Non-equivalent terminating roles of hydroxyl and fluorine in hydroxy/fluorooxo-borate optical functional materials

Abstract

Nonlinear optical/birefringent crystals as important optical functional materials have been widely applied in optical communication, laser information processing, and laser polarization technology. In recent decades, hydroxyborates, fluorooxoborates and hydroxyfluorooxoborates have attracted significant interest as key branches of the borate family because of their outstanding (deep-) ultraviolet nonlinear optics or other optical functional performances. Negatively charged terminal groups OH⁻ and F⁻ can regulate the structure and the material characteristics. OH⁻ and F⁻ have the same valence and both appear in the terminal position of the anionic framework, which can be substituted by each other theoretically. However, it is found that the introduction of OH⁻ and F⁻ is apparently different in structure and properties. In this mini-review, a variety of theoretical and experimental characterization methods to identify OH⁻ and F⁻ was discussed. The different role of OH⁻ and F⁻ in involved systems’ microstructures and macro-properties including nonlinear optics was comprehensively analyzed. We aim to provide guidance for designing and synthesizing novel materials with balanced properties used in deep-ultraviolet applications.