Crystal structure of the Mg1-xAlxB2 superconductors near x≈0.5 -: art. no. 014518

Precise structural information on the Mg1-xAlxB2 superconductors in the vicinity of xapproximate to0.5 is derived from high-resolution synchrotron x-ray powder diffraction measurements. We find that a hexagonal superstructure, accompanied by doubling of the c axis, ordering of Mg and Al in alternating layers, and a shift of the B layers towards Al by similar to0.12 Angstrom, is formed. The unusually large width of the (00 1/2) superlattice peak implies the presence of microstrain broadening, arising from anisotropic stacking of Al and Mg layers and/or structural modulations within the ab plane. The ordered phase survives only over a limited range of compositions away from the optimum x=0.5 doping level.