Investigation on the Structure of a LiBO-LiMoO High-Temperature Solution for Understanding the LiMoO Flux Behavior.

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