Effective destabilization of both mono- and dihydride phases in TiVZrNbHf by Mo addition

TiVZrNbHf bcc high entropy alloy shows promising hydrogen storage capacity, but unfavourable thermodynamics of the hydride phases i.e., too stable hydrides requiring high temperatures for recovering the stored hydrogen. Mo addition in this composition ((TiVZrNbHf) 100- x Mo x x = 5, 10 and 16.666) preserves the bcc lattice, decreases the lattice parameter and improves the hydrogen absorption kinetics at room temperature. Moreover, it effectively destabilizes both the bct intermediate and full fcc hydride phases without significant affecting the maximum storage capacity (∼ 2.1 wt. %). The temperatures of successive phase transitions ( fcc → bct → bcc ) during deuterium desorption strongly reduce with increasing Mo content, as demonstrated by in situ neutron powder diffraction. Several entangled factors can be invoked to explain this thermal destabilization along with electronic structure, steric and electronegativity effects. Therefore, Mo can be proposed as one of the most effective boosting elements to be added in HEAs for hydrogen storage.