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

Gustav Ek,Øystein S Fjellvåg,Ponniah Vajeeston,Jeff Armstrong,Martin Sahlberg,Ulrich Häussermann

doi:10.1016/j.jallcom.2021.160320

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 TiVNbHx 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.

Highlights

High entropy alloys (HEA) have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications
All hydrides can be indexed as either fcc (Fm3m) or bct (I4/mmm) which is indicative of CaF2-type structures common for metal hydrides with hydrogen per metal ratios close to
The present work has probed the vibrational properties of a series of HEA-based metal hydrides by inelastic neutron scattering

Summary

Introduction

High entropy alloys (HEA) have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications. A good indication of site preference in the typical metal lattices has been suggested by Hauck, by comparing the electronegativity of the host metal atom to that of hydrogen [13]. From earlier works on transition and rare-earth metal hydrides, it has been shown that the vibrations of hydrogen occupying the oc tahedral tetrahedral and interstitials are well separated in the energy loss ranges of 40 < −70 meV and 100–140 meV respectively [17,18,19]. The vibrational properties of hydrogen have been studied using INS in a series of HEA-based metal hydrides starting with the quaternary TiVNbHx and subsequently adding Zr and Hf to increase the chemical disorder in the system. TiZrHf was chosen due to its ability to form a metal hydride with bodycentered tetragonal (bct) lattice

Experimental

Results and discussion

Conclusions