Iron-rich rare-earth transition-metal alloys are of interest as constituents for permanent magnets. Ytterbium containing materials have not yet been studied in great detail because of problems with sample preparation. Our recent X-ray diffraction studies on single crystals [1,2] and powders [3] having compositional ratios close to Yb/(Fe,Al) or Yb/(Fe,Ga)=2/17 revealed four distinctly different structures, two having hexagonal symmetry (LuFe_9.5 and TbCu₇ type) at low and intermediate Al or Ga and Yb contents, respectively, and two having rhombohedral symmetry (PrFe₇(A) and PrFe₇(A+B) types) at high Al or Ga and Yb contents. All have a common hexagonal TbCu₇ type subcell and contain chains made up of Yb and Fe(Al)₂ or Fe(Ga)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 the 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 or Fe(Ga)₂ dumbbell ordering, irrespective of structure type, as the probability of finding two dumbbells within a chain that are separated by one Yb atom only.

[1] Cerny R., Pacheco V., Yanson T., Manyako M. and Bodak O.: Z. Kristallographie, 218 (2003) 802-810

[2] Filinchuk Ya. E., Birkedal H., Cerny R., Hostettler M., Yanson T.I., Bodak O.I. and Yvon K.: Chemistry – A European Journal, 10 (2004) 2972-2976

[3] Bodak O., Tokaychuk Y., Manyako M., Pacheco V., Cerny R. and Yvon K.: J. Alloys and Compounds 354 (2003) L10-L15

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