Papers by Gary S. Collins
Defect and Diffusion Forum, Nov 14, 2022
Nuclear relaxation caused by diffusion of 111In/Cd probe atoms was measured in four phases having... more 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.
Crystals
The Al4Ba crystal structure is the most common structure among binary intermetallic compounds. It... more The Al4Ba crystal structure is the most common structure among binary intermetallic compounds. It is well suited for accommodating large atoms of group II elements and is often the intermediate phase closest to the terminal phase. It is, therefore, of interest to characterize diffusion properties of compounds with this tetragonal crystal structure. In the present study, 111In perturbed angular correlation spectroscopy was used to study solute site occupation and atom movement in In4Ba, Al4Ba, Al4Eu, Al4Sr, and Ga4Sr. The indium tracer and its daughter cadmium were found to occupy only the two Al-type sublattices in these compounds through detection of nuclear quadrupole interactions with axially symmetric EFGs. Measurements with increasing temperature revealed merging of signals due to dynamical averaging of these interactions as Cd atoms jumped at increasing rates between alternating sublattices. The jump rates were estimated to be between 8 kHz and 2 MHz at about 350 °C for Al4Eu ...
MRS Proceedings, 1998
ABSTRACTUsing perturbed angular correlation of gamma rays (PAC), nuclear relaxation has been dete... more ABSTRACTUsing perturbed angular correlation of gamma rays (PAC), nuclear relaxation has been detected at high temperature in highly ordered B2 intermetallics that is attributed to stochastic motion of vacancies near 111In/Cd probe atoms. The relaxation is of quadrupole interaction signals due to transition-metal vacancies in the first atomic shells of the probes. Possible relaxation mechanisms are reorientation of the vacancy with respect to the probe, detrapping of the vacancy, or trapping of a second vacancy. The relaxation reaches values of 10 MHz at temperatures of 1500K, 1200 K and 1170 K, respectively, for NiAI, FeAl and PdIn. For NiAl and FeAI, the onset of relaxation is abrupt, suggesting an activation enthalpy for the associated vacancy motion of several electron-volts. For PdIn, the relaxation has an activation enthalpy of 1.00(19) eV, a value significantly lower than might be expected naively on the basis of diffusion data. This difference gives insight into vacancy motio...
We study the non-relativistic limit of Dirac equation for mixed neutrinos. We demonstrate that su... more We study the non-relativistic limit of Dirac equation for mixed neutrinos. We demonstrate that such a procedure inevitably leads to a redefinition of the inertial mass. This happens because, in contrast to the case when mixing is absent, the antiparticle sector contribution cannot be neglected for neutrinos with definite flavor. We then show that, when a gravitational interaction is switched on, in the weak-field approximation the mass parameter which couples to gravity (gravitational mass) does not undergo the same reformulation as the inertial mass, thus leading to an apparent breakdown of the weak equivalence principle.
A thermodynamic model based on the law of mass action is used to calculate concentrations of elem... more A thermodynamic model based on the law of mass action is used to calculate concentrations of elementary point defects and to determine site preferences of solute atoms in ordered alloys. Combinations of lattice vacancies, antisite atoms and host interstitials that form equilibrium defects are enumerated for the CsCl (B2) and Ni2Al3 structures. For CsCl, in addition to the two substitutional sites, a distorted tetrahedral interstitial site is considered. For Ni2Al3, the Ni site, two distinct Al sites and a vacant, insterstitial-type Ni-site are considered. An equation of constraint among concentrations of elementary defects is derived that is valid for any crystal structure. The concentration of a selected defect can be solved using the equation of constraint in conjunction with mass-action equations for defect combinations. The method leads directly to defect concentrations without the need to evaluate composition-dependent chemical potentials, resulting in a more transparent formal...
Measurements were made of jump frequencies of 111In/Cd tracer atoms on the Sn-sublattice in rare-... more 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. 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 bounda...
Diffusional jumps can produce fluctuating electric field gradients at nuclei of jumping atoms. Us... more Diffusional jumps can produce fluctuating electric field gradients at nuclei of jumping atoms. Using perturbed angular correlation of gamma rays (PAC), jumps of probe atoms cause nuclear quadrupole relaxation that can be fitted to obtain the mean jump frequency. An overview is given of the application of this approach to highly ordered intermetallic compounds having the L12 (Cu3Au) crystal structure. New results are then presented for jump frequencies of 111In/Cd probe atoms in pseudo-binary L12 compounds of the forms In3(La1-xPrx) and (In1-xSnx)3La. For the mixed rare-earth system, jump frequencies are found to scale with composition between jump frequencies of the end-member phases In3La and In3Pr. However, for the mixed sp-element system, a large decrease in jump frequency is observed as Sn atoms substitute for In-atoms. This difference in behavior appears to depend on whether atomic disorder is on the diffusion sublattice (In-Sn substitution), as opposed to a neighboring sublatt...
Diffusion of impurity atoms depends on the sublattices occupied, active diffusion mechanisms, and... more Diffusion of impurity atoms depends on the sublattices occupied, active diffusion mechanisms, and jump frequencies to neighboring sites. The method of perturbed angular correlation of gamma rays (PAC) has been applied over the past decade to study impurity diffusion through measurement of nuclear quadrupole interactions (NQI) at nuclei of 111In/Cd probe atoms. Extensive measurements have been made on highly-ordered compounds having the L12 crystal structure, including In3R, Sn3R, Ga3R, Al3R and Pd3R phases (R= rare-earth element). Measurements in thermal equilibrium at high temperature served to determine lattice locations of 111In parent probe-atoms, through characteristic NQIs, and to measure diffusional jump-frequencies of 111Cd daughter probe-atoms, through relaxation of the NQI. This paper summarizes results of the jump-frequency measurements and relates them to the conventional diffusivity. ... A change in diffusion mechanism was proposed in 2009 to explain jump-frequency syst...
Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of ... more 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 ...
MRS Proceedings, 1986
ABSTRACTFollowing nanosecond-duration laser surface-melting, lattice locations of illIn probe ato... more ABSTRACTFollowing nanosecond-duration laser surface-melting, lattice locations of illIn probe atoms in Ni and Pt samples were studied using the technique of perturbed gamma-gamma angular correlations (PAC). After melting and annealing, no probe atoms were observed to become associated with unique vacancy clusters observed after other methods of damaging, while many probe atoms were found on non-unique sites. 111In probe atoms were observed to move to the surface and to be expelled at anomalously low temperatures. These observations are partially reconciled by the hypothesis that probe atoms diffusing in the molten surface layer become trapped on dislocations during resolidification, and during later annealing return to the surface via pipe diffusion.
Hyperfine Interactions, 2011
A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs informa... more A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs information about the fundamental quadrupole frequency to relate the damping as a function of temperature to the EFG fluctuation rate. When the experiment is unable to access slow electric field gradient (EFG) fluctuations that show the fundamental quadrupole frequency directly, one needs additional information to determine the hyperfine field parameters and thereby the connection between observed damping and EFG fluctuation rates. One way to solve this problem is to estimate the hyperfine parameters from the fluctuation rate for maximum damping (i.e. at the relaxation peak) or from the rate of maximum damping. This work relates both the maximum damping rate and the fluctuation rate at the relaxation peak to EFG magnitudes (or quadrupole frequencies) for five dynamic N-state symmetric models of fluctuating EFGs.
Hyperfine …, 2011
A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs informa... more A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs information about the fundamental quadrupole frequency to relate the damping as a function of temperature to the EFG fluctuation rate. When the experiment is unable to access slow electric field gradient (EFG) fluctuations that show the fundamental quadrupole frequency directly, one needs additional information to determine the hyperfine field parameters and thereby the connection between observed damping and EFG fluctuation rates. One way to solve this problem is to estimate the hyperfine parameters from the fluctuation rate for maximum damping (i.e. at the relaxation peak) or from the rate of maximum damping. This work relates both the maximum damping rate and the fluctuation rate at the relaxation peak to EFG magnitudes (or quadrupole frequencies) for five dynamic N-state symmetric models of fluctuating EFGs.
Defect and Diffusion Forum, 2005
The jump frequency of Cd tracer atoms in pure and Al-doped b-Mn was determined from measurements ... more The jump frequency of Cd tracer atoms in pure and Al-doped b-Mn was determined from measurements of quadrupole relaxation using the method of perturbed angular correlation of gamma rays. The jump frequency of Cd in b-Mn containing about 4.5 at.% Al had an activation enthalpy of 0.67(3) eV. In pure b-Mn at 950 K, the jump frequency was a factor about 8 larger than the value of 9.2 MHz in the alloy. Information was also obtained about the sluggish kinetics of the a-Mn to b-Mn polymorphic transformation.
Diffusion Foundations
Following nuclear decay, a daughter atom in a solid will "stay in place" if the recoil ... more 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...
Diffusion Foundations
Using the method of perturbed angular correlation of gamma rays, diffusional jump-frequencies of ... more 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 ...
Hyperfine Interactions
Two types of solute-solute interactions are investigated in this work. Quadrupole interactions ca... more Two types of solute-solute interactions are investigated in this work. Quadrupole interactions caused by nearby Ag-solute atoms were measured at nuclei of 111 In/Cd solute probe atoms in the binary compound GdAl 2 using the method of perturbed angular correlation of gamma rays (PAC). Locations of In-probes and Ag-solutes on both Gd-and Al-sublattices were identified by comparing site fractions in Gd-poor and Gd-rich GdAl 2 (Ag) samples. Interaction enthalpies between solute-atom pairs were determined from temperature dependences of observed site fractions. Repulsive interactions were observed for close-neighbor complexes In/Gd/+Ag/Gd/ and In/Gd/+Ag/Al/ pairs, whereas a slightly attractive interaction was observed for In/Al/+Ag/Al/. Interaction enthalpies were all in the range ±0.15 eV. Temperature dependences of site fractions of In-probes on locally defect-free Gd-and Al-sites yields a transfer enthalpy that was found to be 0.343 eV in a previous study of undoped GdAl 2. The corresponding values in GdAl 2 (Ag) samples are much smaller. This is attributed to competition of In-and Ag-solutes to occupy sites of the same sublattice. While the difference in site-enthalpies of In-solutes on Gd-and Al-sites is temperature independent, it is proposed that the transfer of Ag-solutes from Gd-to Al-sites leads to a large temperature dependence of degeneracies of levels available to In-solutes, resulting in an effective transfer enthalpy that is much smaller than the difference in site-enthalpies.
Hyperfine Interactions, 2017
Lattice locations of 111 In impurity probe atoms in intermetallic GdNi 2 were studied as a functi... more Lattice locations of 111 In impurity probe atoms in intermetallic GdNi 2 were studied as a function of alloy composition and temperature using perturbed angular correlation spectroscopy (PAC). Measurements were made on a pair of samples that were richer and poorer in Gd. Three nuclear quadrupole interaction signals were detected and their equilibrium site fractions were measured up to 700 • C. Two signals have well-defined electric field gradients (EFGs) and are attributed to In-probes on Gd-and Ni-sites in a well-ordered lattice. A third signal exhibiting strong inhomogeneous broadening was observed at low temperature in Gd-richer samples. This is attributed to segregation of the In-probes to phase boundaries (PB) of minor volume fractions of the neighboring GdNi phase. A measurement made on a stoichiometric GdNi sample exhibited the same inhomogeneously broadened signal, indicating that In-probes prefer to occupy PB and/or grain boundary sites in GdNi rather than the well-defined Gd-and Ni-sites. Changes in site fractions were reversible above 300 • C, indicating that probe atoms equilibrate among all lattice locations within a time period of one day. Thus, PB sites provide lower enthalpy environments for In-probes than either crystallographic site in GdNi 2. Enthalpy differences between the levels were determined from measurements of temperature dependences of ratios of site fractions. The enthalpy of transfer of In-probes from the Gd-to Ni-sublattice, which is coupled to intrinsic This article is part of the Topical Collection on
Scripta Metallurgica et Materialia, 1993
INTRQDUCTIQN There is considerable current interest in the use of mechanical attrition in high-en... more INTRQDUCTIQN There is considerable current interest in the use of mechanical attrition in high-energy ball mills to form alloys.(1) Starting from powders of two constituent elements, A and B, several alternative reaction paths are, in principle, possible to form the alloy AB: A may dissolve in the lattice of B, B may dissolve in A, or A and B may undergo a reaction at their interface to form the alloy directly. In the present study, 57Fe M6ssbauer spectroscopy is applied to determine the reaction path for formation of disordered, equiatomic FeCo. The hyperfine field at the nucleus of an 57Fe probe atom depends on the local environment of atoms within the first few near-neighbor shells. The observed distribution of hyperfine fields therefore reflects the superposition of local atomic environments in which the probes find themselves. In the present work, the distribution of hyperfine fields is monitored at stages during milling of elemental powders to trace the evolution of the local environments. From the non-observation of hyperfine fields of dilute Fe probes in Co metal (2) and of Fe atoms with nearby Co solutes in Fe metal,(3) it is concluded that neither constituent element alloys into the other. By default, it is concluded that alloying takes place by an interracial reaction between the constituents. A similar study of formation of Ni3F¢ led to the same conclusion.(4) EXPERIMENT
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Papers by Gary S. Collins