Solid-state chemistry of early transition-metal oxides containing d⁰ and d¹ cations

Abstract

This paper presents a brief survey of the structures and properties of early transition-metal oxides containing d⁰ and d¹-d⁰ electronic configurations. The metal-oxygen (MO₆) octahedron, which is the essential structure building unit of these materials, exhibits a characteristic out-of-centre distortion for the d⁰ configuration in many instances, the degree of distortion increasing with decreasing HOMO-LUMO gap. Several characteristic properties of d⁰ oxides, which include low-dimensional structures (that give rise to intercalation, ion-exchange and acidity), ferroelectricity and non-linear optical response, arise from the out-of-centre distortion of d⁰ MO₆ octahedra. Oxides containing d¹-d⁰ electronic configurations exhibit an equally impressive array of electronic properties that owe their origin to the nature of d states near the Fermi level. While three-dimensional (3-D) oxides containing 5d and 4d electrons exhibit itinerant electron properties, 3d¹ oxides, especially with low-dimensional (low-D) structures, display localized electron magnetism and semiconduction. Low-dimensional oxides containing 4d electrons, typified by molybdenum bronzes and Mo₄O₁₁, exhibit charge-density-wave (CDW)-driven electronic instabilities arising from electron-phonon interactions.