Thermal desorption spectroscopy studies of hydrogen desorption from rare earth metal trihydrides REH3 (RE=Dy, Ho, Er)

Rare earth (RE) metals form two stoichiometric hydrides, REH2 and REH3, and for the yttrium group of rare earth metals a transformation of a FCC (REH2) into an HCP (REH3) lattice takes place during the second step of hydrogenation REH2 + ½ H2 → REH3. Earlier studies of the hydrogen desorption properties of the rare earth hydrides were limited to Y and RE = La, Ce, Pr, Nd, Sm, Gd, Tb and Er. The present work is focused on the studies of the kinetics and mechanism of hydrogen desorption from trihydrides of heavy rare earths, DyH3, HoH3, and ErH3. The Thermal Desorption Spectroscopy (TDS) studies were performed at pressures below 1 × 10−5 mbar during linear heating from room temperature to 1173 K at different heating rates ranging from 1 to 5.5 K/min. Hydrogen desorption traces show the presence of two main events with the low-temperature peak appearing below 573 K, while the second peak is positioned at 1083–1159 K, with the peak temperatures gradually increasing following the rise of the heating rate. Fitting of the peak temperatures in the TDS spectra using the Kissinger method yielded activation energies of hydrogen desorption for both hydrogen desorption events. For DyH3 and ErH3, the shapes of the TDS spectra appear to be well described by a phase-stuctural transformation following a model of nucleation and growth, while for HoH3 the dehydrogenation includes a phase boundary reaction. This model of phase-structural transformations shows differences in dimensionality and rate-limiting steps as related to the studied compound and the desorption events, REH3 → REH2 or REH2 → RE