Structural stability of Fe5Si3 and Ni2Si studied by high-pressure x-ray diffraction and ab initio total-energy calculations

TL;DR

This study investigates the high-pressure structural stability of Fe5Si3 and Ni2Si using x-ray diffraction and ab initio calculations, revealing isotropic and anisotropic compression behaviors, and predicting a phase transition in Fe5Si3 at high pressure.

Contribution

The paper combines experimental high-pressure x-ray diffraction with ab initio calculations to analyze the structural behavior and phase transitions of Fe5Si3 and Ni2Si, providing new insights into their high-pressure phases.

Findings

Fe5Si3 compresses isotropically up to 75 GPa.

Ni2Si exhibits anisotropic compression with a high c-axis incompressibility.

Predicted phase transition of Fe5Si3 at 283 GPa to a garnet-like structure.

Abstract

We performed high-pressure angle dispersive x-ray diffraction measurements on Fe5Si3 and Ni2Si up to 75 GPa. Both materials were synthesized in bulk quantities via a solid-state reaction. In the pressure range covered by the experiments, no evidence of the occurrence of phase transitions was observed. On top of that, Fe5Si3 was found to compress isotropically, whereas an anisotropic compression was observed in Ni2Si. The linear incompressibility of Ni2Si along the c-axis is similar in magnitude to the linear incompressibility of diamond. This fact is related to the higher valence-electron charge density of Ni2Si along the c-axis. The observed anisotropic compression of Ni2Si is also related to the layered structure of Ni2Si where hexagonal layers of Ni2+ cations alternate with graphite-like layers formed by (NiSi)2- entities. The experimental results are supported by ab initio total-energy calculations carried out using density functional theory and the pseudopotential method. For Fe5Si3, the calculations also predicted a phase transition at 283 GPa from the hexagonal P63/mcm phase to the cubic structure adopted by Fe and Si in the garnet Fe5Si3O12. The room-temperature equations of state for Fe5Si3 and Ni2Si are also reported and a possible correlation between the bulk modulus of iron silicides and the coordination number of their minority element is discussed. Finally, we report novel descriptions of these structures, in particular of the predicted high-pressure phase of Fe5Si3 (the cation subarray in the garnet Fe5Si3O12), which can be derived from spinel Fe2SiO4 (Fe6Si3O12).