Defect Structure, Charge Transport Mechanisms, and Strain Effects in Sr4Fe6O12+delta Epitaxial Thin Films

Abstract

The defect structure and charge transport mechanisms of layered perovskite-related structure Sr4Fe6O12+delta in the form of epitaxial thin films are studied as a function of temperature, oxygen partial pressure, and the type of substrate upon which they were grown. Values for the band gap, oxidation, and reduction enthalpies are extracted by fitting the experimental data to a defect model assuming a high oxygen deficiency. The electrical conductivity was dominated by p-type and n-type electronic conductivity at high and low oxygen partial pressures, respectively, with no evidence of a measurable ionic conductivity. Specimens cooled to below 400 degrees C after being annealed at different pO(2)'s at elevated temperatures exhibited an activated conductivity whose activation energy 1 a increased from similar to 0.2 eV to similar to 1.0 eV as the pO(2) at which the specimens were annealed decreased from 1 atm to 10(-4) atm. These data, which are also examined with respect to the different epitaxial strains induced by selecting substrates with different lattice mismatches, are analyzed in terms of a small-polaron hopping mechanism. Subtle structure modulations, induced in the films by increasing epitaxial strain, are shown to be capable of modifying the intrinsic transport mechanisms in Sr4Fe6O12+delta) and enhancing the electronic conductivity by an order of magnitude.