Maximum Entropy Method Charge Density Distributions of Novel Thermoelectric Clathrates

Abstract

Recently materials with promising thermoelectric properties were discovered among the clathrates. Transport data has indicated that these materials have some of the characteristics of a good thermoelectric, namely a low thermal conductivity and a high electrical conductivity. Based on synchrotron powder and conventional single crystal x-ray diffraction data we have determined the charge density distribution in Sr8Ga16Ge3O using the Maximum Entropy Method. The MEM density shows clear evidence of guest atom rattling, and this contributes to the reduction of the thermal conductivity. Analysis of the charge distribution reveals that Sr8Ga16Ge30 contains mixed valence alkaline earth guest atoms. The Sr atoms in the small cavities are, as expected, doubly positively charged, whereas the Sr atoms in the large cavities appear negatively charged. The MEM density furthermore suggests that the Ga and Ge atoms may not be randomly disordered on the framework sites as found in the conventional leastsquares refinements.

Thermoelectricity of clathrate I Si and Ge phases

“…In contrast, distances between these atoms and Ba (Eu) situated in the large voids are much longer and point towards a weak interaction between host atoms and main framework. These weak bonds are responsible for the rattling mode of the large barium and europium atoms as already anticipated in [17,20,33].…”

“…The important feature of these materials with respect to their thermoelectric application is the cage structure providing a significant reduction of thermal conductivity when filled with big, heavy atoms rattling in the large void [30][31][32][33]. These cages may also be filled by rare-earth atoms particularly with intermediate valence behaviour creating additional possibilities for a decrease of thermal conductivity as well as for a modification of band structure.…”