ENTHALPY STABILIZED CHALCOGENIDES

Chalcogenides, including sulfides, selenides, tellurides, arsenides, and antimonides, are a large group of minerals with important ore forming implications and of great economic importance. However, their thermodynamic stability is commonly not well constrained. In addition, some chalcogenides have high potential for industrial use due to sought-after electrochemical, photoelectric, or catalytic properties. However, their widespread application is often limited due to their lower stability compared to oxides in both oxidizing and reducing conditions alike. In order to stabilize certain phases, the favorable energetics of mixing due to the thermodynamic entropy term is often employed. This has led to the commonly used trendy term of entropy-stabilized compounds. While the enthalpies of mixing can also further stabilize compounds, these are typically insignificant and in the order of less than 1 kJ/g atom for oxides, but can be up to 1.5 orders of magnitude larger for alloys. In contrast to oxides, which exhibit strong ionic characteristics, the bonding behavior in chalcogenides transitions to a more metallic type. This raises the question of the impact of enthalpies of mixing for chalcogenides. To address this question, a case study was conducted using synthetic ternary pentlandite (M9S8, where M is Ni, Co, and Fe) as a model system. The results demonstrated that the enthalpy of mixing can have a pronounced stabilizing effect. The enthalpies of mixing were determined by high-temperature oxidative drop solution calorimetry and combined with evaluation of literature data. It was found that the enthalpy of mixing for certain intermediate pentlandites reached more than 50% of the enthalpy of formation from the elements, at almost 30 kJ/g atom. In comparison, the entropy term only stabilizes the structure for a fraction of the enthalpy. This demonstrates the significance of the enthalpy of mixing in the overall stability of some chalcogenides, which warrants further investigation and consideration when studying chalcogenide stabilities or discussing their industrial application and geological formation.