  The structural properties of MgB2 superconductors have been analyzed by means
of convergent-beam electron diffraction, high-resolution transmission-electron
microscopy, and theoretical simulations. The MgB2 crystal has been identified
to have a hexagonal structure with the space group of P6/mmm. Microstructural
features of MgB6 materials depend evidently upon the synthesis pressures.
Stacking faults and grain boundaries in a sample with the critical current
density greater than 105A/cm2 have been extensively investigated.
