Structure and Properties of MgB2 Bulks, Thin Films, and Wires

This paper describes the established correlations between the characteristics of MgB
₂
-based superconducting materials and their structural features, in particular the oxygen distribution and content. As indicated by SEM and Auger analysis, a sizeable amount of oxygen is usually present in superconducting MgB
₂
-based materials (bulks, thin films, and wires). The matrix phase of bulk MgB
₂
contains rather small amounts of oxygen, but contains a high amount of dispersed inclusions or areas with compositions close to MgBO. X-ray phase analysis with Rietveld refinement (using the FullProf Suite program package) of several highly dense MgB
₂
-based bulk samples (with high critical current densities) showed that the superconducting phase had a composition within the range MgB
_1.68
O
_0.26
and MgB
_1.74
O
_0.32
instead of pure MgB
₂
. Besides, a small amount of a phase with MgO structure was observed in the materials by X-rays as well. The calculation of the enthalpy of formation confirms the possibility of oxygen solubility in MgB
₂
and shows that the formation of MgB
_1.75
B
_0.25
is most favorable. The results of ab initio calculations of the electronic structure and stability of the MgB
₂
compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise.