Pressure-induced Loss of Metallicity in RuO2

The density evolution of the physical properties of the transition-metal oxide RuO2 coupled with a deeper understanding of underlying metastable phases is necessary for correlating universality between similar binary systems. Here, we report the pressure-temperature electrical resistance dependency with the structural evolution of RuO2. Conducting quasi-four-probe electrical transport measurements in a diamond anvil cell, a low-temperature loss of metallicity is observed above 28 GPa. This insulative transition is accompanied by a significant drop in pressure, suggesting the electronic transition is linked to a first-order structural phase transition. This is supported by the observation that the insulative electronic state is retained upon warming to room temperature. Density functional theory simulations indicate that the insulative fluorite-type phase can be favorable around these conditions and would exhibit a similar pressure difference through an isochoric transformation from the metallic HP-PdF2-type phase, however there is insufficient experimental evidence to confirm the presence of the fluorite-type phase. Furthermore, a unique arsenopyrite-type phase of RuO2 is observed with x-ray diffraction of a post-laser-heated sample at 62 GPa.