Ab initio study of pressure-induced structural and electronic phase transitions in Ca2RuO4

Ca2RuO4 is a compound with a Mott insulating ground state, which responds to external pressure in a variety of ways, some of which are expected (an insulator-metal transition) while others are more surprising (an expansion of the c lattice constant). We provide here a comprehensive study of these pressure-induced structural and electronic changes using DFT+U calculations, and demonstrate generally good agreement with experiment. The insulator-metal transition is reproduced and coincides with an isostructural transition, the c lattice expansion, RuO6 octahedral distortions and associated changes to the Ru 4d orbital order. The metallic part of the phase diagram features several competing phases. The high-pressure Bbcm phase is found to be unstable in the ground state over a broad pressure range and suggested to be thermally stabilised instead.