Defects in Neutron-Irradiated Strontium Titanate:Ti3+"Off Center" on aSr2+Site

Abstract

Nominally pure SrTi${\mathrm{O}}_3$ crystals show, after irradiation with fast neutrons, paramagnetic-resonance (EPR) spectra arising from ${\mathrm{Ti}}^{3+}$ ions on ${\mathrm{Sr}}^{2+}$ sites. Because of its smaller radius, the ${\mathrm{Ti}}^{3+}$ sits "off center". In the tetragonal low-temperature phase of SrTi${\mathrm{O}}_3$ ($T<\mathrm{}{T}_c\approx 105$ K) it moves towards one of the twelve surrounding oxygen ions roughly 0. 03 Å 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 $T>\mathrm{}{T}_c$ simultaneously another ${\mathrm{Ti}}^{3+}$ 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 $\varphi$ near $T_c$. For $T=(T_c+4)\pm 0.5$ K a frequency spread of $\Delta \nu =3.5\mathrm{}\times {10}^8$ Hz is obtained corresponding approximately to the width of the central mode.