Edge dislocation and superstructure in MgB2 superconducting crystals

Hexagonal plate-shaped MgB2 crystals have been prepared at ambient pressure. A superconducting transition has been observed at 37 K with a large transition width of DeltaTc~22 K. The lattice strain (0.7%) is calculated from the FWHM of XRD data. Both an edge dislocation with Burgers vector b = -a, and a Mg (nano-agglomerate) self-ordering superstructure with lattice constant 2a0 have been directly detected in the [001] direction by HRTEM. The very broad transition may be caused by these defects in the crystals.