Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations.

We report an ab initio study of the semiconducting Mg(2)X (with X = Si, Ge) compounds and in particular we analyze the formation energies of the different point defects with the aim of understanding the intrinsic doping mechanisms. We find that the formation energy of Mg(2)Ge is 50% larger than that of Mg(2)Si, in agreement with the experimental tendency. From a study of the stability and the electronic properties of the most stable defects, taking into account the growth conditions, we show that the main cause of the n doping in these materials comes from interstitial magnesium defects. Conversely, since other defects acting like acceptors such as Mg vacancies or multivacancies are more stable in Mg(2)Ge than in Mg(2)Si, this explains why Mg(2)Ge can be of n or p type, in contrast to Mg(2)Si. The finding that the most stable defects are different in Mg(2)Si and Mg(2)Ge and depend on the growth conditions is important and must be taken into account in the search for the optimal doping to improve the thermoelectric properties of these materials.

Physical properties of thallium-tellurium based thermoelectric compounds using first-principles simulations.

We present a study of the thermodynamic and physical properties of Tl(5)Te(3), BiTl(9)Te(6), and SbTl(9)Te(6) compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The electronic density of states and band structures are calculated to understand the bonding mechanism in the three compounds. The indirect band gaps of BiTl(9)Te(6) and SbTl(9)Te(6) compounds are found to be equal to 0.256 and 0.374 eV, respectively. The spin-orbit coupling has important effects on the electronic structure of the two semiconducting compounds and should therefore be included for a good numerical description of these materials. The elastic constants of the three compounds have been calculated, and the bulk modulus, shear modulus, and Young's modulus have been determined. The change from ductile to brittle behavior after Sb or Bi alloying is related to the change of the electronic properties. Finally, the Debye temperature and longitudinal, transverse, and average sound velocities have been obtained.