Anharmonic properties inMg2X(X=C,Si

Abstract

Thermal conductivity reduction is one of the potential routes to improve the performance of thermoelectric materials. However detailed understanding of the thermal transport of many promising materials is still missing. In this work we employ electronic-structure calculations at the level of density functional theory to elucidate thermal transport properties of the Mg 2 X (X=C, Si, Ge, Sn and Pb) family of compounds, which includes Mg 2 Si, a material already identified as a potential thermoelectric. All these materials crystallize into the same antifluorite structure. Systematic trends in the anharmonic properties of these materials are presented and examined. Our calculations indicate that the reduction in the group velocity is the main driver of the thermal conductivity trend in these materials, as the phonon lifetimes in these compounds are very similar. We also examine the limits of the applicability of perturbation theory to study the effect of point defects on thermal transport, and find that it is in good agreement with experiment in wide range of scattering parameter value. The thermal conductivity of the recently synthesized Mg 2 C is computed for the first time and predicted to be 34 W/mK at 300 o C.2

Thermoelectric and vibrational properties of Be2C, BeMgC and Mg2C using first-principles method

“…In Mg 2 C, the anti-symmetric mode shows LO-TO splitting of 214.28 cm À1 . This is more than the 153 cm À1 obtained from the PAW method using LDA 43 and 160 cm À1 obtained from the LDA-PP method. 64 It may be noticed that absolute TO frequencies from LDA-PP method are in very good agreement while LO frequencies differ by 48 cm À1 leading to a less LO-TO split.…”

“…However, in the case of Mg 2 C thermal conductivity $34 W mK À1 is reported at 300 K using plane wave pseudopotential (PP) method. 43,44 The comparison with the optimum value obtained in the current calculations give s ¼ 4.4 Â 10 À15 s. 43 It alters the relaxation time by a factor of $10 À1 . Therefore we calculated optimum values of PF and k e also at s ¼ 4.4 Â 10 À15 s, and give in Table 1.…”

“…The published calculations reveal that indirect (direct) band gap lies in the 0.67-0.97 eV (1.34-1.9 eV) range. 28,40,43 So both PBE and PBE0 predict band gaps beyond the limits proposed by earlier workers. The LDA used by earlier workers may be the reason for this discrepancy.…”

“…64 Calculations using LDA in the PAW method found these modes at 406.26 and 559.44 cm À1 . 43 So each calculation, The vibrational frequencies and the active modes change with atoms and symmetry. The optical modes are sensitive to the inter-cellular interaction and therefore generalized trend (u L ) 2 J (u T ), 2 is usually seen in the vibration spectra of the three materials.…”

“…41,42 The thermal conductivity and band gap are proposed without specifying the carrier density or the chemical potential which is mandatory for practical purpose. 43,44 Thus there are minimal efforts on the TE studies of carbides. So the second objective is to deploy the interface between periodic LCAO method and the BoltzTraP to nd the thermoelectric coefficients of the Be 2 C, Mg 2 C and the BeMgC.…”