Pressure-stabilized binary compounds of magnesium and silicon

Abstract

The family of binary compounds composed of magnesium and silicon is rather rich. In addition to the well-known magnesium silicide Mg$_2$Si, other compounds, including MgSi$_2$, Mg$_4$Si$_7$, Mg$_5$Si$_6$, MgSi, and Mg$_9$Si$_5$, have also been identified and/or proposed in precipitated Al-Mg-Si solid solutions. Nevertheless, computational studies show that only Mg$_2$Si is thermodynamically stable at ambient conditions while certain non-zero hydrostatic pressure can stabilize Mg$_9$Si$_5$ so that it can co-exist with Mg$_2$Si. We conduct a comprehensive search for viable binary compounds of Mg$_x$Si$_{1-x}$ ($1/3\leq x \leq 2/3$), discovering numerous new structures for all the compounds. On the one hand, we find that MgSi$_2$, MgSi, and Mg$_9$Si$_5$ are likely pressure-stabilized materials, while, on the other hand, supporting previous studies, raising doubt on the existence of Mg$_5$Si$_6$, and claiming that the existence of Mg$_4$Si$_7$ remains an open question. Therefore, we recommend that (hydrostatic and/or non-hydrostatic) pressure should be explicitly considered when discussing the stability of these solids (and maybe other solids as well) by computations. We also find that MgSi$_2$ can potentially exhibit superconducting behaviors within a wide range of pressure with the critical temperature of up to $7$ K.