Diffusion in Rare Earth Binary and Ternary Intermetallics Studied using PAC

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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.