Thermal and Chemical Expansion of Sr-Doped Lanthanum Cobalt Oxide (La1-xSrxCoO3-δ)

The equilibrium thermal expansivity (βT) and oxygen-vacancy chemical expansivity (βC) of polycrystalline La1-xSrxCoO3-δ (LSC; x = 0.2, 0.4, and 0.7) have been measured at 600 °C < T < 900 °C and 10-4 atm < PO2 < 0.21 atm using controlled-atmosphere dilatometry. These measurements show only a moderate dependence of βT on temperature, suggesting that increases often observed in the coefficient of thermal expansion when measured at constant PO2 are primarily thermally induced chemical expansion associated with changes in oxygen stoichiometry. The dependences of βT and βC on the temperature, oxygen-vacancy concentration, and Sr content (x) were characterized and found to follow a consistent nonlinear trend, which may result from the relaxation of lattice strain with increasing defect concentration. A slowly relaxing secondary expansion effect (and/or expansion hysteresis) was also discovered. Possible causes of this behavior, including phase transition/segregation at high vacancy concentration, are discussed.