Microstructure and the influence of spontaneous strain in LaCoO3, La0.8Sr0.2CoO3 and La0.8Ca0.2CoO3

Transmission Electron Microscopy has been conducted on rhombohedral perovsk ite oxides with composition LaCoO3, La0.8Sr0.2CoO3, and La0.8Ca0.2CoO3. Thi n foils prepared within the first weeks of sintering showed macroscopic str ain and a very high defect density, which included fault-related superlatti ce structures. Samples prepared several months after sintering, showed no s ignificant macroscopic strain and the main defect was found to be domains d ue to reflection twinning in the pseudo-cube plane. The behaviour is likely to be related to accommodation of spontaneous strain arising upon cooling of the material from the sintering temperature. The presence of twin domain s illustrates the lowering of lattice symmetry as compared to the ideal cub ic perovskite structure, and is relevant for explaining recent observations of ferroelastic behaviour of these materials. (C) 2000 Kluwer Academic Pub lishers.