Low-frequency internal friction study of phase transitions in La1/3Sr2/3FeO3−δ ceramics

La1/3Sr2/3FeO3−δ samples with various oxygen contents were investigated by low-frequency internal friction as a function of temperature. Around room temperature, the samples annealed in oxygen and the as-prepared both presented a relaxation peak accompanied by a decrease in modulus. By means of the peak shift method, the activation energies of the relaxation peaks were obtained to be 1.27 and 1.18 eV for the as-prepared and annealed samples, respectively. Above 370 K, both samples exhibited the rapid stiffening on the shear modulus curves on heating, which is related to a phase transition from ferroelastic rhombohedral to paraelastic cubic. The samples annealed in oxygen also showed a local minimum in modulus at about 193 K, indicating the occurrence of an antiferromagnetic ordering and a charge-ordering (CO), which were confirmed by the magnetization and electrical resistivity measurements. The change in modulus indicates that the CO is accompanied by a lattice distortion. However the CO and the anomaly of the modulus were not observed in the as-prepared samples at the corresponding temperature.