The hydrothermal synthesis, growth, and optical properties of γ-LiBO2

Large single crystals of the acentric, tetragonal lithium metaborate, γ-LiBO2, have been grown using the hydrothermal method. Favorable conditions for the synthesis of γ-LiBO2 were found over the course of a phase stability study of lithium borates in supercritical aqueous base. Hydrothermal synthesis was achieved by: (1) conversion of the α phase, (2) direct crystallization from B2O3 and LiOH or (3) recrystallization of previously crystallized γ-phase crystallites. Solubility of γ-LiBO2 in dilute aqueous LiOH was found to be 2–5 wt% from 460 to 75 °C with a positive solubility coefficient. Crystal growth experiments were designed to take advantage of the favorable solubility conditions within this range. Seeded growth occurred at a maximum rate of 2.1 mm/day when the dissolution zone temperature was 535 °C and the growth zone temperature was 495 °C. This permitted the routine growth of centimeter-size crystals in less than 1 week. These crystals possess an interesting combination of optical properties, including a band gap of about 9 eV and high non-linear optical (NLO) susceptibility.