Forthcoming congratulations for students Randy Newhouse, PhD, and Qiaoming Wang, MS.
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. EFGs depend on the
crystal structure, lattice location and (possible) neighboring
point defects. Signal amplitudes give site
fractions of probes, from which can be
determined defect
concentrations and thermodynamic properties, including site
enthalpies of probe atoms, formation and
migration enthalpies of defects, and
interaction enthalpies with solute atoms. Diffusion of atoms
at
jump frequencies in the MHz to GHz range gives rise to detectable
nuclear
relaxation. We mostly
study intermetallic
compounds but our methods are applicable to all
classes of solids. We particularly apply perturbed
angular correlation of gamma rays (PAC), and are currently the principal PAC
group in North America studying solids.
Current interests:
Lattice locations and
energies of
solutes in compounds. Solute atoms "switch"
from one site to another in response to changes in
temperature or composition, as shown by our studies of
indium solutes
in the Laves GdAl2
phase here
and in Ni2Al3
phases here.
More recently, we measured enthalpy and
entropy differences of solutes on inequivalent
sites of an
element, with the
measurements
giving the enthalpy
difference of a solute atom on two sites to a precision of 0.01 eV,
as shown in the
study of indium solutes on inequivalent
Al-sites in compounds
having the Al3Ti
and Al3Zr
crystal structures. These are realizations of
two- and
three-level systems of solutes.
Diffusion of probe
atoms: In a 2004 letter,
we showed how jump
frequencies of PAC probe atoms can be measured through
nuclear
quadrupolar relaxation. Such relaxation
occurs when probe atoms jump among lattice sites having different
orientations or magnitudes
of electric field gradients, and can be fitted to determine diffusional
jump frequencies. Temperature
dependences yield activation enthalpies as
precise as diffusivities measured by the classic sectioning
technique--but
with much
less
effort.
Insight into diffusion
mechanisms in compounds:
In a 2009
letter, we showed that, in a binary AB compound, one can
identify whether A- or B-vacancies are principally responsible
for diffusion of an impurity probe atom. In the same paper,
we discovered a remarkable change in diffusion mechanism along a
series of compounds, from one in which A-vacancies dominate to one in
which B-vacancies dominate.
We also have informed a few experimental results
with
full-potential ab initio
electronic structure calculations using the program WIEN2k . Click
below to learn more about PAC and download
papers. Contact me for more information, to join our
work,
or explore a possible collaboration.
visitors
over a one year period.
