Microstructure and structural defects in MgB2 superconductor

We report a detailed study of the microstructure and defects in sintered polycrystalline magnesium diboride (MgB2). Both transmission electron microscopy and X-ray data reveal that MgO is the major second-phase in our bulk samples. Although MgB2 and MgO have different crystal symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg and B (or O) and lattice spacings in certain crystallographic orientations are very similar. The size of MgO varies from 10-500 nm, and its mismatch with the MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a Burgers vector of < 100 >. 1/3 <1-10 > and 1/3 < 210 > partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the (001) basal plane, flux pinning anisotropy is expected. Diffuse scattering analysis suggests that the correlation length along the c-axis for defect-free basal planes is about 50 nm. (001) twist grain boundaries (GBs), formed by rotations along the c-axis, are major grain boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and B atoms. An excess of Mg was observed in some grain boundaries. High-resolution nano-probe electron-energy loss spectroscopy reveals that there is a difference in near edge structure of the boron K-edge acquired from GBs and grain interiors. The change at the edge threshold may be suggestive of variation of the hole concentration that would significantly alter boundary superconductivity. (C) 2001 Elsevier Science B.V. All rights reserved.