Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

[摘要] An x-ray analysis of MgB2-based materials shows that they contain MgB2and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2(ΔHf= -150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25(ΔHf= -191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2and substituted MgB1.75O0.25was modelled as well, but has shown that such processes are energetically disadvantageous (ΔHfof Mg0.875B2and Mg0.75B1.75O0.25are equal to -45.5 and -93.5 meV/atom, respectively).

[发布机构] V. Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine (NASU), Avtozavodskaya Str., 2, Kiev; 07074, Ukraine^1;Lviv Polytechnic National University, 12 Bandera Str., Lviv; 79013, Ukraine^2;Atominstitut, TU Wien, Stadionallee 2, Vienna; 1020, Austria^3;B. Verkin Institute for Low Temperature Physics of the National Academy of Sciences of Ukraine, Kharkov; 61103, Ukraine^4;Donetsk Inst. for Phys. and Eng. Named after O. O. Galkin of the Natl. Acad. of Sciences of Ukraine, R. Luxemburg Str. 72, Donetsk-114; 83114, Ukraine^5;Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen; 76344, Germany^6;Leibniz Institute of Photonic Technology E. V., Albert-Einstein-Straße 9, Jena; 07745, Germany^7;Technische Universität Ilmenau, Institute of Materials Engineering, Gustav-Kirchhoff-Strasse 6, Ilmenau; D-98693, Germany^8

[关键词] Ab initio modelling;Boron atom;Carbon substitution;Ion etching;Magnesium diborides;Oxidized surfaces;Quantitative Auger analysis;Superconducting properties