First-principles study of pressure-induced phase transformations in thermoelectric Mg2Si

Abstract

The phase transitions of the Magnesium Silicide (Mg2Si) are mainly studied by density functional theory. It generally has three crystal structures: anti-fluorite structure, anti-cotunnite structure and Ni2In-type structure. The performances of three kinds of Mg2Si under different pressures are studied, as well as their properties. We find that phase transitions can take place for three kinds of Mg2Si under different pressures. The processes of the phase transitions about Mg2Si are analyzed, which can be summarized as: the differences of the atomic movements on different crystal faces. For anti-fluorite Mg2Si, the atoms on the (110) shrink a little and move up and down some distances which archive the phase transitions to anti-cotunnite structure Mg2Si. For the phase transition of anti-cotunnite structure to Ni2In type structure, the key point should be the atomic movements on the (010). These findings should be helpful for understanding the sequence of phase transitions of many anti-fluorite structure materials: anti-fluorite structure → anti-cotunnite structure → Ni2In type structure.