Crystal structure of the Mg$_{1-x}$Al$_x$B$_2$ superconductors near   x=0.5
  Precise structural information on the Mg1-xAlxB2 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 ~0.1 A, is formed. The unusually large width of the (001/2) 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. 
S. Margadonna, K. Prassides, I. Arvanitidis, M. Pissas, G.   Papavassiliou, and A.N. Fitch