Abstract
X-ray diffraction studies on eleven single crystals having compositional ratios close to Yb/(Fe,Al) = 2:17 revealed four distinctly different structures, two having hexagonal symmetry (LuFe~9.5~ and TbCu~7~ type) at low and intermediate Al and Yb contents (0–7.5 at Al; 9.6–10.6 at Yb and 11.2–12.5 at Al; 10.5–10.9 at Yb), respectively, and two having rhombohedral symmetry (PrFe~7~(A) and PrFe~7~(A + B) types) at high Al and Yb contents (9.8–19.8 at Al; 10.7–11.6 at Yb). All have a common hexagonal TbCu~7~ type subcell and contain chains made up of Yb and Fe(Al)~2~ dumbbells running along the c-axis that substitute for each other with various ordering schemes and concentrations. The subcell ratio c/a depends on both the concentration of Fe(Al)~2~ dumbbells, for which it generally increases, and the structure type (dumbbell order). We define an order parameter capable of describing the dependency of the subcell ratio c/a on the Fe(Al)~2~ dumbbell ordering, irrespective of structure type, as the probability of finding two Fe(Al)~2~ dumbbells within a chain that are separated by one Yb atom only. Aluminium substitutes for iron mainly on non-dumbbell sites at low Al contents (LuFe~9.5~ and TbCu~7~ type) and enters Fe~2~ dumbbell sites only at high Al contents (TbCu~7~ type and PrFe~7~ type derivatives). The bond distances in the mixed Fe(Al)~2~ dumbbells increase with increasing Al content. Those in the Fe~2~ dumbbells depend on the dumbbell order (within one or between different structure types), and less on the Al content of the crystal.