Synthesis and single-crystal X-ray diffraction studies of new framework substituted type II clathrates, Cs8Na16AgxGe136−x (x<7)

New inorganic type II clathrates with Ag atoms substituting for framework Ge atoms, Cs₈Na₁₆Ag_xGe_136−x (x=0, 5.9, and 6.7), have been synthesized by reaction of the pure elements at high temperature. Structural refinements have been performed using single crystal X-ray diffraction. The materials crystallize with the cubic type II clathrate crystal structure (space group ) with a=15.49262(9) Å, 15.51605(6) Å, and 15.51618(9) for x=0, 5.9, and 6.7, respectively, and Z=1. The structure is formed by a covalently bonded Ag-Ge framework, in which the Cs and Na atoms are found inside two types of polyhedral cages. Ag substitutes for Ge in the tetrahedrally bonded framework positions, and was found to preferentially occupy the most asymmetric 96g site. The proven ability to substitute atoms for the germanium framework should offer a route to the synthesis of new compositions of type II clathrates, materials that are of interest for potential thermoelectrics applications.