TEM observations of rhombohedral and monoclinic domains in LaCoO3-based ceramics

Twin structures in rhombohedrally distorted polycrystalline LaCoO3, La0.8Ca0.2CoO3-δ and La0.5Sr0.5Co0.5Fe0.5O3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO3 and La0.8Ca0.2CoO3-δ. In addition, ferroelastic domains with a one-dimensional superstructure corresponding to a tripling of the pseudocubic lattice parameter, a c, were found in some grains. The monoclinic superstructure vanished over time and was sensitive to external mechanical stress. In La0.5Sr0.5Co0.5Fe0.5O3-δ no normal twinning was observed, but domains with a one-dimensional superstructure of two times a c were observed. The superstructures have been interpreted as atomic scale twinning, which reduce the crystal symmetry from rhombohedral to monoclinic.