Studies of Group-IV Clathrates 

J. H. Ross Group, Department of Physics, Texas A&M University 

Group-IV clathrates are cage-type materials with a crystalline framework of Si, Ge, and Sn encapsulating alkali, alkaline-earth, etc. atoms (larger spheres at right, and in animation above).  Many elements may be substituted for the framework and the cage-center atoms. We have been using NMR, magnetic measurements, transport, and other techniques to better understand these materials.

At right are pictured cages from the type-I structure. Some of our current work involves BaGaSn clathrates, with with Ba ions (large red atoms) enclosed in the cages. We are using NMR and computational studies to analyze the atomic "rattling" or tunneling behavior of the Ba ions, and the resulting anomalous phonon behavior that strongly affects the low-temperature thremal properties.

A portion of the helical cage structure of the chiral-type clathrate is shown at left. Our group is also studying transition-metal and rare-earth substitution in these clathrate materials.

Magnetism in the clathrates offers a number of interesting possibilities. The expanded tetrahedral framework can lead to relatively high-mass charge carriers, which in turn may lead to enhanced superconducting and magnetic behavior. Rare earth substitution in the cages could be used to create Kondo insulator or similar types of behavior. These are some of the interests we have in these systems.

Supported by Robert A. Welch Foundation