Structural properties of MgB2 superconductors with a critical current density greater than 10(5)A/cm(2)

The structural features of MgB2 superconductors prepared under different co nditions have been analysed by means of transmission electron microscopy. T he MgB2 crystal is identified to have a hexagonal structure with the space group of P6/mmm. Superconducting materials synthesized under a high pressur e of 6 Cpa are made of regularly stacked grains as large as a few microns i n sizes. Stacking faults and microstructure properties of grain boundaries, either with or without impurities, appearing in a sample with the critical current density greater than 10(5)/cm(2), have been systematically investi gated.