Paramagnetic resonance and optical absorption of Co⁴⁺ in SrTiO₃

The paramagnetic resonance of low-spin Co4+, 3d5 in the oxygen octahedral coordination of SrTiO3 at 4.2K is reported. The spectrum is described by a spin-Hamiltonian with S=1/2, mod g mod = mod g/sub ///+2gperpendicular to mod /3=1.7414+or-0.0015, and a large hyperfine interaction constant mod A mod = mod A/sub ///+2A perpendicular to mod /3=254+or-810-4 cm -1. Correlated with this centre is an optical absorption band at 2.53 eV. This band is attributed to electron-acceptor charge transfer from the oxygen 2p valence states to the t2g( down arrow ) levels of the Co4+. The EPR and optical results are analysed using theoretical expressions due to Thornley (1968), from which an orbital reduction factor of k=0.64+or-0.04 and an average radial electron distribution, (r-3), reduced by a factor 0.67+or-0.06 with respect to the free ion are derived. These significant reduction factors appear to be partly due to a weak dynamic Jahn-Teller effect. The slight tetragonality of the spectrum is a measure of the coupling, p2/ mu e= mod g perpendicular to-g/sub /// mod /3 epsilon =2.14102, to the intrinsic tetragonal distortion of the crystal. The charge linewidth anisotropy is also accounted for by this coupling to Eg random strains with a distribution alpha =126 kg cm-2.