Tunable Dirac nodal line in orthorhombic RuO₂

The pressure evolution of RuO2 is studied using single-crystal x-ray diffraction in a diamond anvil cell, combined with ab initio band-structure calculations. The tetragonal rutile structure transforms into the orthorhombic CaCl2-type structure above 13 GPa under quasihydrostatic pressure conditions. This second-order transition is ferroelastic in nature and accompanied by tilts of the RuO6 octahedra. Orthorhombic RuO2 is expected to be a paramagnetic metal, similar to ambient-pressure RuO2. It shows the increased t2g-eg crystal-field splitting that is responsible for the pressure-induced color change. It further features the Dirac nodal line that shifts across the Fermi level upon compression. © 2025 authors. Published by the American Physical Society.