Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

The linear thermal expansion coefficients (TECs) of perovskite-type La0.3Sr0.7Fe1−xGaxO3−δ (x=0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10−6 K−1 at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10−4 to 1 atm, the isothermal chemically induced expansion of La0.3Sr0.7Fe(Ga)O3−δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, (εC/Δδ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10−2. The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La0.3Sr0.7Fe1−xGaxO3−δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.