The relation of the electrical conductivity in single crystals of rhenium trioxide to the conductivities of Sr2MgReO6 and NaxWO3

In order to discriminate experimentally between proposed models for the conduction band in the bronze system NaxWO3 (where x > 0.28), the electrical transport properties of ReO3 and of Sr2MgReO6 have been investigated. A single crystal of stoichiometric ReO3 was grown by vapor transport and found to exhibit a metallic conductivity that is remarkably high for an oxide. However, polycrystalline Sr2MgReO6 was found to be a semiconductor with a room temperature resistivity of at least 105Ω-cm. These data indicate that the d-electrons of ReO3 are delocalized throughout the lattice as a whole, whereas those of Sr2MgReO6 are localized on the rhenium ions or within isolated ReO6 clusters. Magnetic susceptibility measurements were made on powder samples of both materials and are shown to be consistent with these conclusions. These results suggest that the most appropriate model for metallic conductivity in transition-metal oxides structurally related to ReO3 is one involving covalent bonding via mixing of cation t2g and anionpπ orbitals.