Jan 10, 2010

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

José Santiso, leader of the PLD & Nanoionics group at CIN2 (CSIC-ICN) published, together with other researchers from Department of Materials Science and Engineering, MIT, 'Defect Structure, Charge Transport Mechanisms, and Strain Effects in Sr4Fe6O12+δ Epitaxial Thin Films' on Chem. Mater.

The defect structure and charge transport mechanisms of layered perovskite-related structure Sr₄Fe₆O_12+δ 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 °C after being annealed at different pO₂'s at elevated temperatures exhibited an activated conductivity whose activation energy E_a increased from 0.2 eV to 1.0 eV as the pO₂ 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 Sr₄Fe₆O_12+δ and enhancing the electronic conductivity by an order of magnitude.

+ DOI: 10.1021/cm902957r