Influence of the starting composition Mg1-xB2 on the structural and superconducting properties of the MgB2 phase

We report the preparation of samples with the nominal composition Mg1-xB2 ( 0 less than or equal to x less than or equal to 0.5). Nearly single-phase M gB2 was obtained for x = 0 with a minor amount of MgO. For 0 < x less than or equal to 0.5, MgB4 coexists with MgB2 and some minor impurity phases. Th e amount of MgB4 increases with x. With increasing x, the lattice parameter c of MgB2 increases, the lattice parameter a decreases, and correspondingl y T. decreases. The results are discussed in terms of the presence of Mg va cancies or B interstitials in the MgB2 structure. This work is helpful to u nderstand MgB2 films With different Tcs, as well as the Mg site doping effe ct for MgB2. (C) 2001 Elsevier Science B.V. All rights reserved.