Critical phonons and R- zero-phonon emission line of Cr³⁺ in Mg-doped SrTiO₃

Dielectric permittivity ε and a "dielectric related" temperature shift of a zero-phonon emission R line (²E→⁴A₂) of the octahedral Cr³⁺ impurity centers was studied for nominally pure SrTiO₃ (STO), Sr_0.99Mg_0.01TiO₃ (SMT) and SrTi_0.99Mg_0.01O₃ (STM) ceramics. In STO, as in crystals, ε increases at cooling revealing only a weak dispersion in the 100 Hz-1 MHz region at lower temperatures and the R line shifts at cooling to lower energies, peaking at ≈ 12605 cm^-1 at 25 K. In SMT, the permittivity reveals qualitatively the same behavior as STO. But the spectral position of the chromium R line appeared to be at lower energies (12575 cm^-1, 70 K) and furthermore the R- line shifts to lower energies at heating. In STM the permittivity reveals a wide maximum at ∼36 K with pronounced relaxation-type dispersion, hinting a possibility of a low-temperature phase transition (PT). The spectral position of the R line in STM is ≈ 12614 cm^-1 at 70 K (close to that one in STO). As in STO, at cooling the R line shifts to lower energies, but at ∼ 28 K the lineshift changes in sign. Such behavior could be associated with a low-temperature soft mode driven PT in STM. However, time resolved THz spectroscopy revealed no ε (T) anomaly in STM, which means that at least below 28 K the temperature dependence of the R line shift is not controlled by the TO1 soft mode.