Polaronic conduction and Anderson localization in reduced strontium barium niobate

Electron transport mechanisms in reduced Sr0.5Ba0.5Nb2O6 (SBN50) are investigated from similar to 100 to 955K through an analysis of the electrical conductivity (sigma) and the Seebeck coefficient (S) with respect to temperature (T). Notably, experimental evidence is presented that supports a scenario of Anderson localization below 600K and carrier excitation across a mobility edge at higher temperature. As a relaxor ferroelectric, stoichiometric SBN has intrinsic disorder associated with both the distribution of Sr/Ba vacancies and the formation of polarized nanoregions. The removal of oxygen through reduction generates conduction electrons in SBN. At the lowest temperatures measured (100-155 K), the electrical conductivity exhibits a temperature dependence characteristic of variable range hopping, followed by a transition to small polaron hopping at intermediate temperatures (250-545 K). In both the variable range and small polaron hopping regimes, a semiconductor-like temperature dependence of the electrical conductivity (d sigma/dT>0) was observed. However, above 615 K, d sigma/dT decreases dramatically and eventually becomes metal-like (d sigma/dT<0). Concomitantly, the Seebeck coefficient exhibits a linear dependence on lnT from 615 to 955K with the same slope (similar to 104 mu V/K) for both polycrystalline SBN50 and single crystalline SBN61 (both reduced), indicating a similar, constant density of states near the Fermi level for both compositions. The application of Seebeck coefficient theory to this inherently disordered material reveals that the excitation of carriers across a mobility edge is likely responsible for the change in d sigma/dT at high temperature. Such findings may have a significant impact in the field of conductive ferroelectrics. (C) 2015 AIP Publishing LLC.