First-principles calculations and thermodynamic modeling of the S-Se system and implications for chalcogenide alloys

The S-Se system is the key to many chalcogenide alloys. A thermodynamic model of the S-Se system is formulated using an integrated approach of first-principles calculations and the CALculation of PHAse Diagram (CALPHAD) method, which is used to predict the vapor-liquid-solid S-Se phase diagram. An existing first-principles approach is modified through considering the contribution from the acoustic Grüneisen parameters to the thermodynamic properties, enabling more accurate prediction of thermodynamic properties of crystals with large unit cells. The approach is applied to all the stable and metastable phases with complex molecule structures within the S-Se system with the exception of an intermediate phase due to its unknown structure. The prediction of the transition temperature between α-S and β-S (428 K) is close to the experimental value (369 K). Given the high complexity of solid solutions containing multiple molecular species, two models based on homogeneous and randomly substituted species are constructed and applied to give an interval estimation of solid mixing enthalpies. The CALPHAD approach is applied using both selected experimental measurements and thermodynamic properties predicted by first-principles calculations, reproducing experimental data and predictions reasonably well.