What we do

We identify local environments of radioactive probe atoms in solids (lattice locations or neighboring point defects) by detecting nuclear quadrupole interactions using hyperfine interactions methods,, especially perturbed angular correlation of gamma rays (PAC). Quadrupole interaction arises from the interaction between the nuclear quadrupole moment of the probe nucleus and the electric field gradient (EFG) produced by nearby charges. Each site is characterized by a quadrupole interaction frequency and EFG asymmetry parameter that gives information about local point symmetry. Once environments have been identified, fractions of probes on different sites can be monitored, for example, to measure defect concentrations or to observe how site preferences change as a function of temperature or composition. Relaxation of the quadrupole interaction caused by jumping of nearby defects, or of the probe atoms themselves, is exhibited as damping of spin rotation patterns and can be fitted to determine jump frequencies precisely. Temperature dependences of site fractions and jump frequencies lead to activation enthalpies and entropies for defect formation, for binding of defects with impurity probes, and for probe and defect migration. Over many years, our work has been associated especially with point defects in intermetallic compounds such as NiAl, a high-temperature structural material.

A special feature of PAC measurements is that the mole fraction of probe or tracer atoms can be extremely dilute, say 10 parts per billion. For detailed information about these and other studies, see recent publications for abstracts and downloads. To consult about how our methods might address your materials problems with intermetallics, semiconductors or insulators, please do not hesitate to contact Professor Gary Collins at collins-at-wsu.edu.

Current interests:

Diffusion of tracer atoms studied via nuclear quadrupole relaxation in PAC spectra. This method, recently pioneered by us, is being used to measure jump frequencies and jump-frequency activation enthalpies and shed light on atomistic diffusion mechanisms. PAC even allows one to measure jump frequencies of probe atoms on inequivalent sublattices of a compound simultaneously, an impossibility in standard diffusivity measurements.

Site preferences of impurity atoms in intermetallic compounds. Impurity atoms have been found by us to "switch" from one lattice site to another in response to a change in composition and/or temperature. Such switching has been observed to occur over very narrow compositional ranges for the very dilute probe concentrations of these measurements. Analysis of the temperature dependence of site fractions of a solute has been used to measure an "enthalpy of transfer" between sites in a compound, to our knowledge for the first time.

Past interests: (An extended description of research interests written in 2001 is available here)

Segregation of dilute probe atoms between phases in a two-phase mixture.
Defects in intermetallics: identification of constitutional and thermal equilibrium defects and properties ( formation, migration and binding enthalpies).
Geometric structures of complexes of impurities with vacancies in intermetallics and pure metals.
Interactions among vacancies.
Onset of vacancy motion and the brittle-to-ductile transition in intermetallics.
Hydrogen-vacancy interactions in metals (hydrogen atoms diffusing interstitially trap in vacancies).
Defect production during plastic deformation or mechanical milling.
Disordering and phase transformations caused by plastic deformation of intermetallics.
Sites of dilute probe atoms diffused into grain boundaries at low temperature.
Shape-memory-effect alloys.
Laser-surface-melted metals.
Hyperfine-field-shifts in disordered magnetic alloys.
Magnetic critical phenomena in metals and alloys: static exponent beta and dynamic exponent z.
Probe dependence of magnetic hyperfine fields and electric-field-gradients.
Temperature dependence of quadrupole interaction in noncubic metals.

Facilities and capabilities:

Measurement and sample preparation laboratories: ~200 m² area.
PAC: two four-counter spectrometers employing barium fluoride scintillators and fast Hamamatsu photomultiplier tubes, with measurements possible down to 20K using a closed-cycle He refrigerator and up to 1500 K using ovens heated resistively and using electron beams.
Mössbauer effect: Ranger MS-1200 spectrometer with laser interferometer, capable of measurements between 30-400K using a closed-cycle Janis He refrigerator.
Positron annihilation: lifetime spectrometer using thin BaF₂ scintillators and very fast Hamamatsu photomultiplier tubes.
Sample preparation lab: three-zone 1200 C tube-furnace for annealing and diffusing radioisotopes; vertical-drop tube-furnace for ultra-rapid quenching from up to 1500 C; miniature arc-melters for preparing radioactive samples, with one including a piston-anvil quencher; two Spex 8000 high-impact-energy vibrator-mills for mechanical milling and alloying.
Computation: DEC 3000 alpha workstation, numerous Wintel PC's, Linux two-processor workstation, PowerMac web server. Extensive routines for fitting PAC and ME spectra have been written by Collins. The widely-used WIEN'97 code for first-principles computation of atom arrangements and electric field gradients in solids is also available.
Other facilities on campus: High-field solid-state NMR facility with magic-angle spinning capability, across the street. One megawatt Triga reactor with thermal neutron flux of 10¹³ cm^-2 s^-1, Siemens x-ray diffractometer, TEM, SEM, XPS, AES, ESR, Luminescence...
Off campus: X-ray absorption measurements in collaboration with the Pacitic Northwest Consortium Collaborative Access Team on the beam-line ID-20 at the Advanced Photon Source at Argonne National Laboratory.

Support:

Research of Professor Collins's group has been supported by Washington State University and by the National Science Foundation (Metals Program) under grants DMR 81-08307, 86-19688, 90-14163, 93-13702, 96-12306, 00-91681, and 05-04843. Starting in December 2005, group research is also supported by the Praveen Sinha Fund for Physics Research through a generous donation from Praveen Sinha, a Ph.D. recipient of the group in 1995.

January 2007 Gary S. Collins, Hyperfine Interactions Group, Washington State University. Back to the group's home page.