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

This paper describes the established correlations between the characteristics of MgB<sub>2</sub>-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<sub>2</sub> -based materials (bulks, thin films, and wires). The matrix phase of bulk MgB<sub>2</sub> 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<sub>2</sub> -based bulk samples (with high critical current densities) showed that the superconducting phase had a composition within the range MgB<sub>1.68</sub>O<sub>0.26</sub> and MgB<sub>1.74</sub>O<sub>0.32</sub> instead of pure MgB<sub>2 </sub>. 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<sub>2</sub> and shows that the formation of MgB<sub>1.75</sub>B<sub>0.25</sub> is most favorable. The results of <italic>ab initio</italic> calculations of the electronic structure and stability of the MgB<sub>2</sub> compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise.

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