The deuterides of cubic (C15) Laves phases have been widely studied for the influence of deuterium absorption on their magnetic properties. The systems YFe₂D_x and YMn₂D_x are particularly interesting due to the large variety of crystal structures depending on the D content [1,2]. When absorbing deuterium the metallic matrix retains the cubic cell (Fd3m) above the deuterium ordering temperature. Below this temperature the symmetry is lowered and a fully ordered coordination of metals by deuterium atoms is obtained in the deuterium rich phases. Complex structures with up to 12 metal atoms and 18 deuteriums in the asymetric unit as for monoclinic YFe₂D_4.2 [3] are formed. Little is known about the local deuterium configuration around the transition metal atoms in the disordered phase even if a considerable amount of diffuse intensity was reported in the neutron powder patterns.

Figure 1. Observed PDF (points) of YFe₂D_4.2 in disordered state: Modelling (solid line) by same local order of deuterium atoms around iron as in the ordered phase.

Analysis of YFe₂D_x and YMn₂D_x by Rietveld and Pair Distribution Function (PDF) methods will be presented. Neutron Time-of-Flight (ToF) data were collected at IPNS, Argonne, and Lujan Center, Los Alamos on series of samples with different deuterium content. Each sample was measured below and above the temperature of deuterium ordering. The PDF in ordered and disordered states look very similar up to the radial distance of ~ 8 Å, which is comparable with the lattice parameter of a cubic Laves phase deuteride. The observed PDF was modeled in the disordered state using the same local model as in the ordered phase (Figure 1).

[1] Latroche M., Paul-Boncour V., Przewoznik J., Percheron-Guégan A. & Bourée-

     Vigneron F.; J. Alloys Compounds 231 (1995) 99-103

[2] Paul-Boncour V., Guénée L., Latroche M., Percheron-Guégan A., Ouladdiaf B. &

      Bourée-Vigneron   F.; J. Solid State Chem. 142 (1999) 120-129

[3] Ropka J., Paul-Boncour V. & Černý R.; in preparation

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