Two a-phase structures, Cr2Ru and Cr2Os, have been studied using X-ray single-crystal diffraction techniques. The ordering determined has been compared with those of other a phases and A15 structures. Although atoms of both kinds occupy every atomic position, a significant departure from complete disorder was detected. This is particularly important in the 14-fold J atomic positions which are associated with bond angles of 180°: the marked preference of the smaller Cr atoms for these sites indicates that their occupancy appears to be determined by the electronic A.]

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