BOND-LENGTH FLUCTUATIONS IN TRANSITION-METAL OXIDES

Abstract

The several origins of bond-length fluctuations found in transition-metal oxides are illustrated by specific examples. Special emphasis is given to the first-order character of the transition from localized to itinerant electronic behavior, which occurs where the operative bonding has a double-well potential energy. This first-order transition is a general phenomenon applying to single-valent and mixed-valent systems with either cation-cation or cation-anion-cation operative bonding. A transition from bond-length fluctuations to long-range ordering of the bond character is found below a Tt where Tt is too low for phase separation by atomic diffusion. Where Jahn–Teller site distortions or low-spin to higher-spin site excitations are also present, competitive bond ordering can give complex phase diagrams.