On the origins of ‘forbidden’ 100-type spots in electron diffraction patterns from the A15 compounds Nb₃AI, Cr₃Si and V₃Si

Abstract

Diffraction experiments in the transmission electron microscope have been used to investigate the origin of the weak 100-type reflections obtained at 〈0kl〉 beam directions from the A15 compounds Nb₃Al, Cr₃Si and V₃Si. It is argued that these spots do not arise from a second phase, by double diffraction or from asymmetric bonding, nor do they correspond to any of the models proposed previously for ordering in A15 structures. They are instead consistent with a new ordered state in which either vacancies or excess majority atoms reside on only one of the minority-atom positions in the basis.