Session S15 - Focus Session: Theory of Nanotubes and Carbon Based Nanostructures I.
FOCUS session, Wednesday afternoon, March 24
511D, Palais des Congres
In addition to their diamond structure, the Group IV atoms can form novel solids called clathrates. In the clathrates, as in the diamond structure, all Group IV atoms are tetrahedrally coordinated and sp^3 bonded with their neighbors. However, in contrast to the diamond structure, the clathrates contain pentagonal rings and their lattices are open frameworks containing large ``cages''. These cages can house ``guest'' atoms, and the choice of guest may be used to tune the material properties. There have been recent advancements in the synthesis and characterization of Si-, Ge-, and Sn-based clathrates and many novel properties have been found. However, the synthesis of C-based clathrates remains unachieved. We report the results of a theoretical investigation of some C-based clathrates, including the pure Types I and II materials C_46 and C_136 as well as C-clathrates containing Li guests. Our study uses an LDA electronic structure method, pseudopotentials, and planewave basis sets. We present results for the structural, electronic and vibrational properties of these materials. We have also investigated the high pressure synthesis of Li guest-containing C-clathrates, starting with Li intercalated graphite, and we predict the critical transition pressure. The possible synthesis of mixed Si-C clathrates will also be discussed.