MnS-Tb2S3 Phase Diagram

The MnS-Tb2S3 phase diagram has been plotted by using physico-chemical methods of analysis. This diagram refers to the transition type from the eutectic system MnS-Gd2S3 to the systems with the formation of intermediate compounds. Two MnTb4S7 and MnTb2S4 complex sulfides are formed in the MnS-Tb2S3 system. The thermal effects of solid-phase decomposition of complex sulfides are as follows: Delta H = 27.4 kJ/mol at 1643 K for MnTb4S7 and Delta H = 16 kJ/mol at 1574 K for MnTb2S4. The solubility of MnS in gamma-Tb2S3 at 1650 K is 16 mol.%, and 5 mol.% of MnS is dissolved in alpha-Tb2S3 at 1170 K. The solubility of Tb2S3 in alpha-MnS-based solid solution (SS) is 6 mol.% at 1650 K and 3 mol.% of Tb2S3 at 1170 K. The temperature of the gamma-Tb2S3 solid solution eutectoid decomposition is equal to 1350 K, and the composition of the eutectoid is 87 mol.% of Tb2S3. The coordinates of the eutectic between MnS-based solid solution and gamma-Tb2S3-based solid solution is assumed to be equal to 27 mol.% of Tb2S3 and 1666 K. Calculations of the liquidus line, carried out using the Van Laar's equation, are in agreement with the experimental data. The Gibbs excess partial energies are calculated for the MnS-Tb2S3 system, and their negative values are correlated with the formation of two complex sulfides MnTb2S4 and MnTb4S7 in the system.