Influence of preparation on structure and properties of La0.67Ca0.33MnO3 bulk material

During growth of Lao(0.67)Ca(0.33)MnO(3) 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-x)MnO3 manganites, the ferromagnetic-paramagnetic (F-P) transition temperature TM 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 TM For the La0.67Ca0.33MnO3 single phase, only one resistance peak was observed at T-R on its R-T curve. For the sample sintered at lower temperatures (< 1100 degrees 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 degrees C in air T-R is well in agreement with T-M and its MR is strongly peaked very close to TM. 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.