Anomalous structural behaviour near a Mott transition in compressed Ca2RuO4

Ca2RuO4 has a rich phase diagram, featuring a metal-insulator transition with a structural distortion upon cooling, antiferromagnetic ordering upon further cooling, and metallisation and ferromagnetic order, followed by a further structural transition, under pressure. It is one end member of the Ca(2-x)Sr(x)RuO4 series that includes the proposed triplet superconductor Sr2RuO4 at the other end. Ca2RuO4 shows intriguing collective phenomena in its own right, several of which are intricately linked to structural distortions of the RuO6 octahedra, which in turn are susceptible to pressure-induced changes. In fact, Ca2RuO4 shows an unusual c-lattice expansion under hydrostatic compression. However, the structural and electronic properties of Ca2RuO4 under pressure have not been investigated in detail before. Here, we study the high-pressure phase evolution of Ca2RuO4 using density functional theory (DFT) and DFT+U calculations. We show that the c-lattice expansion in the metallic high-pressure phase as well as details of the octahedral arrangements can be described if the on-site repulsion U is fine-tuned appropriately. The sensitivity to the size of U highlights how close the metallic phase remains to the Mott insulating phase.