Structural analysis for crystalline and amorphous RFe₂D _x (R: Ho, Tb) by X-ray/neutron diffraction and reverse Monte Carlo modeling

Atomic structures of crystalline (c-)HoFe₂D_3.6 and amorphous (a-)HoFe₂D_3.0 prepared by deuterium absorption of the C15 Laves phase compound HoFe₂ were studied by X-ray and neutron diffraction techniques and these were compared with previous results for c- and a-TbFe₂D_x. The structural parameters such as an interatomic distance and a coordination number are qualitatively in agreement with those for c- and a-TbFe₂D_x. Namely, a marked difference was observed in the arrangement of metal atoms in a-HoFe ₂D_3.0 compared to that in c-HoFe₂D _3.6 from interatomic distances and coordination numbers and there are clusters of Fe and Ho atoms in a-HoFe₂D_x. RDF(r)s observed by the neutron diffraction indicated that the D atoms occupy tetrahedral sites consisting of 2Ho+2Fe in c-HoFe₂D_3.6, while the D atoms occupy selectively tetrahedral sites consisting mainly of Ho atoms in a-HoFe₂D_3.0 because of strong affinity between D and Ho atoms. The reverse Monte Carlo (RMC) modeling method was employed to visualize a three-dimensional atomic arrangement by using combined X-ray and neuron diffraction data of RFe₂D_x (R: Ho, Tb). Chemical fluctuation was visually observed in the RMC configurations. Moreover, it was found that about 40% of the D atoms occupy sites consisting of 4R.