|Description||Charles W. Myles, Ph.D.|
Professor, Department of Physics
Texas Tech University
Novel, Open Framework Crystalline Materials Based on the Group IV Elements
The Group IV elements Si, Ge, and Sn can crystallize in the well-known diamond lattice structure, which is the ground state phase for each. It is less well-known that these elements can also form novel crystalline solids, called clathrates because of structural similarities to clathrate hydrates. Group IV clathrates are metastable, expanded volume phases. As in the diamond structure, in the clathrates, the atoms are tetrahedrally coordinated in sp3 covalent bonding configurations with their near-neighbors. In contrast to the diamond lattice, however, the
clathrates contain pentagonal rings of atoms and their lattices are open frameworks containing large (20-, 24-,28-atom) “cages”. The two common clathrate varieties are Type I, a simple cubic lattice with 46 atoms per unit cell and Type II, a face centered cubic lattice with 34 atoms per unit cell. The cages can contain weakly bound
impurities (“guests”), usually Group I or Group II atoms. A reason that the clathrates are interesting is that the choice of guest may be used to tune the material properties. The guests act as electron donors, but because of their weak bonding, they have only small effects on the host electronic structures. However, they can vibrate
with low frequency vibrational modes, which can strongly affect the vibrational properties. Some laboratory-synthesized, guest-containing clathrates show great promise for thermoelectric applications precisely because the guests only weakly perturb the electronic properties, while strongly affecting the vibrational properties.
In this talk, the clathrates and their lattices will be introduced. The results of calculations of the properties of some Si, Ge and Sn-based Type I and Type II clathrates will then be presented. Where data is available, some
results will be compared with experiments. Our calculations are motivated by experiments performed by the G. Nolas group at the U. of South Florida. The calculations used a density-functional based, planewave, pseudopotential method. The results include equations of state, structural parameters, electronic bands,
vibrational spectra, mean-square atomic displacements, and thermodynamic properties. Some recent results, obtained in collaboration with some of my present and former students will be discussed.
For more information, please contact: Dr. Jay Dittmann 254-710-2275