Functional borates and their high-pressure polymorphic modifications. Review

The article presents the results of many years of studies of the growth of a low-temperature modification of barium borate b-BaB2O4 (R3с) crystals in the Na, Ba, B // O, F quaternary reciprocal system. Barium borate b-BaB2O4 is the most important nonlinear optical crystal of the UV spectrum. The key factor determining the quality of crystals is the choice of an optimal solvent. The article presents phase diagrams and the results of the growth of b-BaB2O4 crystals in several subsystems of the studied quaternary reciprocal system. Using atomistic modeling, we predicted and then experimentally obtained new high-pressure modifications: g-BaB2O4 (P21/n), whose structure includes edge-sharing tetrahedra, and d-BaB2O4 with assumed symmetry Pa3. In our study, we also focused on a solid solution with an “antizeolite” structure, which also crystallizes in the Na, Ba, B // O, F system.