In binary hydrides with a cubic close packed (FCC) structure such as TiH 2 and ZrH 2 , the hydrogen is placed in tetrahedral interstitial sites. The observed H/M ratio of 2.5 in the hydrogenated TiVZrNbHf alloy requires that both tetrahedral sites and about 50% of the octahedral sites are filled with hydrogen. This behaviour is unique and has never been observed before in pure transition metal hydrides. Alloys with both transition metals and rare earth (RE) metals (lanthanides and yttrium) are known to form hydrides with H/M > 2 but our results indicate that using an HEA could be a feasible and useful strategy to avoid inclusion of RE elements in hydrogen storage materials.

The results above clearly show that the TiVZrNbHf alloy has a different behaviour to that of ‘normal’ transition metals. The archetypical structural transition upon creating a hydride is the formation of a FCC lattice in the fully hydrogenated form (MH 2 ). However for intermediate hydrogen content, a distorted BCC phase has been observed in many transition metal-hydrogen systems (BCC → distorted BCC (BCT) → FCC)3,19,20. This type of lattice distortion, illustrated in Fig. 4. with an elongation of one of the cube axes has been observed in the α-β transition in the V-H system, where the c-axis is elongated by about 10%13. The elongation is assumed to be caused by hydrogen initially occupying octahedral sites in the BCC structure followed by a transformation to a fully hydrogenated VH 2 phase with hydrogen in the tetrahedral sites. In the case of Ti, Zr and Hf, a tetragonal distortion of the FCC lattice (with c/a < 1) is observed for H concentrations above the critical values but with H/M < 213. Wang et al.21 investigated the instability of the FCC ZrH 2 lattice using electronic structure calculations. They found a stronger Zr-Zr bond in the FCT lattice compared to the FCC lattice that is explained by a planar type crystal field splitting in the FCT lattice and occupancy of the degenerate d yz and d xz orbitals.

Figure 4 Different routes for hydrogen absorption in metals. The BCC route (V-type behaviour, BCC → distorted BCC (BCT) → FCC) up to H/M = 2. The RE route (RE = La, Ce, Pr, Nd, dHCP → FCC → distorted FCC (BTC)) with H/M>2. HEA shows a combination of the two routes (BCC → distorted FCC (BCT)). Full size image

For the light rare-earth metals such as La, Ce, Pr, and Nd, which are able to form hydrides with H/M ratio >2, another transformation route is observed (see Fig. 4). In this case, the pure metal has a double hexagonal close packed (dHCP) structure without hydrogen. Upon hydrogenation, a FCC structure is initially formed for H/M < ~2.3. At higher hydrogen contents, a tetragonal distortion is observed (dHCP → FCC → distorted FCC (BTC))13. This tetragonal distortion has been shown by neutron diffraction on NdD 2.36 to result from formation of a super cell that is doubled in the c-direction22. In the case of CeH 2.48 the additional hydrogen atoms were found to occupy the octahedral sites in the structure23.

An astonishing observation is that the TiVZrNbHf alloy based only on transition metals indicates that an HEA can exhibit a combination of these two routes. Starting with a BCC-type structure we observe a distorted FCC lattice in our fully hydrogenated HEA, where the c-axis is larger than expected ( , see Fig. 4), and the cube is elongated in the c-direction by ~2%, suggesting a hydrogenation path similar to the light rare-earth metals (BCC → distorted FCC (BCT)). This requires that hydrogen can be placed in both octahedral and tetrahedral sites in the HEA in contrast to other transition metal hydrides. Similar intermediate structures as in the normal BCC-case (with initial distortion of the BCC lattice) has been observed during in situ hydrogenations and these results are currently being evaluated, the details of this is however outside the scope of this communication. We suggest that this is due to the presence of strain in the lattice due to variations in atomic radii.

An important parameter in prediction of HEA formation is the parameter δ defined as:

where c i are the atomic composition fractions and r i , the atomic radii, of each component. A high value of δ leads to a large lattice distortion and makes the HEA formation less favourable. Yang and Zhang24 have proposed that HEAs are formed when δ < 6.6% For TiVZrNbHf δ is 6.8% (atomic radius from25), which is slightly above the maximum value defined by Yang and Zhang. Hence, TiVZrNbHf is expected to exhibit a highly strained lattice. Xin et al. have observed hydrogen occupancy in previously unavailable sites in a vanadium thin film under biaxial compressive strain. Even at low hydrogen concentrations, a change from tetrahedral to octahedral occupancy is observed4. The built in strain in an HEA could be the driving force to open up new interstitial sites for hydrogen. This is in agreement with previous results on, for example, the Y-H system where a change from hexagonal to cubic crystal structure is observed at very high pressures (~77 kbar)26. Theoretical modelling using ab initio methods is needed to explain the influence of strain on the stability of hydride formation in these types of alloys.

In summary, we have studied the hydrogenation of the high entropy alloy TiVZrNbHf and observed that extremely large amounts of hydrogen can be absorbed. The observed maximum H/M ratio of 2.5 is similar to that observed in alloys based on rare-earth metals. The formation of a distorted FCC or BCT structure in the fully hydrogenated alloy is also similar to the structure formed in rare-earth compounds. It is suggested that the unprecedented hydrogen storage capacity is an effect of the strain in the distorted HEA lattice, which favours hydrogen occupying both tetrahedral and octahedral sites. The observed H/M ratio of 2.5 corresponds only to about 2.7 wt% H. This is due to the high mass of Hf and Zr. It is likely that much higher storage capacity by weight percent can be achieved with other HEAs by replacing these elements to appropriate lighter ones. Hence, we propose that HEAs can be used as a new class of alloy for hydrogen storage that does not involve any rare-earth metals.