First-Principles Study of Pressure-Induced Amorphization of Fe₂SiO₄ Fayalite

Fayalite (Fe₂SiO₄), which is an end member of the olivine series ((Fe_xMg_{1 − x})₂SiO₄), undergoes a crystal-to-amorphous transformation under high pressure at room temperature conditions. This pressure-induced amorphized fayalite has an interesting feature: it exhibits antiferromagnetism at low temperature regardless of its non-crystalline structure. In spite of this unique property, the first-principles investigation of pressure-induced amorphized fayalite has not been carried out yet. Herein, to clarify the energetic and structural properties of pressure-induced amorphized fayalite, the first-principles molecular dynamics simulations of the compression and decompression processes of fayalite in the pressure range 0–120 GPa are performed. The energetic and structural properties are also compared with those of well-equilibrated melt-quenched amorphous Fe₂SiO₄. Based on structural analysis, it is confirmed that not only sixfold but also fivefold coordinated silicon atoms exist in the amorphous-like structure under high pressure. In addition, it is found that the silicon atoms play the role of network former in the amorphous-like phase under high pressure, but change to a network-modifier role after release to ambient conditions. Moreover, it is found that the obtained amorphous-like phase has a partially ordered structure. It is inferred that the partially ordered structure likely enables the pressure-amorphized fayalite to exhibit antiferromagnetism.