Synthesis and transport properties of type II clathrates

We present electrical resistivity and thermal conductivity for low alkali content NaSi/sub 36/, and compare it to the "fully loaded" Cs/sub 8/Na/sub 16/Si/sub 136/. Our results show that the electrical resistivities of the high and low alkali content materials differ greatly. Na/sub 1/Si/sub 136/ possesses a low thermal conductivity, similar in magnitude and temperature dependence to that of the "empty" clathrate Si/sub 136/. Extensive attempts at synthesizing Na/sub x/Ge/sub 136/ did not produce the clathrate phase in high enough yield for transport measurements, but instead an initially unidentified Na-Ge phase was prevalent. In order to clarify confusion in the literature, we propose a structural model for this novel phase, with resulting composition Na/sub 1-x/Ge/sub 3/. The potential of type II clathrates for thermoelectric applications is reviewed.