Direct observation of nanometer-scale Mg- and B-oxide phases at grain boundaries in MgB2

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 boundari es in polycrystalline MgB2. We find that there is no oxygen within the bulk of the grains but significant oxygen enrichment at the grain boundaries. T he majority of grain boundaries contain BOx phases smaller than the coheren ce length, while others contain larger areas of MgO sandwiched between BOx layers. Such results naturally explain the differences in connectivity betw een the grains observed by other techniques. (C) 2001 American Institute of Physics.