Metallic Re3O2 with mixed-valence states

Abstract

Rhenium (Re) shows the richest valence states from +2 to +7 in compounds, but its mixed-valence states are still missing thus far. In this work, we have explored the Re–O phase diagram with a wide range of stoichiometric compositions under high pressure through first-principles structural search calculations. Besides identifying two novel high-pressure phases of ReO₂ and ReO₃, we reveal two hitherto unknown Re-rich Re₃O₂ and O-rich ReO₄ compounds. Re atoms in Re₃O₂ show mixed-valence states due to their inequivalent coordination environments, the first example in Re-based compounds. Electronic structure calculations demonstrate that the four discovered Re–O phases exhibit metallicity contributed by Re 5d electrons. Among them, ReO₃ has a predicted critical temperature of up to 12 K at 50 GPa, derived from the interaction between Re 5d electrons and Re-derived low-frequency phonons. Our study points to new opportunities to disclose novel transition metal compounds with mixed-valence states.