New solvents for β-BaB2O4 crystal growth by the flux method

•System BBO-NaF-NaCl is investigated.•The concentration and temperature regions of β-BBO crystallization are determined.•The degrees of precipitation are calculated.•The viscosity, evaporation rate and density of some of the solutions are measured.•New solvents for BBO crystal growth are recommended. Several sections of the BaB2O4-NaF-NaCl system with NaF: NaCl molar ratios of 1.0:0.0; 0.8:0.2; 0.6:0.4; 0.5:0.5; 0.4:0.6; 0.35:0.65; 0.3:0.7 and 0.2:0.8 were investigated. The concentration and temperature regions of β-BBO crystallization were determined and the degrees of precipitation were calculated. The viscosity, evaporation rate and density of some of the solutions were measured. The obtained data were compared with those published for the BaB2O4-NaF, BaB2O4-NaCl and BaB2O4-Na2O systems. Significant advantages of the new NaF-NaCl solvent were found, consisting of higher degree of precipitation, near two times wider temperature region and two to three times lower viscosity. Additional benefits are the progressively decreasing viscosity on decreasing the temperature and the possibility to grow β-BBO in this case at a temperature lower by 100 °C (up to 660 °C). These properties of the new solvent are a good basis for optimizing the process of β-BBO single crystals growth by the flux method.