Structure and properties of MgB2bulks: Ab-initio simulations compared to experiment

Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB$_{2}$-based bulk materials (with high level of superconducting characteristics) has AlB$_{2}$ type structure and near MgB$_{1.8-1.68}$O$_{0.2-0.32}$ stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm$^{2}$ jc (at 0 - 1 T, 20 K); up to 15 T B$_{c2}$ (at 22.5 K) and B$_{irr}$ (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgB$_{1.75}$O$_{0.25}$; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB$_{2}$ structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, T$_{c}$, for MgB$_{1.75}$O$_{0.25}$ occurred to be 23.3 K, while for MgB$_{2}$ it was 21.13 K only. Experimental T$_{c}$ of the bulk materials was 35.7-38.2 K