Non-congruence of high-temperature mechanical and structural behaviors of LaCoO₃ based perovskites

This paper presents the mechanical behavior of LaCoO₃ and La_0.8Ca_0.2CoO₃ ceramics under four-point bending in which the two cobaltites are subjected to a low stress of ∼8 MPa at temperatures ranging from room temperature to 1000 °C. Unexpected stiffening is observed in pure LaCoO₃ in the 700–900 °C temperature range, leading to a significant increase in the measured Young's modulus, whereas La_0.8Ca_0.2CoO₃ exhibits softening from 100 °C to 1000 °C, as expected for most materials upon heating. Neutron diffraction, X-ray diffraction and micro-Raman spectroscopy are used to study the crystal structure of the two materials in the RT–1000 °C temperature range. Despite a detailed study, there is no conclusive evidence to explain the stiffening behavior observed in pure LaCoO₃ as opposed to the softening behavior in La_0.8Ca_0.2CoO₃ at high temperatures (above 500 °C).