In situ Raman investigation of a LiB3O5 melt toward understanding the structural memory phenomena

Abstract

Lithium triborate (LiB₃O₅, LBO) is the most widely used nonlinear optical crystal. During the LBO crystal melting process, several interesting and important phenomena, called as structural memory, have been found. However, the essence of these phenomena remains unclear. In order to explain these phenomena, high-temperature Raman spectroscopy was used to investigate in situ the structures of a LiB₃O₅ melt at different temperatures. Two important structural transformations were found during the LBO crystal melting process: (1) when the crystal begins to melt, every two adjacent basic building units (B₃O₄Ø₂) in the crystal structure transforms to a B₄O₅Ø₄ group and a B₂O₃ molecule. (2) Above 860 °C, the B₄O₅Ø₄ units react with the B₂O₃ molecules to form boron–oxygen chains whose minimal repeated unit is the B₃O₄Ø₂ six-membered ring. These results prove that LBO is an incongruently melting compound and provide a rationale for understanding the special structural memory phenomena.