Dielectric relaxations in SrTiO₃ doped with La₂O₃ and MnO₂ at low temperatures

Abstract

In addition to the dielectric relaxation around 170 K in the cubic structure of Sr_1−3x/2La_xTiO₃, a similar relaxation was observed at about 70 K in the tetragonal structure with an activation energy in the range 0.13–0.16 eV which increases as x in Sr_1−3x/2La_xTiO₃ varies from 0.60–3.00 at%. This relaxation is explained by Skanavi's model and is discussed in terms of thermal motions of Ti⁴⁺ between potential minima produced by lattice distortions in the tetragonal structure. In order to provide a direct evidence for the suggestion that the dielectric relaxation around 170 K in the cubic is due to thermal motions of Ti⁴⁺, dielectric properties on manganese-doped specimens, Sr_1−3x/2La_xMn_yTi_1−y),O₃ with x=1.40×10⁻² and y=0.1×10⁻², were investigated because Mn⁴⁺ substitutes for Ti⁴⁺. As well as the relaxation due to Ti⁴⁺, this specimen exhibits another peak due to Mn⁴⁺ with an activation energy somewhat smaller than that of Ti⁴⁺. The activation energies and the relative intensity of these relaxation processes are explained by the difference in ionic radii of Mn⁴⁺ and Ti⁴⁺ and the difference in formation energies of a strontium vacancy adjacent to Mn⁴⁺ and that to Ti⁴⁺, which were calculated theoretically using a shell model.