Nature of dielectric relaxation in SrTiO$_3$:Mn single crystals

Dielectric spectra of SrTiO$_3$ and SrTiO$_3$:Mn single crystals have been studied in the frequency range of 10–3000 cm$^{-1}$ and in the temperature range of 5–297 K using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy. A comparative analysis of the experimental results made it possible to detect a significant broadening of the absorption lines corresponding to the Slater and Last phonon modes, while the parameters of the Axe mode when replacing Ti with Mn (2 at %) stay invariant. This effect is associated with an enhance in structural disorder in the cation subsystem (B-sublattice) of the SrTiO$_3$ crystal. It has been established that doping with Mn ions reduces the antiferrodistortive phase transition temperature by about 20 K, but hardly affects the character of the temperature dependence of the parameters of a ferroelectric soft mode at temperatures of about 60–297 K. It has been found that an additional excitation with the frequency below the frequency of the ferroelectric soft mode should be taken into account for an appropriate model description of the dispersion of the permittivity of SrTiO$_3$:Mn in the terahertz frequency range. The results obtained in this work indicate that dielectric relaxation in the SrTiO$_3$:Mn crystal is due to thermally activated hops of Mn atoms between displaced (noncentral) crystallographic sites; i.e., the mechanism of radiofrequency relaxation in SrTiO$_3$:Mn is hopping rather than polaronic, which is also actively discussed in the literature.