Exploration of Properties from Both the Bulk and Surface of Iron Silicides: A Unified Theoretical Study

Abstract

The properties from both bulk phases (magnetism and thermal stability) and surface (composition, orientation, and stability) of iron silicide model compounds were evaluated by a theoretical study with a combination of density functional theory, ab initio atomistic thermodynamics, and Wulff construction. Magnetism, one of the properties originated from bulk phases, has been chosen as an example to demonstrate how to correlate it with the local chemical environment to provide an in-depth understanding. We found that increasing the number of coordinative Si atoms adjacent to an Fe center would cause a decrease in magnetic moment. To reveal surface structural features relevant to surface properties of iron silicides, we evaluated the influence of Si chemical potential (μSi) on surface thermal stability over a range of temperature (T) and partial pressure of SiH4 (P SiH4 ). It turns out that the thermal stability of the Si-rich surfaces has been improved with increasing μSi under high T and P SiH4 . We then acquired the surface free energy of all possibly exposed facets, built up reasonable crystalline models via Wulff construction, and thereby systematically assessed morphology evolution of all of the Fe–Si models as a function of changing T and P SiH4 . We found that low Miller index facets are more likely to be exposed with the rising T and P SiH4 .

Two-Dimensional Iron Silicide (FeSix) Alloys with Above-Room-Temperature Ferromagnetism

“…Four 2D FeSi x alloys are FM at their ground state with the energy difference between AFM and FM varying from 143.04 to 546.51 meV/Fe, similar to previously reported FM Fe 2 Si (205 meV/Fe) and FeSi 2 (36 meV/Fe), , suggesting that the FM ground state is solid for 2D FeSi x alloys, which is independent of the value of x in the current study. Additionally, for Fe 3 Si 2 , Fe 4 Si 2 -hex, and Fe 4 Si 2 -orth, the FM ground state is consistent with their bulk counterparts, while 2D FeSi 2 exhibits ferromagnetism different from its nonmagnetic bulk counterpart after dimensional reduction …”

“…Additionally, for Fe 3 Si 2 , Fe 4 Si 2 -hex, and Fe 4 Si 2 -orth, the FM ground state is consistent with their bulk counterparts, while 2D FeSi 2 exhibits ferromagnetism different from its nonmagnetic bulk counterpart after dimensional reduction. 44 Next, the calculated electronic band structures and density of states (DOS) projected on the atomic orbital of 2D FeSi x alloys at their FM ground states with the screened hybrid HSE06 functionals are plotted in Figure 3a− S1).…”

“…Additionally, for Fe 3 Si 2 , Fe 4 Si 2 -hex, and Fe 4 Si 2 -orth, the FM ground state is consistent with their bulk counterparts, while 2D FeSi 2 exhibits ferromagnetism different from its nonmagnetic bulk counterpart after dimensional reduction. 44 Next, the calculated electronic band structures and density of states (DOS) projected on the atomic orbital of 2D FeSi x alloys at their FM ground states with the screened hybrid HSE06 functionals are plotted in Figure 3a− plotted DOS indicates that the spin polarization ratios of 2D FeSi x alloys are 76.6%, 65.0%, 96.4%, and 50.9% for Fe 3 Si 2 , Fe 4 Si 2 -hex, FeSi 2 , and Fe 4 Si 2 -orth, respectively, which are larger than the values of their bulk counterparts (Table S1).…”

Indirect magneto-ionic effect in FeSi2/Si nanocomposite induced by electrochemical lithiation and delithiation

“…For this purpose, we investigate the magnetic response of iron disilicide nanocrystals that are embedded in a Si electrode upon its electrochemical lithiation and delithiation of the latter. In the bulk, the stable semiconducting β-FeSi 2 and the metastable metallic α-FeSi 2 phase are both paramagnetic [26]. However, it has been shown that ferromagnetism or superparamagnetism can occur in iron disilicide nanostructures, which has been attributed to lattice distortions and/or chemical disorder at the surface, leading locally to magnetic Fe-rich defective regions [27][28][29][30][31].…”