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 °C, the average Mg content in the MgB₂ crystals systematically decreased while T_c remains essentially unchanged and the superconducting transition slightly broadens from ~0.55 to ~1.3  K. The reduction in the superconducting volume fraction was noticeable already after annealing at 875 °C. Samples annealed at 975 °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 2MgB₂(s)MgB₄(s)+Mg(g). First-principles calculations of the Mg_1−x(V_Mg)_xB₂ electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both and 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. ©2010 The American Physical Society