Giant dielectric relaxation in SrTiO3-SrMg1/3Nb2/3O3 and SrTiO3-SrSc1/2Ta1/2O3 solid solutions

Ceramic samples of (1− x )SrTiO 3 - x SrMg 1/3 Nb 2/3 O 3 and (1− x )SrTiO 3 - x SrSc 1/2 Ta 1/2 O 3 were prepared, and their dielectric properties were studied at x =0.005–0.15 and 0.01–0.1, respectively, at frequencies 10 Hz–1 MHz and at temperatures 4.2–350 K. A giant dielectric relaxation was observed in the temperature range 150–300 K, and not so strong but well-developed relaxation was found in the temperature range 20–90 K. The activation energy U and the relaxation time τ 0 were determined to be 0.21–0.3 eV and from 10 −11 to 10 −12 s for the high-temperature relaxation and 0.01–0.02 eV and 10 −8 –10 −10 s for the low-temperature relaxation, respectively. The additional local charge compensation of the heterovalent impurities Mg 2+ and Nb 5+ (or Sc 3+ and Ta 5+ ) by free charge carriers or the host ion vacancies is suggested to be the underlying physical mechanism of the relaxation phenomena. On the basis of this mechanism, the Maxwell-Wagner model and the model of reorienting dipole centers Mg 2+ (or Sc 3+ ) associated with the oxygen vacancy are proposed to explain the high-temperature relaxation with some arguments in favor of the latter model. The polaron-like model with the Nb 5+ -Ti 3+ center is suggested as the origin of the low-temperature relaxation. The reasons for the absence of ferroelectric phase transitions in the solid solutions under study are also discussed.