Influence of Preparation on Structure and Properties of La_0.67Ca_0.33MnO₃ Bulk Material

During growth of La_0.67Ca_0.33MnO₃ in an usual solid state reaction process, the diffusion of La-site metal ions is a rate-limiting factor. In the initial stage of the growth process, La-deficient manganite with rhombohedral structure was easily formed. With the repetition of the grinding– pressing–sintering process, the metal content on the La-site was gradually increased, and the phase was changed from rhombohedral to cubic structure. For slightly La-deficient (La,Ca)_1–xMnO₃ manganites, the ferromagnetic–paramagnetic (F–P) transition temperature T_M is less sensitive to the La-site deficiency, but their electric transport properties are greatly affected. The slightly La-site deficient compound was characterized by double bumps on its resistivity versus temperature (R–T) curve, one of which was located at higher temperature very near to its temperature T_M. For the La_0.67Ca_0.33MnO₃ single phase, only one resistance peak was observed at T_R on its R–T curve. For the sample sintered at lower temperatures (<1100 °C) in air T_R is much less than T_M, and no magnetoresistance (MR) peak was observed within the temperature range of 80 K < T < 300 K. But for the sample sintered at 1200 °C in air T_R is well in agreement with T_M and its MR is strongly peaked very close to T_M. All the bulk samples showed substantial MR in a magnetic field of 0.6 T at low temperatures that increased almost linearly with decreasing temperature.