Abstract
Electron paramagnetic resonance (EPR) spectra of quasi-tetrahedrally coordinatedCr5+ ions(named CrII5+ in contrast to the octahedrally coordinatedCrI5+) in powderedceramic 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 phasewith rhombohedral symmetry, two peaks in the powder spectrum of the1/2-spinCrII5+ centre show a frequency-dependent doublet splitting which is explained assuming the existence of twoCrII5+ centres withslightly different g tensors. The spontaneous polarization in the ferroelectric domains induces changes in thepeak positions in the spectra and generates alignment effects of the off-centredCr5+ ions.These effects are caused by the linear coupling of the electric dipole moment, associated with the off-centredCr5+ ion within the octahedronof surrounding O2− ions, and the polarization field. At 75 K a dynamic reorientation of the defect ions amongthe possible positions in the unit cell broadens the powder peaks. Above 150 K only anisotropic single-line spectrum is observed, the line width of which increases with risingtemperature.
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