  Here we describe the results of an atomic resolution study of the structure
and composition of both the interior of the grains, and the grain boundaries in
polycrystalline MgB2. We find that there is no oxygen within the bulk of the
grains but significant oxygen enrichment at the grain boundaries. The majority
of grain boundaries contain BOx phases smaller than the coherence length, while
others contain larger areas of MgO sandwiched between BOx layers. Such results
naturally explain the differences in connectivity between the grains observed
by other techniques.
