Electronic Properties and Oxygen Nonstoichiometry of Mixed-conducting Oxide La1-χSrχCoO3-δ Thin Films at High Temperature

Abstract

Thin films of the perovskite-type oxide La1-χSrχCoO3-δ were deposited onto Ce0.9Gd0.1O1.95, SiO2 and single crystal MgO (100) substrates surface by a pulsed laser deposition (PLD) using a XeCl excimer laser to study the difference of the electronic properties and nonstoichiometry. It was shown that the oxygen nonstoichiometry of thin film is smaller than that of bulk for La1-χSrχCoO3-δ. The conductivity of the films were measured by a DC four probe method in 10-4-1 bar O2 gas at temperatures of 25-800{degree sign}C. The activation energy of the conductivity for thin films strongly depends on substrates due to the mismatch of the thermal expansion coefficient between the thin film and the substrate. Although the temperature coefficient of conductivity of La1-χSrχCoO3-δ bulk was negative for χ≥0.4 and was positive for 0≤χ≤0.3, that of thin film was positive irrespective of χ. At the condition with the Co mean valence of 3.0 for χ=0.4, 0.5, the conductivity shows large values for bulk, on the other hand, the conductivity is close to 0 for thin film. These facts suggest the electron hole in bulk is delocalized and metal like, while that in thin film is localized on cobalt ion.