  Impurities in crystalline materials introduce disorder into an otherwise
ordered structure due to the formation of lattice defects and grain boundaries.
The properties of the resulting polycrystal can differ remarkably from those of
the ideal single crystal. Here we investigate a quasi-two-dimensional system of
colloidal spheres containing a small fraction of aspherical impurities and
characterise the resulting polycrystalline monolayer. We find that, in the
vicinity of an impurity, the underlying hexagonal lattice is deformed due to a
preference for 5-fold co-ordinated particles adjacent to impurities. This
results in a reduction in local hexagonal ordering around an impurity.
Increasing the concentration of impurities leads to an increase in the number
of these defects and consequently a reduction in system-wide hexagonal ordering
and a corresponding increase in entropy as measured from the distribution of
Voronoi cell areas. Furthermore, through both considering orientational
correlations and directly identifying crystalline domains we observe a decrease
in the average polycrystalline grain size on increasing the concentration of
impurities. Our data show that, for the concentrations considered, local
structural modifications due to the presence of impurities are independent of
their concentration, while structure on longer lengthscales (i.e. the size of
polycrystalline grains) is determined by the impurity concentration.
