Class: Metal Oxide
Related Materials: Perovskite, Prussian Blue, Tungsten Bronze
Notable Properties: Parent Structure Type, Metallic Conductivity
Rhenium Trioxide is very similar to the perovskite structure type without a body-centered cation in the MO3 lattice. Because of this, ReO3 is a remarkably simple primitive cubic structure. What makes this material interesting however is its electronic structure.
While most metal oxides are excellent electronic insulators or sometimes semiconducting, rhenium trioxide displays metallic conductivity at room temperature. In this structure, the metal center is in the Re6+ oxidation state. Referring to the periodic table, we see the rhenium metal center has one electron left sitting in the d-orbital manifold and is therefore referred to as a d1 metal. The metal ion is coordinated to six neighboring oxide ions with octahedral geometry. This results in a ‘local’ orbital splitting diagram as shown below with this single d1 electron lying in a triply degenerate set of very slightly antibonding 2p-5dπ orbitals. Importantly, the diffuse 5d-orbitals of the third-row transition metals results in enough dispersion to yield a fully delocalize, partially filled band with exceptional charge mobility.
Finally, rhenium’s neighbor in the periodic table forms an analogous alkali ion insertion material known as tungsten bronze. WO3 with no 5d-electrons to populate a conduction band is an electronic insulator. Upon reduction and concomitant insertion of sodium ions the materials transitions from an insulator to a semiconductor to a metal and under certain conditions is superconducting at low temperature. Technologically, the tungsten bronzes are mostly of interest for the dramatic color change that follows reduction from WO3 (white) to MxWO3 (very dark blue). This is currently used commercially as a cathode material in electrochromic windows.