Structural stability of Fe5 Si3 and Ni2 Si studied by high-pressure x-ray diffraction and ab initio total-energy calculations

Abstract

We performed high-pressure angle dispersive x-ray diffraction measurements on Fe5 Si3 and Ni2 Si 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, Fe5 Si3 was found to compress isotropically, whereas an anisotropic compression was observed in Ni2 Si. The linear incompressibility of Ni2 Si 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 Ni2 Si along the c axis. The observed anisotropic compression of Ni2 Si is also related to the layered structure of Ni2 Si, 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 Fe5 Si3, the calculations also predicted a phase transition at 283 GPa from the hexagonal P 63/mcm phase to the cubic structure adopted by Fe and Si in the garnet Fe5 Si3 O12. The room-temperature equations of state for Fe5 Si3 and Ni2 Si 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 additional descriptions of these structures, in particular, of the predicted high-pressure phase of Fe5 Si3 (the cation subarray in the garnet Fe5 Si3 O12), which can be derived from spinel Fe2 Si O4 (Fe6 Si3 O12). © 2008 The American Physical Society.