Structural instability and lattice site occupation of Mn2+ ions in the SrTiO3 quantum paraelectric

Strontium titanate ($\mathrm{SrTi}{\mathrm{O}}_3$) is the most known material from the family of quantum paraelectrics. Thanks to its extremely “soft” lattice, its functionality can be easily tuned by applying both external stimuli (pressure, strain, electric field) and through doping or isotope exchange. In this paper, we present the results of a detailed study of two $\mathrm{M}{\mathrm{n}}^{2+}$ centers in Mn-doped $\mathrm{SrTi}{\mathrm{O}}_3$ single crystals using both continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy at frequencies from 9.5 to 427 GHz and temperatures from 5 to 296 K. The first center is created by a $\mathrm{M}{\mathrm{n}}^{2+}$ ion at the $\mathrm{S}{\mathrm{r}}^{2+}$ lattice site in an off-center position. Its spectroscopic characteristics were determined for both fast and slow motion regimes of the impurity ion. In particular, all spin transitions allowed by the $\mathrm{M}{\mathrm{n}}^{2+}$ spin were well resolved in the slow motion regime. The second center is created by a $\mathrm{M}{\mathrm{n}}^{2+}$ ion at the $\mathrm{T}{\mathrm{i}}^{4+}$ position in the center of the oxygen octahedron. It has been established that the surrounding of this ion undergoes strong distortion when cooled below the phase transition temperature $T_c=105$ K, stimulated by the rotation of the oxygen octahedron. The present data also perfectly explain the previously obtained EPR data from measurements of $\mathrm{SrTi}{\mathrm{O}}_3$:Mn ceramics at low microwave frequencies (9–10 GHz).