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

关于钙钛矿型氧化物的 B 位、骨架上空位形成的可能性

基本信息

批准号：
10450239
负责人：
MIZUSAKI Junchiro
金额：
$ 8.06万
依托单位：
TOHOKU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
1998
资助国家：
日本
起止时间：
1998 至 2000
项目状态：
已结题

项目摘要

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.

在钙钛矿型氧化物ABO_3中，氧空位容易形成，而金属空位不易形成。特别是，B位空位几乎不存在，因为它们通过氧离子紧密相连，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乏金属的非化学计量比。与这种非化学计量相关的缺陷结构一直是一个悬而未决的大问题。中子和X射线衍射研究不能揭示缺陷的位置，无论是仅在A位还是在A位和B位都是如此。此外，目前还没有明确的实验证据表明主要缺陷是氧间隙缺陷还是金属空位。本项目旨在阐明钙钛矿型氧化物中缺陷的性质，特别是LaMnO_3的缺金属非化学计量比。用膨胀仪测量了致密的LaMnO_3在0.0001~1atm的氧分压范围内的体积变化。15 mm长的样品在10天内随氧化膨胀，随还原收缩10~20μm。高温X射线衍射分析给出了晶格体积逆方向的变化。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，以比较两位离子的缺陷浓度。选择YCrO_3作为缺氧型钙钛矿型氧化物，测量了La在A位和Mn在B位的扩散系数。出乎意料的是，La和Mn的扩散系数基本相同，表明A位和B位上的空位浓度是相同的。对于缺金属的LaMnO_3，B位的杂质弥散比A位的弥散小几个数量级。在缺乏金属的LaMnO_3中，空位主要形成在A位。