Thermal transition behavior of La1−xCaxMnO3−δ oxides

Unusual thermal transitions of the La1−xCaxMnO3−δ (x=0.25, 0.33, 0.375) ceramic oxides have been found using differential scanning calorimetry in the temperature range from 293 to 173 K. However, such transitions do not occur in the samples with x=0.125 and 0.5 in the broader temperature range from 293 to 123 K. Interestingly, the thermal transition temperature reflects the Curie temperature and the enthalpy change of the transition varies inversely proportional to the transition temperature. These results indicate that the La1−xCaxMnO3−δ (x=0.25, 0.33, 0.375) oxides undergo first-order exothermic phase transitions from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. It is suggested that the transition result from a local lattice structure change, which is correlated with lattice distortion.