ON THE POSSIBILITY OF VACANCY FORMATION ON THE B-SITE, THE FRAMEWORK, OF PEROVSKITE-TYPE OXIDES

在钙钛矿型氧化物ABO_3中，容易形成氧空位而不容易形成金属空位。特别地，B位空位被认为几乎不存在，因为它们通过氧离子彼此紧密连接以形成刚性晶体结构。LaMnO_3基钙钛矿型氧化物在富氧气氛下表现出缺金属的非化学计量比。与这种非化学计量有关的缺陷结构一直是一个未解决的大问题。中子和X射线衍射研究不能揭示缺陷的位置，无论是在A-网站上，或在A-和B-网站。此外，没有明确的实验证据，是否占主导地位的缺陷是氧间隙或金属空位。本项目旨在阐明钙钛矿型氧化物中缺陷的本质，特别是LaMnO_3中的缺金属非化学计量比。用氧分压计测量了致密LaMnO_3在0.0001 ~ 1atm氧分压范围内的体积变化。长15 mm的试样在10天内氧化膨胀，还原收缩10 ~ 20 μm。高温XRD分析给出了晶格体积在相反方向上的变化。由于离子的扩散是通过空位进行的，因此测量了A位和B位离子的扩散系数，以比较两个位置的缺陷浓度。作为缺氧钙钛矿型氧化物的实例，选择YCrO_3，并测量La在A位和Mn在B位的杂质扩散系数。出乎意料的是，La和Mn的扩散系数基本相同，这表明A位和B位上的空位浓度相同。对于缺金属的LaMnO_3，B位的杂质扩散比A位的小几个数量级。在贫金属LaMnO_3中，空位主要形成在A位上。

In the perovskite-type oxides, ABO_3, oxygen vacancies are easily formed while metal vacancies are not. Particularly, B-site vacancy is considered hardly exists because they are closely linked each other via oxygen ions to form a rigid crystal structure. LaMnO_3-based perovskite-type oxides are known to display metal deficient nonstoichiometry under oxygen rich atmospheres. The defect structure related to this nonstoichiometry has been a big unsolved problem. Neutron and X-ray diffraction studies could not reveal the position of defects whether on A-site only or on both A- and B-site. Moreover, there is no clear experimental evidence whether the predominant defect is oxygen interstitial or metal vacancy. This project aims to clarify the nature of defects in the perovskite-type oxides with particular focus on the metal deficient nonstoichiometry of LaMnO_3.Measurements were made on volume variation of dense LaMnO_3 with the variation of oxygen partial pressure between 0.0001 and 1 atm by a dilatometer. A sample of 15 mm in length expands with oxidation and contracts with reduction for 10 〜 20 μm in 10 days. A high temperature XRD analysis gives the change of lattice volume in the inverse direction. This result infers the predominant defect of metal deficient LaMnO_3 as metal vacancy.Since the diffusion of ions proceeds via vacancy, the diffusivity of A- and B-site ions are measured to compare the defect concentration of both sites. As an example of oxygen deficient perovskite-type oxides, YCrO_3 was selected and impurity diffusion coefficient of La on A-site and Mn on B-site were measured. Unexpectedly, the diffusion coefficient of La and Mn was essentially the same, suggesting that the concentration of vacancy on A- and B-site are the same. As to the metal deficient LaMnO_3, impurity difusion on B-site was found smaller than that on A-site by several orders of magnitude. In the metal deficient LaMnO_3, vacancies are predominantly formed on A-site.