 Mo3 O10 melt, the crystal structure collapsed into polymeric chains of [B3 O4 Ø2 - ]n . Its basic structural unit, the B3 O4 Ø2 - ring, coordinated with the Mo3 O10 2- group to form a MoO3 ·B3 O4 Ø2 - complex and a Mo2 O7 2- group. On the basis of the LiB3 O5 -Li2 Mo3 O10 solution structure, we discuss the LiB3 O5 crystal growth mechanism and the compositional dependence of the solution viscosity.