Neutron spectroscopic comparison of β-phase rare earth hydrides

Abstract

The β-phase hydrides β-RH2 + x of a variety of rare earth metals (R ≡ Tb, Y, La, Ce and Dy) were investigated by incoherent inelastic neutron scattering spectroscopy. The broad, complex, vibrational density of states (DOS) of the tetrahedrally coordinated (t-site) hydrogen (Ht) typically found for these systems is a manifestation of the dynamic coupling within the Ht sublattice due to significant Ht-Ht interactions. A breakdown of this dynamic coupling via isotopic dilution of the Ht atoms with larger mass deuterium atoms is observed spectroscopically by a collapse of the complex DOS into a single sharp feature. For x > 0, the extra hydrogen atoms reside in the octahedral (o-site) interstices. Typically, the Ho DOS is found to be sensitive to the temperature- and concentration-dependent arrangement of the Ho atoms in the o-site sublattice. For small x, the Ho atoms are isolated in a local cubic environment and possess a sharp DOS. As x increases, the transformation to short-range then long-range order occurs as shown by the development of a broadened complex DOS. The presence of Ho atoms also perturbs the Ht DOS. Typically, Ho-Ht interactions cause the affected Ht vibrations to stiffen as illustrated by the formation of a higher energy feature split from the main Ht DOS. In short, hydrogen-hydrogen interactions in the rare earth β-phase hydrides lead to interesting structural and dynamic behavior which can be probed nicely by neutron vibrational spectroscopy.