摘要
具有ABO3结构的钙钛矿锰氧化物RE1-xTxMnO3 ( RE为稀土元素,T为碱土元素)由于其庞大的磁电阻效应和广阔的应用前景,成为十几年来的一个研究热点。应用Zener提出的双交换机制可定性地解释这类化合物的电磁特性。
在本论文中,我们主要对两个基本问题进行了探讨,结合理论计算和实验结果给出了满意的解释。
1、钙钛矿结构中的A位究竟会不会存在空位。本文用溶胶-凝胶法制备了La0.7Sr0.3MnO3中A位高价和低价自掺杂的2个系列样品。使用X’pert Pro型X射线衍射仪测量了各系列样品的衍射谱,发现所制备样品多为复合材料。对于样品中的钙钛矿相,假设在样品中不存在空位,A位阳离子的不足,由Mn2+离子来弥补,所有样品都形成正分的ABO3结构,计算出了A、B和O位各种离子的比例,并直接用于XRD谱的Rietveld拟合,代表精修效果的误差参数Rp、Rwp、s都处于理想值范围内。验证了A位不存在空位。此外,利用我们提出的结合能计算方法计算了这2个系列样品中钙钛矿相的结合能随掺杂量变化的规律,其变化趋势分别与通过Rietveld拟合计算出的晶胞体积随掺杂量的变化趋势相同,从而说明晶胞体积随掺杂量的变化是由其结合能决定的。从而为我们提出的结合能计算方法提供了新的有力支持。采用Lake Shore M7310型振动样品磁强计(VSM)对样品进行磁性分析,结果发现:其室温磁化率和饱和磁化强度都随着自掺杂浓度增加而增大。居里温度TC随着样品钙钛矿相中Mn4+离子含量的变化基本符合La1-xSrxMnO3等二价掺杂系列材料的变化规律。从而得出结论:利用溶胶-凝胶法最终在800°C形成的复合体系的钙钛矿相中基本不存在A位空位,而是由二价Mn2+离子进入到A位,形成ABO3的稳定结构,其A、B和O位的离子数目比近似为标准的1 : 1 : 3。其中A位阳离子的不足,由Mn2+离子来弥补,正分氧含量是按A、B位阳离子的比例在样品成相之前的热处理过程中形成的。
2、Ag究竟能不能进入到钙钛矿结构中。我们通过在母体材料La0.7Sr0.3MnO3中以Ag替代A位高价和低价离子,制备了2个系列样品,使用X’pert Pro型X射线衍射仪测量了各系列样品的衍射谱,得到由菱面体类钙钛矿相、金属Ag相和Mn3O4相组成的三相复合材料;假设样品的钙钛矿相中“基本不存在A位空位”,结合对样品成分的分析,计算出了A、B和O位各种离子的比例,并直接用于XRD谱的Rietveld拟合,代表精修效果的误差参数Rp、Rwp、s都处于理想值范围内。根据我们提出的结合能计算方法计算了钙钛矿相的结合能。计算结果显示,钙钛矿相的结合能随掺杂量变化的规律,与通过Rietveld拟合计算出的晶胞体积随掺杂量的变化趋势相同,从而说明晶胞体积随掺杂量的变化是由其结合能决定的。采用Lake Shore M7310型振动样品磁强计(VSM)对样品进行磁性分析,其室温磁化率和饱和磁化强度都随着Ag的掺杂浓度增加而增大。居里温度TC随着样品钙钛矿相中Mn4+离子含量的变化基本符合La1-xSrxMnO3等二价掺杂系列材料的变化规律。通过上述研究得出结论:利用溶胶-凝胶法最终在800°C形成的名义成分为La0.7Sr0.3MnO3的复合体系钙钛矿相中,部分Ag能够以离子形式进入到ABO3型钙钛矿结构中,其余Ag形成金属Ag相。
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.
引文
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