KKR–CPA study of electronic structure and relative stability of Mg2X ([formula omitted] = Si, Ge, Sn) thermoelectrics containing point defects

Abstract

Different types of chemical disorder like vacancies, interstitials and anti-site defects are investigated in binary Mg2X (X=Si, Ge or Sn) and pseudo-ternary Mg2Si1-xSnx thermoelectric materials from the charge self-consistent electronic structure calculations, using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR–CPA). This approach allows to directly compare computational results for possible types of crystal imperfections (treated as random), since the symmetry of the unit cell is kept unchanged in all considered cases. The influence of the aforementioned point defects on electronic structure and relative crystal stability is investigated from computed densities of states (DOS) and formation energy (Eform) analysis, respectively. It is found that the anti-site disorder in stoichiometric Mg2X compounds is highly unfavourable due to constitution of huge and non-bonding impurity peaks at the Fermi level (EF), the conclusion which remains in line with electronegativity and bonding criteria. On the other hand, the small amount of constituent elements (Si or Mg) located in the interstitial void (within ideal stoichiometry) results in strong modification of electronic states on both sides of the energy gap, yielding in principle either p-type (Mg) or n-type (Si) doping. In off-stoichiometric Mg2-xX1-y, the vacancy defects may lead to either p-type (Mg-site) or n-type (X-site) doping, due to EF shifting into the valence or conduction band edge, respectively. However, the total energy analysis indicates that the site preference of such defects’ onset tends to change with increasing atomic number of X. Si vacancy defects energetically favourable in Mg2-xSi1-y (whatever the considered chemical potential limits) are no more valid in Mg2-xSn1-y, where Mg vacancy defects are expected to be preferred. Mg2-xGe1-y exhibits intermediate behaviours.