THE DEVELOPMENT OF NEW NLO CRYSTALS IN THE BORATE SERIES

It is well known that the inorganic borates exist in numerous structural types, and some crystals such as KB5 (KB5O8·4H2O) and BBO(β-BaB2O4) are excellent nonlinear optical (NLO) materials, particularly in the ultraviolet region. In this review, we are going to approach our topic from the anionic group theory, which we have begun to develop in the late 1960's. A systematic classification has been made of the borate series in terms of the structural types of the anionic groups. Calculations have been made for the linear and NLO properties for most of the important borate anionic groups, including second-order susceptibilities and absorption edges in the ultraviolet spectral region. We have therefore been led to the formulation of a series of structural criteria, which serve as useful guidelines for the screening, characterization, and development of certain borate crystals, such as BBO (β-BaB2O4, i.e., BaB2O4 in its low-temperature modification) and LBO (LiB3O5) as new materials. The characteristic features of BBO and LBO crystals, excellent NLO materials developed during the past few years in China at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, are also discussed. © 1990, Elsevier Science Publishers B.V.. All rights reserved.