Welcome! We study the local structure of
ordered solids through measurements of
nuclear hyperfine
interactions. Interactions between
nuclear quadrupole moments and electric field gradients (EFG) "flag"
local environments of radioactive probe atoms, including possible
neighboring point defects. Signal amplitudes give site
fractions of probes on the various sites, with and without nearby
defects, from which can be
determined defect
concentrations and thermodynamic properties such as formation and
migration enthalpies of defects and
interaction enthalpies with solute atoms. Atomic motion at
jump frequencies in the the MHz to GHz range gives rise to measurable
nuclear
relaxation. We mostly
study intermetallic
compounds but the methods are applicable to all crystalline
solids. We especially apply
the spectroscopy perturbed
angular correlation of gamma rays (PAC), and are one of few
PAC groups actively studying solids in North America. Current
interests:
Lattice locations and
energetics of
solutes in compounds. We recently discovered systems
in which dilute solutes "switch" from one site to another in response
to changes in
temperature or composition. See our studies of
indium solutes
in the Laves
phase GdAl2 and Ni2Al3 phases in
the"papers" pages. More recently, we are measuring enthalpy and
entropy differences of solutes on different sites in compounds through
temperature dependences of site fractions. When two sites are of
the same element, the measurements yield directly the enthalpy
difference of the solute atom in the two sites with a precision of
order 0.01 eV, as shown in our study of indium solutes in compounds
having the Al3Ti and Al3Zr crystal structures.
Diffusion of probe
atoms: Recently, we showed that jump
frequencies of PAC probe atoms can be measured through
relaxation
of
the nuclear
quadrupolar interaction. Such relaxation
occurs when probe atoms jump among lattice sites having different
orientations or magnitudes
of electric field gradients. Jump frequencies in the MHz to GHz
range can be determined by fitting observed nuclear
relaxation. Temperature
dependences have given activation enthalpies as or more precise than
those
obtained via the classic method of measuring diffusion concentration
profiles--and with much
less
effort. See our papers on
jump frequencies of Cd-impurities in rare-earth indides and other
intermetallics.
Click
below to learn more about us or PAC and to
download
papers. Contact
Collins for more information, to participate in our work,
or to explore collaboration in a project.
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