Jahn-Teller and off-center defects in BaTiO3: Ni+, Rh2+, Pt3+ and Fe5+ as studied by EPR under uniaxial stress

Abstract

Application of uniaxial stress reveals the Jahn-Teller (JT) properties of the defect ions Ni+ (3d(9)), Rh2+ (4d(7)) and Pt3+ (5d(7)), all incorporated on Ti4+ sites in BaTiO3. In all cases the vibronic ground states are stabilized by an E circle times e Jahn-Teller effect, here leading to tetragonally elongated defect-O-6 octahedra. Orbitals of (x(2) - y(2)) type are thus lowest for the hole-like Ni+, and (3z(2) - r(2)) for the electron-like strong-field cases Rh2+ and Pt3+. The three systems are characterized by motionally averaged isotropic spectra at elevated temperatures. At low temperatures strong dependences on the distribution of internal strains are found, leading to a coexistence of isotropic averaged and quasi-static tetragonal situations for Ni+, becoming more static under uniaxial external stress. The latter behavior is also identified for Rh2+. For Rh2+ and Pt3+ the coupling of the JT systems to the external stress could be determined quantitatively, predicting rather low JT energies by a simple model calculation. The observation of tetragonal distortions of these defects thus points to the presence of regions with strong internal strains. The impurity ion Fe5+ (3d(3)), also replacing Ti4+, is found to go spontaneously off-center along [111] type directions. The defect axis orients parallel to the stress direction, indicating an axially compressed Fe5+ -O-6 oxygen arrangement in the off-center situation.