Tunable Dirac nodal line in orthorhombic RuO$_2$

The pressure evolution of RuO$_2$ 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 CaCl$_2$-type structure above 13 GPa under quasihydrostatic pressure conditions. This second-order transition is ferroelastic in nature and accompanied by tilts of the RuO$_6$ octahedra. Orthorhombic RuO$_2$ is expected to be a paramagnetic metal, similar to ambient-pressure RuO$_2$2. It shows the increased 𝑡$_2𝑔$−𝑒$_𝑔$ 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.