The crystal structure of the R phase, Mo–Co–Cr

Abstract

The crystal structure of the R phase in the Mo-Co-Cr system, with atom ratio 30. 4-51.3-18.3, has been determined by single-crystal X-ray diffraction analysis. The crystal has space group C~i-R-3, with 53 atoms per rhombohedral cell. The rhombohedral lattice constants are a 0 = 9.011 /~, a = 74 27"5', hexagonal lattice constants a 0 = 10.903, c o = 19.342 A. The structure is closely related to those of other transition-group phases such as the a phase (Fe-Cr and many other systems), P phase (Mo-lXli-Cr and Mo-Ni-Fe), Z phase (~-Mn, Mo-Fe-Cr), and/~ phase (Mo-Co and other systems). The/~ phase field lies directly between the ~ and/~ phase fields on the Mo-Co-Cr ternary phase diagram. The R phase exhibits 12-fold (icosahedral), 14-fold, 15-fold, and 16-fold atomic coordination, which are the coordinations shown in various combinations by the other phases mentioned. Like the others (except X), the R phase has exclusively tetrahedral interstices. As with the P and phases, but to a somewhat lesser degree, interatomic distances in the R phase agree with sums of characteristic radii, which are different for five-and six-coordinated ligands for each type of coordination. As in the case of the P phase, the molybdenum content of the atomic sites increases with coordination from approximately zero for coordination 12 to approximately 100~/o for coordination 16.