Synthesis and characterization of framework-substituted Cs8Na16Cu5Ge131

We report the synthesis and temperature-dependent transport properties of a new type II germanium clathrate, Cs8Na16Cu5Ge131 (space group Fd3¯m; a = 15.42000(9) Å). In comparison to the parent compound Cs8Na16Ge136, Cu substitution for Ge on the clathrate framework was found to increase in magnitude the electrical resistivity and Seebeck coefficient, while reducing the thermal conductivity. The results are consistent with partial charge compensation due to Cu substitution for Ge. In addition to presenting possibilities for new compositions, framework substitution in type II clathrates may offer a route to control the transport properties of these materials, and could offer potential candidates for thermoelectric applications.