Observation of heat flow at transition temperature in La1–xCaxMnO3 + delta Oxides

Heat-flow changes of La1–xCaxMnO3 + delta (x=0.25, 0.33, and 0.375) magnetoresistive oxides at their Curie temperatures have been detected using differential scanning calorimetry in the temperature range from 173 to 293 K. However, the transition does not occur in the samples of x=0.125 and 0.5. It was found that the heat flow reflected the transition behavior and the enthalpy change of the transition decreased as the transition temperature increased. These results indicate that the La1–xCaxMnO3 + delta oxides (x=0.25, 0.33, and 0.375) undergo an endothermic phase transition from a low-temperature ferromagnetic metal to a high-temperature paramagnetic insulator. It is assumed that the transition is strongly associated with a local structure change, which is correlated with the metal–insulator transition.