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

Abstract

A comprehensive study of conduction polarons in purified BaTiO3 crystals, containing about 100-ppm Nb as extrinsic ions, is presented. Nb5+ is compensated by Ti3+ (3d(1)) ions, many of them isolated. Small Ti3+ polarons, stabilized by crystal strains, and polarons of intermediate size in less strained crystal regions are identified. Both species break the point symmetry, indicating stabilization by a tetragonal T(2)xe Jahn-Teller distortion. There is indirect evidence for the presence of bipolarons in the crystal ground state. They have a rather small dissociation energy, 0.01 eV. The investigations are based on electron paramagnetic resonance (EPR) performed at T<20 K under application of uniaxial stress. This allows to obtain local information on the intrinsic Jahn-Teller properties of the conduction states of an oxide perovskite. For the small polarons stress has the following effects: (i) aligns the tetragonal Jahn-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 Jahn-Teller axes orient perpendicular to the stress axis. Several of the polaron features are elucidated by comparison with the stress-dependent Jahn-Teller properties of the impurity ion Mo5+ (4d(1)), where the d electron is prevented from tunneling to its Ti4+ neighbors. The EPR of Ti3+ in reduced BaTiO3 is attributed to polarons bound to doubly filled oxygen vacancies.