Short-Range Structure of Ti0.63V0.27Fe0.10D1.73 from Neutron Total Scattering and Reverse Monte Carlo Modelling

Abstract

Ti-V-based body-centered cubic (BCC) alloys have potential for large-scale hydrogen storage if expensive vanadium is substituted with much cheaper Fe-containing ferrovanadium. Use of ferrovanadium reduces the alloys' hydrogen storage capacity. This is puzzling since the amount of Fe is low and hydrogen atoms are accommodated in interstitial sites which are partly coordinated by Fe in many intermetallic compounds. The present work is aimed at finding a structural explanation for Fe-induced capacity loss in Ti-V alloys. Since such alloys and their hydrides are highly disordered without long-range occupational order of the different metal species, it was necessary to employ a technique which is sensitive to local structure. Neutron total scattering coupled with reverse Monte Carlo modelling was thus employed to elucidate short-range atomic correlations in Ti 0.63 V 0.27 Fe 0.10 D 1.73 from the pair distribution function. It was found that Fe atoms form clusters and that the majority of the vacant interstitial sites are within these clusters. These clusters take the same face-centered cubic structure as the Ti-V matrix in the deuteride and thus they are not simply unreacted Fe which has a BCC structure. The presence of Fe clusters is confirmed by transmission electron microscopy. Density functional theory calculations indicate that the clustering is driven by thermodynamics.

The Influence of Fe on the Structure and Hydrogen Sorption Properties of Ti-V-Based Metal Hydrides

“…Furthermore, the maximum hydrogen content decreases with increasing Fe-content. This is in agreement with the reported observation that Fe form hydrogen-free clusters within the alloy [15]. The gravimetric and manometric values are in agreement for z ≤ 0.1 when the initial pressure is limited to 20 bar H 2 .…”

“…For the Ti-rich part of the system it was recently demonstrated by neutron total scattering and transmission electron microscopy (TEM) that Fe form clusters in (Ti 0.7 V 0.3 ) 0.9 Fe 0.1 D 1.7 [15]. Thus, the maximum hydrogen storage capacity is reduced due to the fact that Fe does not form stable hydrides at ambient conditions.…”