Interest in rare earth halide-containing systems has recently been stimulated by their use as additives in high-pressure metal halide lamps. Knowledge of phase diagrams and thermodynamic activities of components in the condensed phases are necessary to predict the vapor composition over the metal halide mixtures for the systems of different phase and chemical compositions.
Here we report for the first time on the experimental study of phase diagram and thermodynamic activities in the NaBr-DyBr3 system. Twenty three binary samples over the entire composition range were prepared for the present study. Careful investigations of the phase transformations were performed by use of differential thermal analysis (DTA). Two eutectics involving DyBr3(s) + 3NaBr*DyBr3(s) (710 ± 3 K) and 3NaBr*DyBr3(s) + NaBr(s) (730 ± 3 K) were concluded.
The vaporisation of the phases of the NaBr-DyBr3 samples was studied in the temperature range of 703 K-1025 K by the use of Knudsen effusion mass spectrometry (KEMS). The gaseous species NaBr, (NaBr)2, DyBr3, (DyBr3)2, NaDyBr4, and Na2DyBr5 were identified in the equilibrium vapour. Thermodynamic activities of NaBr and DyBr3 were evaluated over the complete concentration range of the system at 863 K by use of heterogeneous equilibria and by the ion intensity ratio integration method. The data obtained are discussed and compared with the known phase diagram of the NaBr-DyBr3 system. Gibbs energy of formation of 3NaBr*DyBr3(s) at 703 K, -13.1 ± 2.0 kJ mol-1, resulted from thermodynamic activities of components.
Compatibility of the thermodynamic data obtained in the present study for the condensed phases with the phase diagram of the NaBr-DyBr3 system was checked by the optimisation procedure BINGSS of H.L. Lukas.