Session V9 - Advanced Solid State Electronic Cooling and Power Generation I.
FOCUS session, Thursday morning, March 15
Room 201, Washington State Convention Center
Using an ab-initio local density approximation (LDA) method, we have theoretically studied the structural properties, the equations of state, the electronic bandstructures, and the vibrational properties of several tin-based clathrate materials. We find that the pure clathrate frameworks, type-I (Sn_46) and type-II (Sn_136) are metastable, semiconducting, expanded-volume phases of Sn. Further, the binary Cs encapsulated type-I Sn clathrate is metallic if full stoichiometry (Cs_8Sn_46)is assumed. However, the LDA calculations suggest that vacancy formation is energetically favored, and we find that the stable vacancy compound Cs_8Sn_44\Box_2 is semiconducting. The binary vacancy compound is also compared with two ternary clathrate compounds Cs_8Ga_8Sn_38 and Cs_8Zn_4Sn_42, which are potential thermoelectric materials. We have also determined the vibrational modes of these materials, which are important in engineering the thermal conductivity, and have identified the infrared and Raman-active modes. Chemical trends in the structural, electronic, and vibrational properties of these Sn clathrates are discussed, and our theoretical results are compared with experimental data where possible.