Transport and optical properties of the Type II clathrates Cs8Na16Si136 and Si136

In recent years there has been a thrust in interest on novel open structured materials. Many of these materials are characterized by their ability to host guest atoms in interstitial "voids," giving rise to a range of interesting physical properties. In particular, compounds with crystal structures isomorphous to the type I and II clathrate hydrates are being studied due to their potential for use as thermoelectric materials. Of the two polymorphs the type II clathrates have been investigated less extensively, both theoretically and experimentally. In this paper we present an overview of the temperature dependent electrical and thermal transport properties as well as Raman scattering; spectra for the type II clathrates Cs8Na16Si136 and Si-136. Prospects for applications and future directions are also discussed.