Papers by Keyword: Nuclear Relaxation

Abstract: Nuclear relaxation caused by diffusion of ¹¹¹In/Cd probe atoms was measured in four phases having the tetragonal FeGa₃ structure (tP16) using perturbed angular correlation spectroscopy (PAC) and used to gain insight into diffusion processes in phases having more than one diffusion sublattice. The three indide phases studied in this work have two inequivalent and interpenetrating In-sublattices, labeled In1 and In2, and nuclear quadrupole interactions were resolved for probes on each sublattice. The phases are line-compounds with narrow field-widths. Diffusional relaxations, fitted using an exponential damping ansatz, were measured at the two opposing boundary compositions as a function of temperature. “High” and “low” relaxation regimes were observed that are attributed to In-poorer and In-richer compositions, under the reasonable assumption that the atomic motion occurs via an indium-vacancy diffusion mechanism. Relaxation was observed to be greater for tracer atoms starting on In2 sites in the indides immediately following decay of ¹¹¹In into ¹¹¹Cd, which is attributed to a preference of daughter Cd-tracer atoms and/or indium vacancies to occupy In1 sites. Activation enthalpies for relaxation are compared with enthalpies for self-diffusion in indium metal.

Abstract: Following nuclear decay, a daughter atom in a solid will "stay in place" if the recoil energy is less than the threshold for displacement. At high temperature, it may subsequently undergo long-range diffusion or some other kind of atomic motion. In this paper, motion of ¹¹¹Cd tracer probe atoms is reconsidered following electron-capture decay of ¹¹¹In in the series of In₃R phases (R= rare-earth). The motion produces nuclear relaxation that was measured using the method of perturbed angular correlation. Previous measurements along the entire series of In₃R phases appeared to show a crossover between two diffusional regimes. While relaxation for R= Lu-Tb is consistent with a simple vacancy diffusion mechanism, relaxation for R= Nd-La is not. More recent measurements in Pd₃R phases demonstrate that the site-preference of the parent In-probe changes along the series and suggests that the same behavior occurs for daughter Cd-probes. The anomalous motion observed for R= Nd-La is attributed to "lanthanide expansion" occurring towards La end-member phases. For In₃La, the Cd-tracer is found to jump away from its original location on the In-sublattice in an extremely short time, of order 0.5 ns at 1000 K and 1.2 ms at room temperature, a residence time too short to be consistent with defect-mediated diffusion. Several scenarios that can explain the relaxation are presented based on the hypothesis that daughter Cd-probes first jump to neighboring interstitial sites and then are either trapped and immobilized, undergo long-range diffusion, or persist in a localized motion in a cage.

Abstract: Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of probe atoms can be measured through relaxation of the nuclear quadrupole interaction. This was first shown in 2004 for jumps of tracer atoms that lead to reorientation of the local electric field-gradient, such as jumps on the connected a-sublattice in the L1₂ crystal structure. Studies on many such phases using the ¹¹¹In/Cd PAC probe are reviewed in this paper. A major finding from a 2009 study of indides of rare-earth elements, In₃R, was the apparent observation of two diffusional regimes: one dominant for heavy-lanthanide phases, R= Lu, Tm, Er, Dy, Tb, Gd, that was consistent with a simple model of vacancy diffusion on the In a-sublattice, and another for light-lanthanides, R= La, Ce, Pr, Nd, that had no obvious explanation but for which several alternative diffusion mechanisms were suggested. It is herein proposed that the latter regime arises not from a diffusion mechanism but from transfer of Cd-probes from In-sites where they originate to R-sites as a consequence of a change in site-preference of ¹¹¹Cd-daughter atoms from In-sites to R-sites following transmutation of ¹¹¹In. Support for this transfer mechanism comes from a study of site-preferences and jump-frequencies of ¹¹¹In/Cd probes in Pd₃R phases. Possible mechanisms for transfer are described, with the most likely mechanism identified as one in which Cd-probes on a-sites transfer to interstitial sites, diffuse interstitially, and then react with vacancies on b-sites. Implications of this proposal are discussed. For indides of heavy-lanthanide elements, the Cd-tracer remains on the In-sublattice and relaxation gives the diffusional jump-frequency.

Abstract: Measurements were made of jump frequencies of ¹¹¹In/Cd tracer atoms on the Sn-sublattice in rare-earth tri-stannides having the L1₂ crystal structure via perturbed angular correlation spectroscopy (PAC). Phases studied were Sn₃R (R= La, Ce, Pr, Nd, Sm and Gd). Earlier measurements on isostructural rare-earth tri-indides showed that the dominant diffusion mechanism changed along that series [4]. The dominant mechanism was determined by comparing jump frequencies measured at opposing phase boundary compositions (that is, more In-rich and more In-poor). Jump frequencies were observed to be greater at the In-rich boundary composition in light lanthanide indides and greater at the In-poor boundary composition in heavy-lanthanide indides. These observations were attributed to predominance of diffusion via rare-earth vacancies in the former case and indium vacancies in the latter. Contrary to results for the indides, jump frequencies found in the present work are greater for the Sn-poor boundary compositions of the stannides, signaling that diffusive jumps are controlled by Sn-vacancies. Possible origins of these differences in diffusion mechanisms are discussed.

Abstract: Jump frequencies of Cd tracer atoms were measured in three phases having the orthorhombic Al11R3 structure, with R= La, Ce, or Pr. The structure has four inequivalent Al-sites and two inequivalent R-sites. 111In/Cd tracer atoms were observed to occupy several sites via the nuclear quadrupole interaction using perturbed angular correlation of gamma rays (PAC). Time-domain PAC spectra became damped as the temperature increased, which is attributed to nuclear relaxation caused by diffusional jumps of Cd tracer atoms leading to changes in orientations and/or magnitudes of electric field gradients (EFG’s). Maximum relaxations were observed near 770 K. A method is proposed for estimating the mean jump frequency at that temperature, giving a mean jump frequency w averaged over all sites of about 100 MHz. At still higher temperatures, damping decreased due to motional averaging, and the quadrupole perturbations evolved into unique signals having lower frequencies and corresponding in each phase to the averages of EFG tensors of all sites visited by the Cd tracer atoms. For Al11La3, the jump frequency at 1073 K was estimated to be 1.9 GHz. Such jump frequencies imply unusually high diffusivities in these phases.

Abstract: Jump frequencies of Cd tracer atoms were measured in three lanthanide gallides having the L12 structure: DyGa3, ErGa3 and LuGa3. 111In/Cd impurity probe atoms were observed to occupy the non-cubic Ga-sites through the nuclear quadrupole interaction using the method of perturbed angular correlation of gamma rays (PAC). Measurements at elevated temperatures exhibited nuclear relaxation (damping) of quadrupolar perturbation functions attributed to diffusional jumps of the probes among orientationally inequivalent Ga-sites. Accurate values of jump frequencies were determined from fits of the measured perturbation functions using a model of stochastically fluctuating electric-field gradients, as in previous work [e.g., Matthew O. Zacate, Aurélie Favrot and Gary S. Collins: Physical Review Letters Vol. 92 (2004) p. 225901]. Arrhenius plots of jump frequencies for the three systems exhibited jump-frequency activation enthalpies in the range 0.86-1.05 eV and prefactors of about 2 THz. The activation enthalpy for ErGa3, 0.86(2) eV is compared with those for ErAl3, 1.40(4) eV, and ErIn3, 1.34(5) eV.

Abstract: The jump frequency of Cd tracer atoms was measured as a function of temperature in seven rare-earth tri-indide intermetallic compounds having the L12, or Cu3Au, structure. The frequency, proportional to the diffusivity, was detected by relaxation of nuclear quadrupole interaction at Cd nuclei caused by reorientation of the electric field gradient in each diffusive jump. Measurements were made using perturbed angular correlation of gamma rays, sensitive to jump frequencies in the range 1-1000 MHz. Results are as follows. (1) Jump frequencies measured in LaIn3 and CeIn3 were observed to be 10-100 times greater at the more In-rich boundary composition than the less In-rich boundary composition, even though the phases appear as line compounds in phase diagrams. (2) Arrhenius plots of the jump frequency were fitted to activation enthalpies that increase from 0.535 to 1.80 eV across the series of phases LaIn3, CeIn3, PrIn3, and NdIn3.