Session G25 - Focus Session: Novel Thermoelectric Materials and Phenomena I.
FOCUS session, Tuesday morning, March 04
Room 16B, Austin Convention Center
We have studied the electronic and vibrational properties of some of the Type-I and Type-II Si, Ge, and Sn-based semiconductor clathrates using LDA electronic structure methods. The lattices in these framework materials have open cages which can contain guest impurities, and these guests can produce local (``rattling'') vibrational modes. These modes may scatter the extended, heat carrying acoustic modes of the framework, potentially reducing the thermal conductivity. We present results for the vibrational spectra of the Type I clathrate compound Cs_8Ga_8Sn_38 and of the Type II clathrate compounds Na_16Cs_8Si_136, Na_16Cs_8Ge_136, Cs_8Sn_136, and Na_16Cs_8Sn_136. We have also theoretically identified the infrared- and Raman-active modes in these materials and have computed their Raman spectra. For the Type II materials, we discuss the trends in the vibrational frequencies of the guest modes as the host material changes from Si to Ge to Sn. Finally, we compare the experimental Raman spectra of Cs_8Ga_8Sn_38, Na_16Cs_8Si_136, and Na_16Cs_8Sn_136 with the theoretical spectra. By this means, we are able to unambiguously identify the low frequency, Cs and Na guest-related ``rattling'' vibrational modes in these materials.