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Crystal structure of the Mg$_{1-x}$Al$_x$B$_2$ superconductors near x=0.5

S. Margadonna·K. Prassides·I. Arvanitidis·M. Pissas·G. Papavassiliou·A. N. Fitch
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Abstract

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.

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S. Margadonna, K. Prassides, I. Arvanitidis, M. Pissas, G. Papavassiliou, A. N. Fitch. 2002-03-31. Crystal structure of the Mg$_{1-x}$Al$_x$B$_2$ superconductors near x=0.5. https://doi.org/10.1103/physrevb.66.014518

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