First-Principles Study of Pressure-Induced Amorphization of Fe(2)SiO(4)Fayalite

Fayalite (Fe2SiO4), which is an end member of the olivine series ((FexMg1 - x)(2)SiO4), 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.