Equilibrium Grain Boundary Segregation and Clustering of Impurities in Colloidal Polycrystalline Monolayers

We investigate the
segregation of impurities to grain boundaries
in colloidal polycrystalline monolayers using video microscopy. A
model colloidal alloy is prepared by embedding large spherical impurities
in a polycrystalline monolayer of small host colloidal hard spheres,
which stops grain growth at a finite grain size. The size ratio between
the impurities and the host particles determines whether they behave
as interstitial or substitutional impurities in the bulk crystal,
akin to those in real alloys. We find that the partitioning of impurities
between the grains and the grain boundaries is in very good agreement
with the Langmuir–McLean adsorption model for equilibrium grain
boundary segregation. This enables the direct measurement of the free
energy of adsorption for the two types of impurities. Near saturation,
we characterize the spatial distribution of the adsorbed impurities
and find that it strongly depends on their interstitial or substitutional
nature. This is because the relative importance of clustering and
mixing due to nonadditivity is determined by geometrical constraints
imposed by the crystalline host lattice.