An atomic-resolution study was made of the structure and composition of the grain interiors and grain boundaries in polycrystalline samples. It was found that there was no O within the bulk of the grains, but there was significant O enrichment at the boundaries. Most of the grain boundaries contained BOx phases which were smaller than the coherence length. Others contained larger areas of MgO, which were sandwiched between BOx layers.

Direct Observation of Nanometer-Scale Mg- and B-Oxide Phases at Grain Boundaries in MgB2. R.F.Klie, J.C.Idrobo, N.D.Browning, K.A.Regan, N.S.Rogado, R.J.Cava: Applied Physics Letters, 2001, 79[12], 1837-9