Review Borates—Crystal Structures of Prospective Nonlinear Optical Materials: High Anisotropy of the Thermal Expansion Caused by Anharmonic Atomic Vibrations Rimma Bubnova 1,2,*, Sergey Volkov 1, Barbara Albert 3 and Stanislav Filatov 2 1 Institute of Silicate Chemistry, Russian Academy of Sciences, Makarov Emb. 2, St. Petersburg 199034, Russia;
[email protected] 2 Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034, Russia;
[email protected] 3 Eduard Zintl-Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany;
[email protected] * Correspondence:
[email protected]; Tel.: +7-981-181-3262 Academic Editors: Ning Ye and Rukang Li Received: 7 February 2017; Accepted: 16 March 2017; Published: 22 March 2017 Abstract: In the present study the thermal structure evolution is reviewed for known nonlinear optical borates such as β-BaB2O4, LiB3O5, CsLiB6O10, Li2B4O7, K2Al2B2O7, and α-BiB3O6, based on single-crystal and powder X-ray diffraction data collected over wide temperature ranges. Temperature-dependent measurements of further borates are presented for the first time: α-BaB2O4 (295–673 K), β-BaB2O4 (98–693 K), LiB3O5 (98–650 K) and K2Al2B2O7 (98–348 K). In addition to the established criteria for nonlinear optical (NLO) properties of crystals, here the role of the anisotropy and anharmonicity of the thermal vibrations of atoms is analysed as well as changes in their coordination spheres and the anisotropy of the thermal expansion of the crystal structure. Non-centrosymmetric borates, especially those that have NLO properties, often show distinct anisotropies for each cation in comparison to centrosymmetric borates.