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As indicated by SEM and Auger analysis, a sizeable amount of oxygen is usually present in superconducting MgB2-based materials (bulks, thin films, and wires). The established correlations between the characteristics of MgB2-based superconducting materials and their structural features, in particular the oxygen distribution and content are under the consideration. The matrix phase of bulk MgB2 contains rather small amounts of oxygen, but a high amount of close to MgBO dispersed inclusions or regions. X-ray phase analysis with Rietveld refinement of several highly dense MgB2-based (with high Jc) bulk samples (even that prepared under special conditions for preventing oxygenation) showed that the superconducting phase had a composition MgB1.68-1.8O0.2-0.32 instead of pure MgB2. Besides, a small amount of a phase with MgO structure was observed in the materials by x-rays. The calculation of the enthalpy of formation and Gibbs energy confirms the possibility of oxygen solubility in MgB2 and shows that the formation of MgB1.75O0.25 is most favorable. The results of ab initio calculations of the electronic structure and stability of the MgB2 compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise or form chains. In the case of MgB1.75O0.25 it is preferable oxygen incorporation into each second boron plane and thus the undisturbed boron plane is alternated by the boron plane with oxygen atoms. The experimental TEM study gave support for these calculations.
The numerical calculations in the DFT approximation showed that in the case of substitution of boron atoms to carbon the energetically preferable is their homogeneous distribution. The small amount of the carbon atoms in MgB2 structure can dramatically affect the superconducting characteristics because it will lead to the essential changes in the electron density distribution.