Jahn-Teller effect in BaTiO3:Cr5+: an electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of Cr-I(5+) defects incorporated on Ti4+ sites in single-crystal and powdered ceramics of BaTiO3 were investigated in the temperature range 50-220 K at 34 GHz (Q band). Single-crystal and powder measurements allow the unambiguous determination of the full g tensor in the ferroelectric rhombohedral phase whereas in the orthorhombic phase only the determination of the principal values of the Cr-I(5+) g tensor was successful. The T-2 circle times e Jahn-Teller effect stabilizes the vibronic ground state of the 3d(1) electron of the Cr-I(5+) ion and leads to a tetragonally compressed defect-O-6 octahedron with the point symmetry D-4h. The spontaneous electrical polarization present in the ferroelectric phases of BaTiO3 appears as a further perturbation. The quadratic field effect reduces the D-4h symmetry of the Cr-I(5+) defect centre and produces an additional g tensor breaking the tetragonal symmetry of the Zeeman term. For symmetry reasons one of the principal axes of the rhombic g tensor must be directed along one < 110 > direction. The angles of the other two principal axes with respect to the < 001 > axes are dependent on the absolute value of the spontaneous polarization. The difference in the temperature behaviour of the single-crystal and powder spectra can be explained by the presence of internal stress in the grains of the ceramic samples which increases the JT energy of the 3d(1) electron.