Growth of barium borate crystals from sodium fluoride solutions

NaF is suggested as a new solvent for top-seeded solution growth of beta-BaB2O4 single crystals. The BaB2O4-NaF pseudobinary system has been found to form a simple eutectic phase diagram with the eutectic composition corresponding to 61 mol% NaF and eutectic temperature of 754 degrees C. Viscosity, density and volatility of the BaB2O4-NaF solution along the liquidus line have been determined experimentally. Lower viscosities in the entire temperature range of interest for crystal growth and a shallower liquidus slope as compared with BaB2O4-Na2O solutions an the main factors allowing the growth of beta-BaB2O4 crystals of appreciable size. The formation and distribution of solvent inclusions has been studied as well and discussed in terms of their influence on the optical quality of the crystals.