The Jahn-Teller effect in Cr5+-doped PbTiO3: a multi-frequency electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of Cr5+ defects incorporated on Ti4+ sites in powdered ceramics of PbTiO3 were investigated in the temperature range 50-400 K at 9 GHz (X), 34 GHz (Q) and 94 GHz (W band). The T-2 circle times e Jahn-Teller effect stabilizes the vibronic ground state of the 3d(1) electron of the Cr5+ ion and leads to a tetragonally distorted defect-O-6 octahedron with the point symmetry D-4h. The spontaneous electrical polarization present in the ferroelectric phase of PbTiO3 appears as a further perturbation producing an additional g-tensor contribution by the quadratic field effect. Its symmetry is dependent on the orientation of the electrical polarization with respect to the Jahn-Teller distortion axis, the tetragonal axis of the defect-O-6 octahedron. If the polarization of a domain is anti-or parallel to this axis, the local tetragonal symmetry of the Cr5+ ion persists whereas it is reduced by a perpendicular orientation. Anisotropic EPR spectra of tetragonally and orthorhombic distorted Cr5+O612- are detected at low temperatures. Increasing the temperature, the peaks of the two spectra are broadened and a motionally averaged isotropic spectrum appears at 200 K.