Direct observation of ferroelastic domain effects in LSCF perovskites

Polycrystalline La0.58Sr0.4Co0.2Fe0.8O3–δ (LSCF) shows in bending tests a non-linear deformation behavior at room temperature and a linear one at elevated temperatures (≥ 800 °C). The non-linearity originates from ferroelastic domain switching in grains with rhombohedral symmetry. Ferroelastic domains cannot form in the cubic structure that exists at elevated temperatures. The critical stress of domain switching determined on the basis of bending tests from the onset of non-linear load-deflection behavior decreases from ~ 30 MPa at room temperature to ~ 5 MPa at 700 °C. Electron microscopic methods have been used to gain insight in formation, switching effects and stability of the domains under mechanical and thermal loading conditions. Direct SEM observations of the same surface area before and after compressive loading are used to verify the influence of the stress on the domain switching. Complementary TEM in-situ observations that verified the disappearance of domains at higher temperatures are discussed in terms of phase stability and oxygen partial pressure.