HEA synthesis. Due to the low melting temperature of Mg and Al and the high vapour pressure of Mg, the selected compositions were directly synthesized as hydride phases following a two step procedure: first, pre-alloys of refractory elements were prepared by arc melting followed by reactive ball milling (RBM) of the pre-alloy with the Mg/Al in powder form under high H 2 pressure. The pre-alloys TiVCr, TiVCrNb and TiVZrNbHf were synthesized by arc melting under Ar atmosphere from lumps of Ti (Neyco, 99.99% metals basis), V (Neyco, 99.9% metals basis), Cr (Alfa Aesar, 99.99% metals basis), Zr (Neyco, 99.95% metals basis), Nb (Alfa Aesar, 99.95% metals basis) and Hf (Alfa Aesar, 99.9% metals basis). They were flipped and remelted 12 times to ensure chemical homogeneity. The pre-alloys were crushed in pieces by hammering and further used in the second step of the synthesis. The reactive ball milling of Mg (Alfa Aesar, 99.8% metals basis) and Al (Strem Chemicals, 99% metals basis) powders and the pre-alloy was performed under 70 bar H 2 pressure for 120 minutes process following the procedure reported elsewhere. During the initial filling of the RBM vial with H 2 gas, the pre-alloys TiVCr and TiVCrNb spontaneously absorb hydrogen at room temperature forming hydrides. Thus, the hydrogen content in the final compositions MgAlTiVCr and MgTiCrNb is calculated based on two separate reactions: the hydrogenation of the pre-alloy at room temperature and the hydrogen absorption during the RBM process. The TiVNbZrHf pre-alloy did not show any hydrogen absorption during the initial filling of the vial, in agreement with our previous results in reference. For this last sample, we have demonstrated the repeatability of the twostep synthesis by performing the same procedure several times and obtaining similar results in terms of both crystalline lattice of the hydride and its hydrogen content.