Abstract:One of the many singularities of Frank-Kasper phases is their ability to accommodate extremely large composition ranges by atom mixing on the different sites of the crystal structures. This phenomenon will be reviewed in the present paper with special emphasis on the experimental demonstration of this phenomenon, the theoretical calculation of disordered structures and the modeling of these phases.

Y 0.9 Gd 0.1 Fe 2 (H z D 1-z) 4.2 compounds crystallize in a monoclinic structure at room temperature, with an increase of the cell parameters versus the H content. These compounds undergo a ferro-antiferromagnetic first-order transition, the transition temperature of which increases from 98 to 144 K for z going from 0 to 1, due to a strong magnetovolumic effect. Above room temperature (290-340 K), they display an order-disorder (O-D) transition from monoclinic towards a cubic structure, which has been studied by DSC and XRD versus temperature. This transition occurs via the presence of an intermediate phase, the structure of which depends on the H content. For z = 0 and 0.5 the intermediate phase is monoclinic, whereas an orthorhombic phase is observed for z = 0.75 and 1. In addition, for the H-rich compounds the orthorhombic phase disappears at a much lower temperature upon cooling than it appears upon heating. DFT band structure calculations for YFe 2 H x compounds showed that for 4 < x < 4.5, although the monoclinic phase is the more stable one, the energy of formation of the orthorhombic phase is only 0.1 kJ higher. For 4.5 < x ≤ 5 the orthorhombic phase becomes more stable. The sensitivity of the O-D transition to the H/D content could be related to a volume effect. At higher temperatures (T > 400 K), the thermal desorption studied by TGA shows a multipeak behavior that is not sensitive to the (H, D) isotope effect. Laves phases / Isotope effect / X-ray diffraction / Order-disorder transition / DFT calculation

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