  Amorphous grain boundary complexions lack long-range crystalline order but
are not featureless, as distinct gradients in structural short-range order have
been reported through their thickness. In this work, we test the hypothesis
that the distribution of short-range order is determined by the confining
crystals using atomistic simulations of both Cu-Zr bicrystals and a random
polycrystal. Voronoi polyhedra with structures similar to that of perfect
face-centered cubic serve as signatures of high structural order and are only
found at the amorphous-crystalline transition regions. The density of the
ordered structural motifs within a specific amorphous-crystalline transition
region is found to not be directly determined by the orientation and symmetry
of the grain which touches it, but rather by the incompatibility between the
two confining grains. Ordered polyhedra density is found to be inversely
related to grain incompatibility, meaning that large incompatibilities between
the confining crystals lead to less order in the amorphous-crystalline
transition region. The finding that the entire grain-film-grain system must be
considered to understand local structure unequivocally demonstrates that
amorphous complexions are not simply a collection of independent phases which
happen to nucleate at a grain boundary. Rather, an amorphous grain boundary
complexion is a single entity that finds a local equilibrium configuration.
