Violations of Coordination: Exploring Metastable Diborides via Energetic Transition Metals

The ideal aeronautical solid-state fuel should possess a high gravimetric heat of combustion (more energy for less weight) and a high volumetric heat of combustion (more room for mission-critical items). In this work, manganese diboride (MnB₂) demonstrates a high gravimetric heat of combustion of 39.26 kJ/g and the highest volumetric heat of combustion of any known fuel of 208.08 kJ/cm³. When compared to the currently used fuel in Space Shuttle rocket boosters and the Space Launch System, aluminum metal, MnB₂ represents a 26% increase in gravimetric heat of combustion and a 148% increase in volumetric heat of combustion. Surprisingly, the local topology of the inner coordination sphere controls energetic output. A model cluster system analyzed by density functional theory shows that the local environment can contribute to the bulk properties even without physical manifestations in the periodic structure. This high enthalpic performance comes from the metastability of MnB₂ and demonstrates that transition metals, typically shunned as solid-state fuels, can store potential energy from their high-temperature synthesis through 'overcoordination' and violation of their valence shell.