Dielectric anomalies in bismuth-dopedSrTiO3: Defect modes at low impurity concentrations

Abstract

The dielectric relaxation modes in low bismuth doped ${\mathrm{SrTiO}}_3$ were studied at temperatures 1.5–300 K and frequencies up to 1.8 GHz. We observe two modes, at 8 K (mode I) and 65 K (mode V), which also occur in single-crystalline ${\mathrm{SrTiO}}_3.$ With Bi doping, two further dielectric peaks, 18 K (mode II) and 30 K (mode III), are induced in both real and imaginary parts of the permittivity on the quantum paraelectric background; the large paraelectric permittivity of pure ${\mathrm{SrTiO}}_3$ is strongly reduced. This fact signals the suppression of quantum fluctuations and concomitantly the appearance of polar clusters. The dynamics of these modes is studied and the physical nature is discussed. Mode I, which has been previously explained in terms of the coherent quantum state, is possibly due to an unknown defect mode. Mode V seems to be induced by the ferroelastic domain-wall dynamics. We attribute mode III to the existence of the noninteracting polar clusters, which may be coupled with some intrinsic mechanism of the host lattice. Mode II is tentatively explained in terms of polaronic relaxation.