Tunable Dirac nodal line in orthorhombic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>RuO</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:math>

The pressure evolution of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>RuO</a:mi><a:mn>2</a:mn></a:msub></a:math> is studied using single-crystal x-ray diffraction in a diamond anvil cell, combined with band-structure calculations. The tetragonal rutile structure transforms into the orthorhombic <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>CaCl</b:mi><b:mn>2</b:mn></b:msub></b:math>-type structure above 13 GPa under quasihydrostatic pressure conditions. This second-order transition is ferroelastic in nature and accompanied by tilts of the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>RuO</c:mi><c:mn>6</c:mn></c:msub></c:math> octahedra. Orthorhombic <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>RuO</d:mi><d:mn>2</d:mn></d:msub></d:math> is expected to be a paramagnetic metal, similar to ambient-pressure <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>RuO</e:mi><e:mn>2</e:mn></e:msub></e:math>. It shows the increased <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:msub><f:mi>t</f:mi><f:mrow><f:mn>2</f:mn><f:mi>g</f:mi></f:mrow></f:msub><f:mtext>−</f:mtext><f:msub><f:mi>e</f:mi><f:mi>g</f:mi></f:msub></f:mrow></f:math> 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.