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

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 (mu(Si)) 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 mu(Si) 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.