Structural instability and lattice site occupation of Mn2+ ions in the SrTiO3</mml:msub…

Strontium titanate (SrTiO₃) 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 Mn²⁺ centers in Mn-doped SrTiO₃ 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 Mn²⁺ ion at the Sr²⁺ 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 Mn²⁺ spin were well resolved in the slow motion regime. The second center is created by a Mn²⁺ ion at the Ti⁴⁺ 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 Tc=105 K, stimulated by the rotation of the oxygen octahedron. The present data also perfectly explain the previously obtained EPR data from measurements of SrTiO₃:Mn ceramics at low microwave frequencies (9--10 GHz).