Electronic Conduction in Slightly Reduced Strontium Titanate at Low Temperatures

The electronic transport properties of slightly reduced pure strontium titanate have been studied at low temperatures between 2 and 300 °K. The temperature dependence of resistivity, Hall coefficient, and Hall mobility showed different features from previous results for doped and reduced crystals with higher carrier concentrations. Specifically, the mobility at liquid-helium temperatures is small (5-700 ${\mathrm{cm}}^2$/V sec) and increases with carrier concentration. Optical-absorption measurements showed the existence of several compensating acceptor levels and that the dominant mechanism of electron scattering was by longitudinal optical phonons at room temperature. Further, the data show that a specialized form of ionized-impurity scattering may play a role at low temperatures. A discussion of our experimental results is presented and it is concluded that an explanation of these in terms of a model of impurity-band conduction is appropriate.