Thermal transition behavior of La1-xCaxMnO3-delta oxides

Unusual thermal transitions of the La1-xCaMnO3-delta (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 rang e from 293 to 123 K. Interestingly, the thermal transition temperature refl ects the Curie temperature and the enthalpy change of the transition varies inversely proportional to the transition temperature. These results indica te that the La1-xCaxMnO3-delta (x = 0.25, 0.33, 0.375) oxides undergo first -order exothermic phase transitions from a high-temperature paramagnetic in sulator to a low-temperature ferromagnetic metal. It is suggested that the transition result from a local lattice structure change, which is correlate d with lattice distortion. (C) 2001 Elsevier Science Ltd. All rights reserv ed.