Dielectric anomalies in bismuth-doped SrTiO3: Defect modes at low impurityconcentrations

The dielectric relaxation modes in low bismuth doped SrTiO3 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 SrTiO3. With Bi doping, two further dielectric peaks, 18 K (mode IT) and 3 0 K (mode III), are induced in both real and imaginary parts of the permitt ivity on the quantum paraelectric background; the large paraelectric permit tivity of pure SrTiO3 is strongly reduced, This fact signals the suppressio n of quantum fluctuations and concomitantly the appearance of polar cluster s. The dynamics of these modes is studied and the physical nature is discus sed. Mode I, which has been previously explained in terms of the coherent q uantum 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 w ith some intrinsic mechanism of the host lattice. Mode II is tentatively ex plained in terms of polaronic relaxation. [S0163-1829(99)03506-7].