Consideration about the synthesis pressure effect on lattice defects of Mg2Si using 1st principle calculations

In order to understand the observed decrease in both of the absolute value of the Seebeck coefficient and the electrical conductivity of n-type Mg2Si synthesized at high pressure, we have studied pressure dependence of several kinds of lattice defect formation enthalpies of Mg2Si and also calculated their densities of states and band structures. Under the Mg-rich condition, the Mg atoms which occupy the interstitial sites of Mg2Si lattice, IMg, is the most dominant defect which causes the n-type conductivity, and this defect seems to be energetically more favorable in the higher pressure. On the contrary, divacancy of MgSi, VMgSi, which is electrically neutral, and Si substitution of the Mg site, whose contribution to the Seebeck coefficient would not be so large, are energetically favorable under the Si-rich conditions, though IMg is still probable. Increase in pressure is favorable for stabilizing of IMg and unfavorable for that of VMgSi and trivacancy (VMg2Si), the latter of which can be the possible origin of the p-type conductivity, though the probability that three vacancies (two VMg's and one VSi) associate would be small enough so as to cancel the effect of not-so-large defect formation enthalpy. The observed decrease in the n-type defects in that material synthesized under higher pressure would probably be caused by lowering of the synthesis temperature, which will be favorable for relaxation of the n-type defects.