Structural investigation of Li2O–B2O3–MoO3 glasses and high-temperature solutions: toward understanding the mechanism of flux-induced growth of lithium triborate crystal

Abstract

MoO₃ is an important flux for lithium triborate (LiB₃O₅) crystal growth from high-temperature solutions. Although it has been widely used, the mechanism of the MoO₃ flux-induced growth of LiB₃O₅ crystals is still not very clear. In this paper, we present a spectroscopic investigation of the Li₂O–B₂O₃–MoO₃ ternary glasses/solutions, which were prepared from high-temperature MoO₃-based solutions for LiB₃O₅ crystal growth. By combining all the experimental data of Raman and MAS NMR, the types of structural species and the interactions between flux and solute are discussed to understand the MoO₃ flux-reduced mechanism of LiB₃O₅ crystallization from the high-temperature solution. Considering the activities of lithium cations, an isomerization reaction is proposed to describe the structural evolution of the BØ₄ tetrahedron into the BØ₃ triangle in boroxol due to the MoO₃ flux. The transition between the boron oxide species is essential for the LiB₃O₅ crystal growth. On cooling, the formed boroxol rings are polymerized by the re-formation of BØ₄ tetrahedrons again, and gather together to form the LiB₃O₅ crystal phase. Finally, the MoO₃ flux-induced LiB₃O₅ crystallization may be elucidated with the decrease of the concentration of the BØ₄ tetrahedron in high-temperature solutions.