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.
Diffusion, or atom movement, in intermetallic compounds was studied using perturbed angular correlation of gamma rays (PAC). PAC allows one to determine the local atomic environments of probe atoms in crystals. Atom movement is generally made possible in solids through the presence of missing atoms into which neighboring atoms can jump. Diffusion occurs especially at high temperature and has a major effect on the mechanical strength, ductility and stability of materials. PAC is a specialized technique and the group at WSU is the only one in the US currently using PAC to study any aspect of solids. Ten years ago the group pioneered application of PAC to study diffusion by showing how it could be used to measure frequencies at which atoms jump in the solid. This is possible when the local environment of the jumping atom changes during the jump. The method does not work for common, simple crystal structures such as NiAl, but is ideal for investigations of diffusion in more complex crystal structures, in which there is increasing interest. Studies were carried out on many compounds having the Cu3Au crystal structure shown in Figure 1. An atom jumping from the top of the unit cell to the side leads to a change in the orientation of the local environment. This change leads to decoherence in precessions of the nuclear spins that can be measured and fitted to obtain the jump frequency. Systematic studies have provided information about diffusion mechanisms. PAC measurements also give information about the lattice locations occupied by impurity probe atoms, for example the face-centered and corner sites in the structure shown. Intellectual merit. There is no other technique comparable to PAC. Exploratory studies were made on many systems, giving insight into novel diffusion phenomena. Specific outcomes outcomes include the following. Jump-frequencies were measured along many series of compounds. A remarkable correlation was identified between site-preferences and jump frequencies of indium impurities in a series of Pd3R compounds (R=rare-earth), with jump-frequencies increasing as site preferences changed. Such a correlation had never been observed previously. In3R phases having mixtures of two rare-earth elements were studied to see the effect on diffusion of an energy landscape with random "speed bumps" due to the differing local environments of jumping atoms. Diffusion was studied in many Al4Ba phases. For these phases, jumps do not reorient the local environment but instead lead to a change in precessional frequency. Frequencies were observed to merge at high temperature due to "frequency averaging" caused by more rapid jumping. Diffusion was studied in a series of layered compounds LanCoIn3+2n phases, with n=1,2,3. Here, the probe atoms jump only in indium layers, and jump-frequencies were found to increase as indium layers became thicker. Some experimental results were checked by electronic structure calculations of precessional frequencies and of the energies of impurity atoms on the different sites. In an unanticipated finding, the maximum amounts of indium that can be dissolved in liquid or solid gallium metals were determined. For solid gallium, the amount found was only one atom per hundred billion, a record low number. Experiments have been completed for jump frequencies in many series of Cu3Au phases, with results summarized in Figure 2. The figure displays a measure of jump-frequencies for 30-odd phases plotted as a function of lattice parameters. Roughly speaking, the higher the vertical position of a data point on the plot, the higher the jump frequency. While large variations can be seen between jump frequencies and lattice parameters among the palladium and indium phases, the variation across all five alloy series, taken together, shows no clear variation with lattice parameter. Theoretical work is clearly needed to explain these interesting findings. Broader impacts. One faculty member and five graduate, seven undergraduate and one high-school student received training, including three women and one Native American student. Two PhD and two MS degrees were awarded. The high-school student was recognized as a semifinalist in the Intel Science Talent Search 2010 based on her research. Contact was made between this research and a broader audience through seminars at undergraduate and graduate levels and in presentations at meetings. Findings were also disseminated in 9 presentations at international and regional conferences and in about 28 other talks or poster presentations by students and myself on campus. PAC offers new insight to diffusion studies. Traditional measurements of diffusivities of tracer isotopes are very laborious. Generally, one wishes to measure the temperature dependence of the jump frequency in order to obtain the activation energy for diffusion, but that requires measurements to be made at perhaps ten different temperatures. The traditional approach requires a new sample for measurement at each temperature whereas one sample can suffice for an entire run of PAC measurements.
