  MgB2 samples prepared by solid-state reaction were investigated using
high-resolution transmission electron microscopy (HREM), X-ray
energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS),
and energy-filtered imaging. Large amounts of coherent precipitates with a size
range from about 5 nm up to about 100 nm were found in the MgB2 crystallite
matrices. The precipitates are of different shapes including sphere, ellipsoid,
and faceted polyhedron depending on the size of the precipitates. EDX and EELS
analyses confirm that smaller precipitates contain magnesium, boron and oxygen
while larger faceted precipitates contain mainly magnesium and oxygen, implying
that the oxygen content increases with precipitate size. HREM and electron
diffraction investigations found that the precipitates have the same crystal
lattice structure as that of MgB2 but with various composition modulations
depending on the composition of the precipitates. The precipitates transform to
the MgO phase after long exposure to residual oxygen in flowing Ar gas at high
temperatures. The effect of the precipitates in different size ranges on flux
pinning is discussed.
