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

The effects of high-temperature vacuum-annealing-induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 degrees C, the average Mg content in the MgB2 crystals systematically decreased while T-c remains essentially unchanged and the superconducting transition slightly broadens from similar to 0.55 to similar to 1.3 K. The reduction in the superconducting volume fraction was noticeable already after annealing at 875 degrees C. Samples annealed at 975 degrees C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB(2)(s) --> MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(V-Mg)(x)B-2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence T-c, in agreement with observations. The significant reduction in the superconducting volume fraction without noticeable T-c reduction indicates the coexistence, within the same crystal, of superconductive and nonsuperconductive electronic phases, associated with regions poor and rich in Mg vacancies.