Strain coupling mechanisms and elastic relaxation associated with spin state transitions inLaCoO3

Abstract

Advantage is taken of the wealth of experimental data relating to the evolution with temperature of spinstates of Co3 + in LaCoO3 in order to undertake a detailed investigation of the mechanisms by which changes inelectronic structure can influence strain, and elastic and anelastic relaxations inperovskites. The macroscopic strain accompanying changes in the spin state inLaCoO3 is predominantly a volume strain arising simply from the change in effective ionic radius of theCo3 + ions. This acts to renormalize the octahedral tilting transition temperature in a mannerthat is easily understood in terms of coupling between the tilt and spin order parameters.Results from resonant ultrasound spectroscopy at high frequencies (0.1–1.5 MHz) revealstiffening of the shear modulus which scales qualitatively with a spin order parameterdefined in terms of changing Co–O bond lengths. From this finding, in combination withresults from dynamic mechanical analysis at low frequencies (0.1–50 Hz) and data from theliterature, four distinctive anelastic relaxation mechanisms are identified. The relaxationtimes of these are displayed on an anelasticity map and are tentatively related to spin-spinrelaxation, spin-lattice relaxation, migration of twin walls and migration of magneticpolarons. The effective activation energy for the freezing of twin wall motion below ∼ 590 K at low frequencieswas found to be 182 ± 21 kJ mol − 1 (1.9 ± 0.2 eV) which is attributed to pinning by pairs of oxygen vacancies, though thelocal mechanisms appear to have a spread of relaxation times. It seemsinevitable that twin walls due to octahedral tilting must have quite differentcharacteristics from the matrix in terms of local spin configurations ofCo3 + . A hysteresis in the elastic properties at high temperatures furtheremphasizes the importance of oxygen content in controlling the properties ofLaCoO3.