Phase stability prediction in Ti–Zr–Hf–Ni–Fe–Co high entropy alloys for hydrogen storage: a comprehensive DFT and thermodynamic assessment

Abstract

Abstract High-entropy alloys (HEAs) based on early transition metals have attracted growing interest for solid-state hydrogen storage due to their compositional flexibility, lattice distortion, and tunable thermodynamics. However, the interplay between phase stability, crystal structure, and hydrogen storage performance in multicomponent HEAs remains only partially resolved, particularly for the specific stoichiometric compositions and structural prototypes examined here. In this work, a comprehensive density functional theory (DFT) and thermodynamic investigation is conducted on the Ti–Zr–Hf–Ni–Fe–Co system across four technologically relevant intermetallic prototypes: AB, A 2 B, AB 2 (Laves phase), and AB 5 . First-principles calculations combined with equation-of-state analysis and quasi-harmonic Debye modelling are employed to evaluate phase stability, elastic behaviour, electronic structure, and hydrogenation thermodynamics. The results reveal that systematic changes in stoichiometry strongly influence lattice distortion, mixing enthalpy, valence electron concentration, and hydrogen affinity. Among the investigated structures, AB 2 Laves-type phases exhibit the most favourable balance between thermodynamic stability, elastic compliance, and hydrogen binding enthalpy (− 22 to − 30 kJ mol −1 H 2 ), while maintaining moderate volume expansion upon hydrogenation. Simulated X-ray diffraction patterns confirm that hydrogen absorption in AB 2 alloys proceeds via elastic lattice expansion without phase transformation. Electronic density-of-states and band-structure analyses show pronounced H-1s–metal-d hybridization and preserved metallic conductivity, enabling efficient charge redistribution during hydrogen cycling. Vibrational thermodynamic analysis further demonstrates that AB 2 hydrides achieve an optimal balance between entropy stabilization and lattice integrity over a wide temperature range. Overall, this study identifies AB 2 -type Ti–Zr–Hf–Ni–Fe–Co high-entropy alloys as the most promising structural platform for reversible hydrogen storage among the four stoichiometric prototypes examined, under near-ambient thermodynamic conditions relevant to mobile storage applications, and provides a predictive framework for rational phase design in multicomponent hydride-forming alloys.