The average and local structure of TiVCrNbDx (x = 0, 2.2, 8) from total scattering and neutron spectroscopy

Abstract

The volumetric hydrogen density of 160 kg H/m 3 in TiVCrNbH 8 is among the highest for interstitial hy- drides, but the reported reversible capacity is only about 2/3 of the full theoretical capacity at room temperature. In the present work we have investigated the local structure in TiVCrNbD x , x = 0 , 2.2, 8 with the aim to unravel how the remaining sites can be destabilized with respect to hydrogen/deuterium occupation using total scattering measurements and Reverse Monte Carlo (RMC) structure modelling. Our analysis indicates that the partially desorbed deuteride ( x = 2 . 2 ) adopts a body-centred tetragonal struc- ture ( I4 /mmm ) where the deuterium atoms occupy both tetrahedral and octahedral interstices with low occupancies. There is a significantly higher portion of occupied sites with nearest-neighbour metals with low valence-electron concentration VEC . This observation is used to motivate strategies for further desta- bilization of the hydride. Inelastic neutron scattering (INS) and density functional theory (DFT) calcula- tions indicate that the vibrational density of states is very diverse in TiVCrNbH 2 . 4 , and it is suggested that the hydrogen atoms might be mobile between nearby interstices.