Clathrate Semiconductor Research
Sn (Tin) Clathrate Research Page

      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 Research Lab), 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 I                                                              Type II
TypeI                                  TypeII
      The cages can contain loosely bound impurity atoms or "guests", usually from Column I or II of 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.
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 II Si & Ge materials. More recently, I've looked at the (so far hypothetical) C clathrates. 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 in INVITED 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).

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