Chemical State Analysis of Transition-metal Ions in Complex Oxides with Spinel Structure

许多含有过渡金属离子的氧化物尖晶石具有令人感兴趣的物理和/或化学性质，并且被广泛使用。研究晶体的生成条件与晶体中过渡金属离子的化学状态之间的关系是非常重要的。在本研究中，我们在各种条件下制备了铁尖晶石、铝酸镍铁氧体、镍锌铁氧体和锰锌铁氧体。通过X射线衍射、EXAFS和穆斯堡尔谱分析确定了阳离子的分布。我们制作了Rietveld分析的计算机程序，并通过粉末X射线衍射测定了阳离子分布。X射线衍射通常是分析阳离子分布的最有效方法。然而，当晶体中含有几种具有相似原子散射因子的过渡金属离子时，很难通过X射线衍射区分过渡金属离子。相反，X射线吸收边彼此分离，EXAFS可以独立地分析某一过渡金属离子的分布。我们编制了EXAFS分析的计算机程序，并将其用于确定尖晶石型氧化物的阳离子分布，尖晶石型氧化物的阳离子分布主要由静电能、晶场稳定能和极化能决定。根据这些能量，引入了Mn^<3+>和Ni^<2+>的八面体位置优先，Zn^<2+>和Fe^<2+>的四面体位置优先，以及Mn^<2+>和Fe^<3+>的非位置优先，这与上述分析结果基本一致。认为这些能量可用于估计过渡金属离子的化学状态。

Many oxide spinels containing transition-metal ions have technologically interesting physical and/or chemical properties and are used widely. It is important to investigate the relation between the conditions of the production of the crystals and the chemical state of the transition metal ions in the crystal.In this study, we produced iron spinels, nickel aluminate ferrites, nickel-zinc ferrites, and manganese-zinc ferrites under various conditions. We determined the cation distributions by the x-ray diffraction, EXAFS analysis and Mossbauer analysis. We produced the computer program for Rietveld analysis and determined the cation distribution by powder x-ray diffraction. X-ray diffraction is usually the most effective method of analyzing the cation distribution. When, however, the crystal contains several kinds of transition-metal ions with somewhat similar atomic scattering factors, it is difficult to distinguish among the transition-metal ions by x-ray diffraction. On the contrary, the x-, ray absorption edges are well separated from each other, the distribution of a certain transition-metal ion can be analyzed independently by EXAFS. We produced the computer program for the EXAFS analysis, and applied it to determining the cation distributions.The cation distribution of spinel oxides are determined mainly by the electrostatic energy, the crystal-field stabilization energy, and the polarization energy. By these energies, the octahedral site preference of Mn^<3+> and Ni^<2+>, the tetrahedral site preference of Zn^<2+> and Fe^<2+>, and the no site preference of Mn^<2+> and Fe^<3+> are introduced, which were fair coincident with the result of the above analysis. It is considered that the chemical state of the transition-metal ions can be estimated practically by these energies.