Synthesis and characterization of fullerene-like Si and Ge clathrate compounds

Si and Ge clathrates can be viewed as covalent fullerene solids. In this paper we describe the synthesis and structural characterization of several metal-containing clathrates. These compounds were synthesized by either thermal decomposition of metal silicides and germanides or direct reaction between the individual constituents. A new Ba-containing type I Ge clathrate was obtained which is isostructural with the superconducting Na2Ba6Si46, with a simple cubic lattice (a = 10.651 Angstrom). The unit cell is composed of face-sharing Ge-20 dodechahedral and Ge-24 tetrakaidecahedral fullerenes which encapsulate the Ba atoms. Detailed analysis shows the compound has significant amount of Ge vacancy. Type II KxBayGe136 clathrate was also observed in a multi-phase sample. Na8Si46 and superconducting K2Ba6Si46 were prepared and studied at 300K and 10K. X-ray diffraction and structural analysis show that although Si atoms are four-fold coordinated, the bends are significantly distorted fi om the sp(3) configuration. No structural transition was observed down to 10K. Experiments were also carried out to explore the possibility of replacing the divalent Ba with other alkaline earth metals.