Time and frequency dependent mechanical properties of LaCoO₃-based perovskites: Internal friction and negative creep

The internal friction and creep deformation behavior of La_0.8Ca_0.2CoO₃ and pure LaCoO₃ mixed ionic electronic conducting perovskite ceramics have been studied by Dynamic Mechanical Analysis and uniaxial compression under constant applied load, respectively. It was found that both the internal friction and creep strain were almost an order of magnitude higher for Ca²⁺ doped LaCoO₃ as compared to pure undoped LaCoO₃. The difference in Ca²⁺ doped LaCoO₃ behavior was attributed to the much higher concentration of point defects (e.g., oxygen vacancies) in the structure and their interaction with other mobile defects, such as ferroelastic domain/twin walls, stacking faults, dislocations, etc. Such interactions of numerous point defects with domain walls produce energetic barriers and slow down the movement of ferroelastic domain walls under applied stress. At the same time, the defects' interactions increase the internal friction resulting in a much higher creep strain of La_0.8Ca_0.2CoO₃ as compared to pure LaCoO₃, as the creep strain is determined by the distance between the domain wall and its equilibrium position at the onset of the creep process. Therefore, the high friction will result in the larger distance the wall has to move to reach the equilibrium which in turn results in higher creep strain. The expansion of LaCoO₃ under constant applied compressive stress, named here as negative creep, was also discovered to occur during room temperature creep experiments.