Metallic Oxides and the Overlooked Role of Bandwidth

Oxides exhibiting metallic conduction are crucial for various applications, including fuel cells, battery electrodes, resistive and magnetoresistive materials, electrocatalysts, transparent conductors, and high-temperature superconductors. Oxides that approach metallicity also play significant roles in switching applications, where the metal-insulator transition phenomenon is utilized across a range of technologies. This perspective, motivated by the question of when oxides are metallic, employs electronic structure calculations on metallic oxides to identify typical features in the electronic structure that promote metallic behavior. The critical factor of the bandwidth of the electronic energy bands near the Fermi energy is emphasized since it has been somewhat overlooked in the literature. For example, bandwidth considerations would suggest that the recently proposed phosphate "LK-99" would never be a suitable target for superconductivity. By relating the crystal structure and electronic band features obtained through density functional theory calculations, we present the general heuristic that crystals with conduction bands narrower than 1 eV (as obtained from routine electronic structure methods) are unlikely to be metallic. We further examine the origins of narrow or flat bands to distinguish between structural properties that are conducive or detrimental to physical behavior like superconductivity. This survey of representative oxide metals highlights the essential chemical and structural ingredients that contribute to extended covalent interactions and ultimately wide electronic bands. A key takeaway is that oxyanion compounds such as borates, carbonates, silicates, sulfates, nitrates, and phosphates are unlikely to exhibit metallic conduction at ambient pressure. While the focus here is on oxides, the general findings should apply across various material families, extending to conducting organic crystals, conducting polymers, and hybrid and framework materials.