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

Raman and infrared measurements of the LiB 3 O 5 (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 (BO 3 ) 3− and tetrahedral (BO 4 ) 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 (BO 4 ) 5− ions above 200 cm −1 are stronger than those of the (BO 3 ) 3− ions if compared with the Raman spectra of BaB 2 O 4 crystals. These are caused due to the slope and distortions of the B 3 O 7 rings and their BO 3 and BO 4 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.