Thermoelectric materials allow for a more sustainable and energy efficient future by converting a temperature gradient into electricity, and vice versa.  Clathrates, which have a crystal structure that can trap atoms within a cage of other atoms, are likely to be good thermoelectrics since the surrounding atomic cage allows electrons to move freely while the trapped guest atoms scatter phonons.  The mobility of electrons in combination with the scattering of phonons permit the ideal properties of thermoelectric materials: excellent electrical conductivity and poor thermal conductivity.  We examined type -VIII BaGaSn clathrates, in which the gallium and tin atoms form a cage around the barium species, from a computational perspsective.  The effects of the distribution of gallium and tin atoms on the thermoelectric properties were investigated.  Structures with varying numbers of Ga-Ga bonds were generated and analyzed using first-principles methods.  We used the Vienna Ab Initio Simulation Package (VASP) [1-4] to relax and model the electronic properties of the BaGaSn clathrates.  The VASP results were then used in calculating transport coefficients using the BoltzTrap program [5].  Results show that structures with the fewest Ga-Ga bonds are the most stable and have greater charge mobility.  Structures with no Ga-Ga bonds have the highest Seebeck coefficients and powerfactors, which are measures of efficiency for thermoelectrics.  Defects in the structures were introduced and are shown to decrease the powerfactor of BaGaSn clathrates.  More intensive calculations must be done to model the thermal conductivities of the clathrates in order to fully determine the efficiency of BaGaSn clathrates as a thermoelectric material.    

References

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2. G. Kresse and J. Hafner. Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B, 49:14251, 1994.

3. G. Kresse and J. Furthmüller. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mat. Sci., 6:15, 1996.

4. G. Kresse and J. Furthmüller. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B, 54:11169, 1996.

5. Madsen, G. K. H.*; Singh, D. J. Comput. Phys. Commun. 2006 175, 67