Electrical and thermal properties of La0.2Sr0.8Cu0.1Fe0.9O3−δ and La0.2Sr0.8Cu0.2Fe0.8O3−δ

La0.2Sr0.8Cu0.1Fe0.9O3−δ and La0.2Sr0.8Cu0.2Fe0.8O3−δ both exhibit cubic perovskite structures with lattice parameters a=0.38711 and 0.38694 nm, respectively. The initial oxygen non-stoichiometry, δ, of the as-synthesized materials was 0.24 for the sample with 10% Cu and 0.34 for 20% Cu. La0.2Sr0.8Cu0.1Fe0.9O3−δ has a lower pO2 stability limit at 1×10−4 atm, whereas La0.2Sr0.8Cu0.2Fe0.8O3−δ starts to decompose at 1×10−3 atm. The conductivity is ∼10 S cm−1 and it decreases with decreasing pO2 from 1 atm to the lower pO2 stability limit, following a 1/4 power pO2 dependence for temperatures above 800 K. The activation energy in O2 is 1–2 kJ mol−1 and this increases to about 10 kJ mol−1 for the lower pO2 limit. The Seebeck coefficient is very small, usually within ±15 μV K−1 of zero. Below 650 K, the weight is constant with temperature, whereas above 750 K δ increases with increasing temperature, causing an increase in thermal expansion. The thermal expansion coefficient below 650 K is approximately 15×10−6 K−1, and approximately 30×10−6 K−1 above 750 K.