The structure of nanocrystalline Ti:Ru:Fe (2 -y:(1+ y)/2:(1 + y)/2) obtained by high-energy ball milling has been studied by X-ray and neutron diffraction as a function of the Ti content, for y varying from 0.00 to 1.00 by step of 0.25, using Rietveld refinement analysis. When y ) 0.00, a nanocrystalline metastable B2 cubic phase is formed, with most of the 1a site occupied by Ti atoms and the 1b site occupied by either Fe or Ru atoms. When decreasing the Ti content, the B2 structure becomes less stable. A preferential replacement of Ti by Fe on the 1a occurs and leads to the precipitation of hcp Ru. In situ X-ray diffraction measurements under hydrogen at high pressure were also made. In all cases, a shift of the diffraction peaks of the B2 structure toward the smaller 2θ values was observed. This shift is totally reversible upon removing hydrogen. It indicates that hydrogen is absorbed into the materials. The volume increase of the B2 structure varies according to the Ti content, reflecting the fact that less hydrogen is absorbed when Ti is reduced. Assuming that the volume occupied by a single hydrogen atom is ∼2.5 D 3 , the hydrogen content of the various nanocrystalline Ti:Ru:Fe (2 -y:(1 + y)/2:(1 + y)/2) is calculated from the volume increase of the unit cell of the corresponding materials.

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