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The effects of high-temperature vacuum-annealing induced Mg deficiency inMgB2 single crystals grown under high pressure were investigated. As theannealing temperature was increased from 800 to 975 C, the average Mg contentin the MgB2 crystals systematically decreased, while Tc remains essentiallyunchanged and the superconducting transition slightly broadens from 0.55 K to1.3 K. The reduction of the superconducting volume fraction was noticeablealready after annealing at 875 C. Samples annealed at 975 C are partiallydecomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-growncrystals. Annealing at 1000 C completely destroys superconductivity. X-raydiffraction analysis revealed that the main final product of decomposition ispolycrystalline MgB4 and thus the decomposition reaction of MgB2 can bedescribed as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of theMg1-x(VMg)xB2 electronic structure, within the supercell approach, show a smalldownshift of the Fermi level. Holes induced by the vacancies go to both sigmaand pi bands. These small modifications are not expected to influence Tc, inagreement with observations. The significant reduction of the superconductingvolume fraction without noticeable Tc reduction indicates the coexistence,within the same crystal, of superconductive and non-superconductive electronicphases, associated with regions rich and poor in Mg vacancies.

Influence of Mg deficiency in MgB2 single crystals on crystal structure and superconducting properties