A relatively new area
of interest for me
is exploring the basic microscopic
physics properties (electronic, structural,
vibrational) of exotic ("new")
materials, such as the clathrate semiconductors, which have potential
applications as thermoelectrics. As is discussed below, the CLATHRATES,
which have open-framework,
cage-like lattice structures, are extremely
interesting from the fundamental physics viewpoint.
I started working on clathrate problems
while I was on sabbatical at Arizona
State U. in the Spring of 2000. There, I began
collaborating with Otto
Sankey & his then post-doc Jianjun
(JJ) Dong (now at Auburn U.). In addition to our theory collaboration,
we also
collaborate with George
Nolas
(U.
of South
Florida), Chris
Kendziora
(US
Naval ResearchLab),
and Jan
Gryko (Jacksonville State U.).
They are experimenters who
do clathrate materials synthesis & characterization.
In addition to their ground state, diamond crystal structure, the Group
IV elements Si, Ge, &
Sn can
form novel crystalline solids, which are
called CLATHRATES because of structural similarities to the clathrate
hydrates. These materials are metastable, expanded volume phases. As in
the diamond structure, in the clathrates, all Group IV atoms
are
tetrahedrally coordinated in sp3
covalent bonding configurations with
their neighbors. However, in contrast to the diamond structure, the
clathrates contain pentagonal rings and their lattices are open
frameworks containing large (20-, 24-,
and 28-atom)
"cages". The
figures shown here schematically show the cubic unit cells of the Type
I (46 atoms per cell) & the Type II (136
atoms per cell) clathrate
lattices.
Type IType II
The cages can contain
loosely bound impurity atoms or "guests",
usually fromColumn IorIIof the periodic table, and the
choice of guest may be
used to tune the properties of the material. Because of their weak
bonding, these guests have minimal effects on the material electronic
properties. However, they can produce low frequency vibrational
("rattling")
modes which can strongly affect the vibrational
properties. Some guest-containing clathrates have been shown to be
excellent candidates for thermoelectric applications precisely because
the guests only weakly perturb the electronic transport properties,
while strongly affecting the vibrational (heat
transport) properties.
Our calculations have utilized the Vienna Austria ab-initio Simulation Package(VASP).
This is based on the local density approximation (LDA) to density
functional theory. It uses a planewave basis with ultrasoft
pseudopotentials along with the Ceperley-Alder
functional to
approximate the exchange-correlation energy. This method has been
extensively tested on a wide variety of systems. The implementation we
use is particularly efficient for the large clathrate unit cells.
Earlier, Otto
Sankey & JJ
Dong used this method
to calculate some of the properites of Si & Ge
clathrates.
Our collaborative effort has focussed primarily on the Sn
clathrates,
but we have also looked at the Type IISi
& Ge materials. More
recently, I've looked at the (so far hypothetical)
Cclathrates. We've
calculated equations of state, structural parameters, electronic
bandstructures, and vibrational spectra and have compared the results
with experiment where possible.
The results for the Sn clathrates
are qualitatively similar to those
for all clathrates. Some representative results for these materials
are Here. Some results for the C clathrates
may be posted
later.
This has been a very
productive
collaboration! Recent
publications which have resulted from this work are
#4,5,6,8,10,11
& 19 on
the "Recent
Publications"
link below. This effort has also resulted inINVITED
talks
& several conference presentations. My 2002 presentation ("Study
of Rattling Atoms in Type I and
Type II Clathrate Semiconductors") at the Motorola
Workshop on Computational
Materials & Electronics (Tempe, AZ)
is Here (Power Point, 1.79 MB). My 2003 presentation
("Theoretical
Investigation of Carbon-Based
Clathrate
Materials") at the same workshop (Austin,
TX) is Here
(Power Point, 2.69 MB). Our presentation at the 2003 March National
American Physical Society Meeting (Austin, TX) is Here (Power Point,
1.21 MB). See also
#4,6,8-10,12-15,18,20,21,27 & 29 on the
"Recent
Conference Presentations" link below.
Abstract
for a recent colloquium on clathrates (Word).
That colloquium is Here
(Power Point, 5.21 MB).