  Crystallization represents the prime example of a disorder order transition.
In realistic situations, however, container walls and impurities are frequently
present and hence crystallization is heterogeneously seeded. Rarely the seeds
are perfectly compatible with the thermodynamically favoured crystal structure
and thus induce elastic distortions, which impede further crystal growth. Here
we use a colloidal model system, which not only allows us to quantitatively
control the induced distortions but also to visualize and follow heterogeneous
crystallization with single-particle resolution. We determine the sequence of
intermediate structures by confocal microscopy and computer simulations, and
develop a theoretical model that describes our findings. The crystallite first
grows on the seed but then, on reaching a critical size, detaches from the
seed. The detached and relaxed crystallite continues to grow, except close to
the seed, which now prevents crystallization. Hence, crystallization seeds
facilitate crystallization only during initial growth and then act as
impurities.
