Abstract
Electron paramagnetic resonance (EPR) spectra ofCrI5+ defectsincorporated on Ti4+ sites in single-crystal and powdered ceramics ofBaTiO3 were investigated in the temperature range 50–220 K at 34 GHz (Q band). Single-crystaland powder measurements allow the unambiguous determination of the fullg tensor in the ferroelectric rhombohedral phase whereas in theorthorhombic phase only the determination of the principal values of theCrI5+ g tensor was successful. The Jahn–Teller effect stabilizes the vibronic ground state of the3d1 electronof the CrI5+ ion and leads to a tetragonally compresseddefect-O6 octahedron withthe point symmetry D4h. The spontaneous electrical polarization present in the ferroelectric phases ofBaTiO3 appears as a further perturbation. The quadratic field effect reduces theD4h symmetry of theCrI5+ defect centre andproduces an additional g tensor breaking the tetragonal symmetry of the Zeeman term.For symmetry reasons one of the principal axes of the rhombicg tensor must be directed along one direction. The angles of the other two principal axes with respect to the axes are dependent on the absolute value of the spontaneous polarization. The difference in thetemperature behaviour of the single-crystal and powder spectra can be explained by the presenceof internal stress in the grains of the ceramic samples which increases the JT energy of the3d1 electron.
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