Electrical and thermal properties of La0.2Sr0.8Cu0.1Fe0.9O3−δ and La0.2Sr0.8Cu0.2Fe0.8O3−δ
Abstract La 0.2 Sr 0.8 Cu 0.1 Fe 0.9 O 3− δ and La 0.2 Sr 0.8 Cu 0.2 Fe 0.8 O 3− 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. La 0.2 Sr 0.8 Cu 0.1 Fe 0.9 O 3− δ has a lower p O 2 stability limit at 1×10 −4 atm, whereas La 0.2 Sr 0.8 Cu 0.2 Fe 0.8 O 3− δ starts to decompose at 1×10 −3 atm. The conductivity is ∼10 S cm −1 and it decreases with decreasing p O 2 from 1 atm to the lower p O 2 stability limit, following a 1/4 power p O 2 dependence for temperatures above 800 K. The activation energy in O 2 is 1–2 kJ mol −1 and this increases to about 10 kJ mol −1 for the lower p O 2 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.