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 (TN≈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.