Phase diagram and order-disorder transitions in Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$H$_{x}$ hydrides (x $\le$ 2.9)

TL;DR

This study maps the phase diagram of Y0.9Gd0.1Fe2Hx hydrides, revealing multiple structural phases and order-disorder transitions related to hydrogen content and arrangement, with implications for hydrogen storage materials.

Contribution

It provides a detailed characterization of the structural phases and order-disorder transitions in Y0.9Gd0.1Fe2Hx hydrides up to x=2.9, including the identification of superstructure peaks and phase sequences.

Findings

Multiple hydride phases with distinct crystal structures identified.

Order-disorder transitions observed upon heating with reversible behavior.

Hydrogen content influences phase stability and symmetry changes.

Abstract

Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$, which crystallizes in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$H$_{x}$ hydrides (2.9 $\le$ x $\le$ 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (M1 and M2), and one phase with orthorhombic structure (O), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exists as single phase within a H homogeneity range, and they are separated from each other by two-phase domains. The reductions of crystal symmetry are related to various hydrogen orders into interstitial sites. Weak superstructure peaks were indexed by doubling the cubic cell parameter of the cubic C2 phase. Upon heating, the monoclinic M1 and M2 and the cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered cubic structure CDis. These O-D transitions are reversible with thermal hysteresis effects. The cubic C3 and orthorhombic O phases transform into a disordered cubic phase accompanied by H desorption.