Structure of iron-rich compounds in the Yb–Fe–Al system

Abstract

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 (LuFe9.5 and TbCu7 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 (PrFe7(A) and PrFe7(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 TbCu7 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 (LuFe9.5 and TbCu7 type) and enters Fe2 dumbbell sites only at high Al contents (TbCu7 type and PrFe7 type derivatives). The bond distances in the mixed Fe(Al)2 dumbbells increase with increasing Al content. Those in the Fe2 dumbbells depend on the dumbbell order (within one or between different structure types), and less on the Al content of the crystal.