A Synergistic Approach to Unraveling the Thermodynamic Stability of Binary and Ternary Chevrel Phase Sulfides

State-of-the-art high temperature oxide melt solution calorimetry and density functional theory were employed to produce the first systematic study of thermodynamic stability in a series of binary and ternary Chevrel phases. Rapid microwave-assisted solid-state heating methods facilitated the nucleation of pure-phase polycrystalline MyMo6S8 (M = Fe, Ni, Cu; y = 0, 1, 2) Chevrel phases, and a stability trend was observed wherein intercalation of M y species engenders stability that depends on both the electropositivity and ionic radii of the intercalant species. Ab initio calculations indicate that this stability trend results from competing ionic and covalent contributions, where transition metal intercalation stabilizes the Chevrel structure through increased ionicity but destabilizes the structure through reduced covalency of the Mo6S8 clusters. Our calculations predicted that over intercalation of high-valent M-y, species leads to slight destabilization of the Mo-6 octahedral cores, which we confirm using calorimetry and X-ray absorption spectroscopy. Our combined computational and calorimetric analysis reveals the interplay of the foundational principles of ionic and covalent bonding characteristics that govern the thermodynamic stability of Chevrel and other inorganic phases.