Nanoparticles of
iron and nickel are promising candidates as nanosized
soft magnetic materials and as catalysts for carbon nanotube synthesis
and CO methanation, among others. To understand geometry- and size-dependent
properties of these nanoparticles, phase diagram of Fe/Ni alloy nanoparticles
was calculated by density functional theory and cluster expansion
method. Ground state convex is presented for face-centered cubic (FCC),
body-centered cubic (BCC), and icosahedral (ICO) particles. Previous
experimental observations were explained by using multiscale model
for particles with realistic size (diameter ≥2 nm). At size
1.5 nm, geometry changes from BCC at low X(Ni) to icosahedral at high
X(Ni). FCC is stabilized over icosahedral geometry by increasing number
of atoms from 561 to 923. In large FCC particles, there is enrichment
of Fe atoms from core to shell beneath surface, while surface and
core are enriched by Ni atoms. Catalytic enhancement effect in CO
methanation was found to be due to Ni incorporating in the active
sites which brings adsorption energy of oxygen closer to the optimum.
The predicted phase diagrams and implications on catalysis are expected
to help rationalization of experimental results and provide guidance
for design of Fe/Ni-based nanomaterials.