| Perovskite manganite RE1-xTxMnO3 (RE for Rare earth element, T for alkaline-earth element) with ABO3 structure, has attracted much attention, because of its colossal magnetoresistance effect and broad application prospects for the past 10 years. The double exchange mechanism proposed by Zener can be used to explaine qualitatively the electromagnetic and magnetic properties of such compounds.In this paper, the two problems were discussed on the based of theoretical calculations and experimental results, and the satisfactory explanation was given.1. Whether there will be vacancies at A site in the perovskite structure. In this paper, the two series of samples were prepared using sol-gel method (the highest heat treatment temperature was at 800°C), in which La3+ and Sr2+ ions in perovskite La0.7Sr0.3MnO3 were substituted respectively by vacancies. It is found by X-ray diffraction results, that there are two phases in the samples: dominating perovskite phase and second Mn3O4 phase. For the perovskite phase in samples, assuming that there were no vacancies, the lacking cations at A site were filled by Mn2+ ions, and all the samples possessed standard ABO3 structure, calculated the ion ratios of at A, B and O sites. The ion ratios were used to the Rietveld fitting for XRD spectra, obtained error parameters Rp, Rwp and s were acceptable, and therefore our the above assumption were made sure. In addition, the dependences of the cohesive energies on the substitute level for the perovskite phase in the two series of samples were calculated using the method proposed by us, including the ionic cohesive energy and a small additional metallic cohesive energy. The dependences were similar to those of the unit cell volume on the substitute level, obtained by the Rietveld fitting. Therefore, the dependence of the unit cell volume changes on the substitute level was determined by the cohesive energy. It provides a new strong support for the cohesive energy calculation method propsoed by us.The magnetic properties of samples were measured by Lake Shore M7310 Vibrating Sample Magnetometer (VSM). The results showed that: magnetic susceptibility and saturation magnetization of the samples in the room temperature increase with substitute level increasing. The dependences of the Curie temperature TC on the content of Mn4 + ion at B site, are similar to those of the typical perovskite La1-xSrxMnO3. Thus , the author concludes that, because of Mn2+ entering into A site of the perovskite structure, the samples synthesized by sol-gel process form an ABO3 structure at 800°C, in which the ionic ratio of A,B and O site is approximately 1:1:3 and there is less vacancies.2. Whether Ag can enter into the perovskite structure. In this paper, the two series of samples were prepared using sol-gel method (the highest heat treatment temperature was at 800°C), in which La3+ and Sr2+ ions in perovskite La0.7Sr0.3MnO3 were substituted respectively by Ag. It is found by X-ray diffraction results, that there are three phases in the samples: dominating perovskite phase, second Mn3O4 phase and third metal Ag phase.For the perovskite phase in samples, assuming that there were no vacancies, the lacking cations at A site were filled by Mn2+ ions, and the perovskite phase in all the samples possessed standard ABO3 structure, calculated the ion ratios of at A, B and O sites. The ion ratios were used to the Rietveld fitting for XRD spectra, obtained error parameters Rp, Rwp and s were acceptable. The dependences of the cohesive energies on the Ag substitute level for the perovskite phase in the two series of samples were calculated using the method proposed by us, including the ionic cohesive energy and a small additional metallic cohesive energy. The dependences were similar to those of the unit cell volume on the Ag substitute level, obtained by the Rietveld fitting. Therefore, the dependence of the unit cell volume changes on the substitute level was determined by the cohesive energy.The magnetic properties of samples were measured by Lake Shore M7310 Vibrating Sample Magnetometer (VSM). The results showed that: magnetic susceptibility and saturation magnetization of the samples in the room temperature increase with substitute level increasing. The dependences of the Curie temperature TC on the content of Mn4+ ion at B site, are similar to those of the typical perovskite La1-xSrxMnO3. It could concluded through above study, for our samples prepared using sol-gel method at 800°C, that a part of Ag ions can enter into the ABO3 type perovskite structure, and the rest of the Ag were formed metal Ag phase.
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