Electronic and structural optimisation of compressed Ca2RuO4

Ca2RuO4 is a compound with a Mott insulating ground state and metallic high-temperature state, which responds to external pressure in a variety of ways. Some of these are expected (a metallisation transition to the high-T phase) while others are more surprising (expansion of the c axis). A delicate coupling between structural and electronic effects has long been acknowledged in layered Ruthenates, but computational studies have remained restricted to experimental crystal structures. Here, we present a study of the pressure-induced structural and electronic evolution of Ca2RuO4 using DFT+U calculations, which marks the first reported attempt at a fully self-consistent optimisation of Ca2RuO4. Our results generally agree well with experiment. In particular the coupling between crystal structure and Ru-4d orbital order is made abundantly clear, and the anomalous structural behaviour near the Mott transition is reproduced. Results are sensitive to the on-site repulsion U, which highlights the proximity to the Mott transition, even in the metallic phase.