Vacancies in magnesium silicide - stoichiometric vacancies preferred?

Magnesium silicide (Mg2Si) in its calcium fluorite (CaF2) structure is a hardly investigated material up to now. However, it may play an important role ill upcoming new materials for photovoltaic applications due to its semiconducting properties and the fact that it is consisting of non-poisonous elements. On the other hand metastable phases Of Mg2Si play an extremely important role in the hardening of 6xxx (AlMgSi-based) aluminum alloys. Hence, it seems worth investigating the formation energy of vacancies in Mg2Si. In this paper we calculate the formation [GRAPHICS] Mg2Si in CaF2-structure energy for vacancies on the different sublattices using the ab-initio method SIESTA. While the values for the silicon (1.04 eV) and the magnesium vacancy (1.74 eV) lie in the expected range, the formation energy of the di-vacancy and the stoichiometic tri-vacancy turn out to be just 0.68 eV and 0.78 eV, respectively. This is smaller than the value for both mono-vacancies. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim