The influence of domains on tetrahedrally coordinated Cr5+ in ferroelectric BaTiO3: an electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) spectra of quasi-tetrahedrally coordinated Cr5+ ions (named Cr-II(5+) in contrast to the octahedrally coordinated Cr-I(5+)) in powdered ceramic samples of BaTiO3 were investigated in the temperature range 50-220 K at 9.4 and 34.0 GHz (X and Q band). At 50 K, in the ferroelectric low-temperature phase with rhombohedral symmetry, two peaks in the powder spectrum of the 1/2-spin Cr-II(5+) centre show a frequency-dependent doublet splitting which is explained assuming the existence of two Cr-II(5+) centres with slightly different g tensors. The spontaneous polarization in the ferroelectric domains induces changes in the peak positions in the spectra and generates alignment effects of the off-centred Cr5+ ions. These effects are caused by the linear coupling of the electric dipole moment, associated with the off-centred Cr5+ ion within the octahedron of surrounding O2- ions, and the polarization field. At 75 K a dynamic reorientation of the defect ions among the possible positions in the unit cell broadens the powder peaks. Above 150 K only an isotropic single-line spectrum is observed, the line width of which increases with rising temperature.