Samples synthesized by solid-state reaction were found to contain several types of crystalline defect, including Mg deficiency, coherent precipitates, screw dislocations and O impurity; depending upon the synthesis conditions. The Mg deficiency introduced significant strain into the crystalline lattice. A high density of screw dislocations lying on (00▪1) planes, with a Burgers vector of <11▪0>, partially relieved the lattice strain. The remnant strain in the MgB2 lattice directly determined the critical superconducting temperature; which decreased with strain. A small amount of O dissolved in the matrix was not responsible for the lower TC observed in the samples.

Strain Effect on the Critical Superconducting Temperature of MgB2. X.Z.Liao, A.Serquis, Y.T.Zhu, D.E.Peterson, F.M.Mueller, H.F.Xu: Superconductor Science and Technology, 2004, 17, 1026-30