Diffusion in Rare Earth Binary and Ternary Intermetallics Studied using PAC

技术概述：Collins教授及其同事率先使用摄动角相关光谱（PAC）来测量高有序金属间化合物中探针原子的扩散跳跃频率。跳跃是通过电场梯度（EFG）方向或大小的随机波动引起的核四极相互作用的松弛来检测的。该方法已特别应用于？行化合物?由稀土和三元素（Al, Ga, In）组成。观察到异常高的频率是由于空位的快速运动造成的。通过对具有相反边界成分的二元合金对样品的测量，也阐明了扩散机制。测量跳跃频率的比较已用于确定主要负责扩散的空位类型（稀土或三元素）。提出了一系列二元、伪二元和三元系化合物的PAC实验。例如Al4Ba，其所有位点的EFGs都是共线的，伪二元化合物In3（La1-xPrx）都具有L12结构，以及与In3La结构相关的三元相LaCoIn5。为了检验Cd杂质原子和In主原子的跳变频率的差异程度，我们安排了短期的协同作用。非技术概要：固体中的扩散或原子运动对于理解材料的机械强度和稳定性很重要，并影响许多其他性质，如晶体有序度和磁性。越来越复杂的材料对技术变得越来越重要，人们对提高我们对这些材料中的扩散现象的理解非常感兴趣。柯林斯教授和他的同事们率先使用核物理学衍生的光谱学来直接测量高有序化合物中探针原子的跳跃频率。在大多数固体中，原子通过跃入空位（缺失的原子）来移动。在某些种类的金属间化合物中发现了异常高的跳跃频率。通过比较两种成分稍有不同的样品的测量结果，柯林斯和他的同事们已经能够确定A元素和B元素的空位是扩散的主要原因。这为扩散现象提供了一种全新的见解。提出了一系列二元和三元化合物的研究。该项目将涉及研究生和本科生的参与和培训。超过12名研究生、本科生和高中生在之前的资助下参与了研究，其中包括6名女性。结果将在出版物、会议发言和讨论会中广泛传播。此外，还计划开设有关缺陷和扩散以及使用核方法研究固体的短期课程。在日内瓦的欧洲核子研究中心（CERN）已经建立了一个合作项目，来进行一些在美国无法进行的实验。

TECHNICAL SUMMARY:Professor Collins and coworkers pioneered use of perturbed angular correlation spectroscopy (PAC) to measure diffusional jump frequencies of probe atoms in highly ordered intermetallic compounds. Jumps are detected via relaxation of the nuclear quadrupole interaction caused by stochastic fluctuations in either the orientation or magnitude of electric field gradients (EFG). The method has been applied especially to ?line compounds? formed from rare earth and trielide elements (Al, Ga, In). Exceptionally high frequencies have been observed that are attributed to rapid motion of vacancies. Diffusion mechanisms have also been elucidated experimentally by making measurements on pairs of samples of binary alloys having the opposing boundary compositions. Comparison of measured jump frequencies has been used to identify the type of vacancy (rare-earth or trielide) that is predominantly responsible for diffusion. PAC experiments are proposed for a range of binary, pseudo-binary and ternary line compounds. Examples include Al4Ba, in which the EFGs at all sites are collinear, the pseudo-binary compounds In3(La1-xPrx), which all have the L12 structure, and LaCoIn5, a ternary phase that is structurally related to In3La. Short term collaborations have been arranged to check the extent to which jump frequencies of Cd impurity atoms and In host atoms differ.NON-TECHNICAL SUMMARY:Diffusion, or atom movement, in solids is important for understanding the mechanical strength and stability of materials and affects many other properties, such as the degree of crystalline order and magnetism. Increasingly complex materials are becoming important to technology and there is great interest in improving our understanding of diffusion phenomena in those materials. Professor Collins and coworkers have pioneered use of a spectroscopy derived from nuclear physics to directly measure frequencies of jumping of probe atoms in highly-ordered compounds. In most solids, atoms move by hopping into vacancies (missing atoms). Unusually high jump frequencies have been found in certain classes of intermetallic compounds. By comparing measurements made on two samples having slightly different compositions, Collins and coworkers have been able to determine whether the vacancy on sites of element A or element B is most responsible for diffusion. This has given an entirely new kind of insight into diffusion phenomena. Studies are proposed on a range of binary and ternary compounds. The project will involve participation and training of graduate and undergraduate students. More than 12 graduate, undergraduate and high-school students participated in research under a previous grant, including six women. Results will be disseminated widely in publications and in conference presentations and seminars. Also, short courses are planned on defects and diffusion and on use of nuclear methods to study solids. A collaboration has been established to carry out some experiments at CERN, the European nuclear physics center in Geneva, that are not possible in the US.