Vibrational properties of High Entropy Alloy based metal hydrides probed by inelastic neutron scattering

Highlights: • The main feature of the vibrational spectra of HEA-based metal hydrides is centered around 150 meV. • Fine structures in the optical peaks are identified by DFT to originate from different phonons. • Lower energy shoulders in the spectra indicate occupation by hydrogen in octahedral sites. • Optical peak energies show that not only valency determines the stability of the metal hydride. -- Abstract: The vibrational properties of several High Entropy Alloy (HEA) based metal hydrides are investigated by inelastic neutron scattering (INS). HEAs have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications such as hydrogen storage. The special properties of HEAs are believed to originate from the disordered lattice and internal strain that is introduced from the differences in atomic radii. This makes HEA hydrides provide an intriguing situation for the local H coordination, of several different transition metals. INS spectra were collected on a series of HEA-based metal hydrides starting with TiVNbH_x and subsequently adding Zr and Hf to increase the atomic size mismatch. A general feature of the spectra are the optical peaks centered around an energy loss of 150 meV that can be attributed to hydrogen vibrations in a tetrahedral environment. Upon the addition of Zr and Hf, a shoulder appears on the optical peak at lower energy transfers that after comparison with in silico calculated INS spectra is indicative of hydrogen also occupying octahedral sites in the structure.