Superexchange Interaction in Double Transition Metal MXenes: A Pathway to Superior Mechanical and Thermal Performance.

MXenes are a large family of 2D materials with tunable electronic, mechanical, and thermal properties. Within this family, double transition metal MXenes, characterized by distinct metal layers, offer opportunities beyond their single transition metal counterparts. However, their structure - property relationships remain largely unexplored. In this study, density functional theory is employed to investigate O-, F-, and OH-terminated double transition metal MXenes and benchmark their properties against single transition metal MXenes. Mechanical properties are obtained using the strain-energy method, while thermal behavior is assessed based on their phonon spectra. Electronic structure analysis reveals that carbon-mediated superexchange interactions between transition metal d-orbitals, facilitated by carbon 2p-states, strengthen the metal-carbon-metal bonding framework. This thereby enhances in-plane bond rigidity, phonon stability, and charge transport. Among different terminations, O-termination is thermodynamically preferred and yields superior Young's modulus and Debye temperature. These findings establish carbon-mediated superexchange interaction as the key mechanism behind the enhanced properties of double transition metal MXenes.