Dielectric spectra and electrical conduction in Fe-doped SrTiO3

The transport and dielectric properties of Sr-0.97(Ti-1-Fe-x(x))O-3-(delta) solid solutions have been investigated at temperatures 1.5 K less than or equal to T less than or equal to 300 K and frequencies 20 Hz less than or equal to v less than or equal to 1 MHz. Depending on x, suppression of the quantum-paraelectric background, dielectric anomalies associated with relaxational processes, variable-range-hopping conduction, and a sublinear power-law increase in the ac conductivity were observed. The results of the present work are the following: (i) For low concentration Fe doping, the suppression of the paraelectric state is smaller than for Bi and La doping; the higher stability of the quantum paraelectric state is due to the fact that, in contrast to Bi and La, Fe substitutes for Ti sites. (ii) The observed variable-range-hopping (VRH) conduction indicates that Fe doping leads to the occurrence of local states in the highly disordered SrTiO3 lattice; this behavior coexists with the stable quantum paraelectric state. (iii) A dielectric relaxation process from 50 to 200 K is observed that is related to some defects; it is evidenced that the dielectric relaxation has no direct relation with the VRH mechanism, however, it is very possibly related to a polaron mechanism.