Isotope effect on structural transitions in Y0.9Gd0.1Fe2(HzD1-z)4.2 compounds

Y0.9Gd0.1Fe2(HzD1-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 YFe2Hx 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.