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

Abstract

Twin structures in rhombohedrally distorted polycrystalline LaCoO₃, La_0.8Ca_0.2CoO_3-δ and La_0.5Sr_0.5Co_0.5Fe_0.5O_3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO₃ and La_0.8Ca_0.2CoO_3-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 La_0.5Sr_0.5Co_0.5Fe_0.5O_3-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.

Keywords:

• LaCoO₃
• TEM
• deformation twinning
• atomic scale twinning

Acknowledgements

This research has been funded by the Norwegian University of Science and Technology, Strategic Area Materials.