Planar faulting in rare earth–Co 2:17 alloys

Planar faulting in R2Co17 intermetallic alloys were studied by high-resolution X-ray diffraction. For the light rare-earth elements, the Th2Zn17 rhombohedral structure is found, while for the heavy rare-earth atoms, the Th2Ni17 structure prevails. For intermediate rare-earth atom, the R2Co17 structure shows planar disorder. The occurrence of planar faulting is associated with the interlayer distance in the structure. A correlation was found between interlayer distance and stacking order. Simple qualitative explanation is given for the energy preference for one particular type of stacking order. Planar disorder does not introduce new nearest-neighbors environments for the rare-earth atoms apart from those already found in the hexagonal and rhombohedral variants, but merely changes the probability of occurrence of each neighborhood. Simple stacking fault models should prove insufficient to explain the observed planar disorder.