A Multipurpose Study of BaZrS ₃ and BaHfS ₃ : Absolute Entropies, Thermal Decomposition, and Prediction of Intrinsic and Extrinsic Thermodynamic Stability

Chalcogenide perovskites have emerged in the past few years as very promising candidates for photovoltaic applications. However, experimental studies on these materials are still relatively scarce and there is a concerning lack of experimentally determined physical properties, which is particularly serious for thermodynamic properties, notwithstanding their importance in the assessment of the suitability of these materials for the proposed applications. In this work, the thermodynamic properties of chalcogenide perovskites BaZrS3 and BaHfS3 were therefore studied with the aim of estimating the intrinsic stability of the two compounds and the thermodynamic tendency to react under relevant synthesis/real world environments. The measurement of heat capacities from 1.8 to 300 K was performed for the first time and absolute entropies of the two materials were derived therefrom. Furthermore, the thermal decomposition of BaZrS3 was investigated by means of Knudsen effusion mass spectrometry up to 1850 K, revealing the release of gaseous sulfur as the only gaseous decomposition product up to about 1600 K. The largely dominant sulfur species were S(g) and S2(g), as expected for a low sulfur-activity phase, with higher oligomers S3–S8 only observed in the very first steps of heating. Above 1600 K, also BaS(g) was identified in the vapor phase, with an activity lower than unity, suggesting that pure solid BaS is not formed upon thermal degradation. XRD, SEM, TEM and Raman analyses performed on the residual sample indicated the formation of the Ruddlesden–Popper phase Ba2ZrS4, confirming previous theoretical predictions. However, no isothermal invariance of the sulfur partial pressure was observed, making it impossible to identify any heterogeneous equilibrium established under the effusion conditions. Finally, by combining the newly determined absolute entropies of the two chalcogenide perovskites with theoretical formation energies available in the literature, the intrinsic thermodynamic stability of BaZrS3 and BaHfS3 and the thermodynamic degradation behavior under oxygen, water, and water + CO2 gaseous atmospheres were predicted. Both BaZrS3 and BaHfS3 were shown to be stable with respect to the binary sulfides BaS, ZrS2, and HfS2 at room temperature, with the entropic term causing further stabilization at higher temperatures.