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

Abstract

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

Phase diagram and order-disorder transitions in Y0.9Gd0.1Fe2Hx hydrides (x ≥ 2.9)

“…We have selected two samples: HYD-M2-A and HYD-M2-B containing 78 % and 90.7 % of monoclinic M2 phase at 300 K respectively and compared their evolution with that of Y0.9Gd0.1Fe2D4.2 as reference compound. It was also done to verify the isotope effect previously observed with a low resolution laboratory XRD device [38]. The evolution of the SR-XRD patterns of DEUT-M2, HYD-M2-A and HYD-M2-B upon heating is compared on the 2D plots in figs.…”

“…The transition from M2 to M'2 phase is observed between 295 K and 330 K, then the disordered cubic phase starts to appear above 320 K. In this sample an additional intermediate orthorhombic phase is observed between 317 and 330 K, as already observed in ref. [38]. It is obvious that this intermediate orthorhombic phase displays a significantly larger distortion than the monoclinic M2 and M'2) phases when compared in an equivalent cubic cell (Fig.…”

“…Y0.9Gd0.1Fe2 intermetallic compound have been prepared by induction melting of the chemical elements followed by three weeks annealing treatment under vacuum at 1100 K. The composition and homogeneity of the alloy was checked by X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA) as described in [38]. Y0.9Gd0.1Fe2Hx and Y0.9Gd0.1Fe2D4.2 samples were prepared by solid-gas reaction using a Sievert apparatus as described in and pressure-composition isotherms (PCI) were measured.…”

“…The analysis of the SR-XRD pattern shows a more complex structural evolution than for the two previous samples. This is the same hydride as in [38], but the better instrumental resolution of the synchrotron allow to separate more easily the different occurring phases and to perform a quantitative refinement of each phase. The transition from M2 to M'2 phase is observed between 295 K and 330 K, then the disordered cubic phase starts to appear above 320 K. In this sample an additional intermediate orthorhombic phase is observed between 317 and 330 K, as already observed in ref.…”

“…This magnetic transition is accompanied by a cell volume variation and is sensitive to any cell volume change induced by an applied external pressure [30,31], D for H isotopic substitution [32,33] or R for Y substitution [34][35][36][37]. For example, the investigation of the monoclinic Y0.9Gd0.1Fe2(H1-zDz)4.2 compounds indicated that upon H for D substitution the FM-AFM transition temperature is shifted to higher temperature, whereas TO-D is shifted to lower value [38].…”