Composition-Controlled Metal-Insulator Transitions in Metal Oxides

Abstract

The family of perovskite-related oxides of the 3d transition metals such as LaBO₃ (B=Ti³⁺, V³⁺, Cr³⁺, Mn³⁺, Fe³⁺, Co³⁺ and Ni³⁺) are interesting from the point of view of metal-insulator (MI) transitions in strongly correlated systemm^1–3. These oxides exhibit enormous range of interesting properties. Thus, LaTiO₃ and LaNiO₃ have low resistivities at room temperature (~10^-2 and ~10^-3 Ω cm respectively). LaNiO₃ has a positive temperature coefficient of resistivity (TCR) over a wide temperature range while LaTiO₃ shows a change in sign of TCR around 130K with metallic behaviour in the high temperature region^3,4. LaVO₃, LaMnO₃, LaCrO₃ and LaFeO₃ are magnetic insulators with well defined magnetic ordering temperatures (T_N≈140, 180, 280 and 750K, respectively) and high resistivities^1–3 (> 10³ Ω cm at 300K). A fascinating member of this series of oxides is LaCoO₃ which shows an intricate temperature-dependence of the electronic configuration of trivalent Co ions including changes in the spin-state and a probable disproportionation to di- and tetra-valent Co ions^2,5,6. Around 1200K, LaCoO₃ undergoes a first-order MI transition. There is an intimate relationship between the electronic configurational changes and the changes in the electrical and magnetic properties preceding the MI transition. In a sense, LaCoO₃ may be looked upon as showing a MI transition arising from thermally induced compositional changes.

Communication No.293 from the Solid and Structural Chemistry Unit.

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