Conduction states in oxide perovskites: Three manifestations of Ti3+Jahn-Teller polarons in barium titanate

Abstract

A comprehensive study of conduction polarons in purified BaTiO3crystals, containing about loo-ppm Nb as extrinsic ions, is presented. Nb5+is compensated by Ti3+(3d1) ions, many of them isolated. Small Ti3+pol arons, stabilized by crystal strains, and polarons of intennediate size in less strained crystal regions are identified. Both species break the point symmetry, indicating stabilization by a tetragonal T2×e lahn-Teller distortion. There is indirect evidence for the presence of bipolarons in the crystal ground state. They have a rather small dissociation energy, 0.01eY. The investigations are based on electron paramagnetic resonance (EPR) performed at T<20 K under application of uniaxial stress. This allows to obtain local infonnation on the intrinsic lahn-Teller properties of the conduction states of an oxide perovskite. For the small polarons stress has the following effects: (i) aligns the tetragonal lahn-Teller axes along the stress direction, and (ii) enlarges the radius of the aligned orbitals, transforming them into intermediate ones. The stress alignment of the intermediate polarons is different: The lahn-Teller axes orient perpendicular to the stress axis. Several of the polaron features are elucidated by comparison with the stress-dependent lahn-Teller properties of the impurity ion Mo5+(4d1), where the d electron is prevented from tunneling to its Ti4+neighbors. The EPR ofTi3+in reduced BaTiO3is attributed to polarons bound to doubly filled oxygen vacancie. © 2002 The American Physical Society.