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 〈1 0 0〉 and 1/3〈1 - 1 0〉 and 1/3〈2 1 0〉 partial dislocations and their associated stacking faults were also observed. Since both dislocations and stacking faults are located in the (0 0 1) 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. (0 0 1) 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.