Hexagonal plate-shaped MgB2 crystals were prepared at ambient pressure. A superconducting transition was observed at 37K, with a transition width of 22K. The lattice strain (0.7%) was calculated from the FWHM of X-ray diffraction data. Both an edge dislocation with Burgers vector b = -a, and a Mg (nano-agglomerate) self-ordering superstructure with a lattice constant 2a0 were directly detected in the [001] direction by high-resolution transmission electron microscopy. The very broad transition was suggested to be caused by these defects.

Edge Dislocation and Superstructure in MgB2 Superconducting Crystals. W.Du, H.Zhang, D.Xu, X.Wang, X.Hou, G.Zhang, Y.Geng, H.Liu, F.Jiang, Y.Wang: Superconductor Science and Technology, 2005, 18, 1513-6