A detailed study was made of the microstructure and defects of sintered polycrystalline material. Transmission electron microscopy and X-ray data revealed that MgO was the

major second phase in the bulk samples. Although MgB2 and MgO had differing crystal symmetries (P6/mmm and Fm¯3m, respectively) the stacking sequences of Mg and B (or O) and the lattice spacings in certain crystallographic orientations were very similar. The size of MgO varied from 10 to 500nm and its mismatch with MgB2 could be a source of dislocations. Dislocations in MgB2 often had a Burgers vector of <100> (figure 1), while 1/3<1¯10> and 1/3<210> partial dislocations and their associated stacking faults were also observed (figure 2). Because both dislocations and stacking faults were located on (001) basal planes, flux-pinning anisotropy was expected. Diffuse scattering analysis suggested that the correlation length along the c-axis for defect-free basal planes was about 50nm. It was noted that (001) twist grain boundaries, formed by rotations along the c-axis, were major grain boundaries in MgB2; as a result of 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 revealed that there was a difference in the near edge structure of the B K-edge acquired from grain boundaries and grain interiors. The change at the edge threshold suggested a variation in the hole concentration that could significantly alter the boundary superconductivity.

Microstructure and Structural Defects in MgB2 Superconductor. Y.Zhu, L.Wu, V.Volkov, Q.Li, G.Gu, A.R.Moodenbaugh, M.Malac, M.Suenaga, J.Tranquada: Physica C, 2001, 356[4], 239-53

Figure 1

Structure of Perfect Edge Dislocation with b = [100]

  (Only surface Mg atoms are shown)

Figure 2

Dissociation of b = [100] Edge Dislocation into

1/3[1¯10] and 1/3[210] Partial Dislocations

(Open circles and graded circles are Mg atoms,

  filled circles are B sub-lattice atoms)