Structure and the nonlinearity of lithium triborate studied by Raman and infrared reflectivity spectroscopy

Raman and infrared measurements of the LiB₃O₅ (LBO) crystals were completed. The experimental results shown here are more and stronger Raman lines and infrared absorption peaks, which implies that the external vibrations of the trigonal (BO₃)³⁻ and tetrahedral (BO₄)⁵⁻ ions, especially the former, in the six-membered boron–oxygen rings at the low wave number are strong and the internal vibrations of the (BO₄)⁵⁻ ions above 200 cm⁻¹ are stronger than those of the (BO₃)³⁻ ions if compared with the Raman spectra of BaB₂O₄ crystals. These are caused due to the slope and distortions of the B₃O₇ rings and their BO₃ and BO₄ units in LBO, which change the structural rigidity of the crystals, intensify the long-range electrostatic force and short-range molecular force, and shorten the ultraviolet absorption edge.