Defects in neutron-irradiated strontium titanate: Ti3+ "off center" on a Sr2+ site

Nominally pure SrTiO3 crystals show, after irradiation with fast neutrons, paramagnetic-resonance (EPR) spectra arising from Ti3+ ions on Sr2+ sites. Because of its smaller radius, the Ti3+ sits "off center". In the tetragonal low-temperature phase of SrTiO3 (T<Tc105 K) it moves towards one of the twelve surrounding oxygen ions roughly 0. 03 in a direction perpendicular to the domain axis. This is proved by the response of the EPR spectra to the application of external electric fields. For T>Tc simultaneously another Ti3+ spectrum is seen, which results from a partial averaging of the low-temperature spectrum. The coexistence of both types of spectra is attributed to the critical dynamics of the fluctuations of the order parameter near Tc. For T=(Tc+4)±0.5 K a frequency spread of Δν=3.5×108 Hz is obtained corresponding approximately to the width of the central mode. © 1973 The American Physical Society.