Crystal structure of the Mg_1-xAl_xB₂ superconductors near x≈0.5

Precise structural information on the Mg_1-xAl_xB₂ superconductors in the vicinity of x≈0.5 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 ∼0.12 Å, 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.