Electric field effect in ESR spectra of Fe$^{3+}$ and Mn$^{4+}$ centers in thin SrTiO$_{3}$ plates

The results of the study of the electric field effect in the electron paramagnetic resonance spectra of the Fe$^{3+}$ and Mn$^{4+}$ impurity ions in thin oriented strontium titanate single-crystal plates are presented. The crystal structure of plates at $T>$ 105 K turned out to be tetragonal, different from both the cubic and the antiferrodistortive tetragonal, inherent to SrTiO$_{3}$ at $T< $105 K. It is shown that for the studied centers, the electric field effect is quadratic in the applied electric field and has the same sign and order of magnitude, indicating the nonpolar origin of the observed high temperature tetragonal state. Analysis of the dependences of the axiality value on the applied field indicates that the electric field effect does not originate from the electrostriction and is due to the modification the wavefunction of the ground state of the impurity centers.