Electronic, phononic, and superconducting properties of FeHx (x = 1–6) at 150 GPa†

Metal hydrides have been studied extensively due to their intriguing superconductivity and potential hydrogen storage capacities. Here, the structural, electronic, phononic, and superconducting properties of iron hydrides FeH_x (x = 1–6) are examined based on density functional theory at a high pressure of 150 GPa. Our calculated formation enthalpy, elastic constants, and phonon spectra indicate that Fmm-FeH, I4/mmm-FeH₂, Pmm-FeH₃, Imma-FeH₄, I4/mmm-FeH₅, and P2/c-FeH₆ are all dynamically and mechanically stable and may be synthesized in experiments under such conditions. Increasing the hydrogen concentration, the shortest H–H distance is shortened from 2.34 Å (FeH) to 0.73 Å (FeH₆). Meanwhile, the resistance to elastic deformation decreases slightly from FeH to FeH₆ and the elastic anisotropies in these systems are evident. Using the Allen–Dynes-modified McMillan equation, the superconducting transition temperatures (T_cs) of FeH, FeH₃, and FeH₅ are estimated to be 0 K, indicating their conventional metal nature. Excitingly, the T_cs of FeH₂, FeH₄, and FeH₆ are predicted to be 1.24, 1.50, and 3.35 K, respectively. The electron–phonon coupling (EPC) values of FeH₂, FeH₄, and FeH₆ are 0.36, 0.36, and 0.40, respectively, indicating that these three systems are weak conventional superconductors. The EPC in them mainly originates from the Fe-3d orbitals and the vibrations of the Fe atoms. Our results reveal that the content of hydrogen has a significant influence on the electronic, phononic, and superconducting properties of iron hydrides and will supply instructions for exploring new binary or ternary hybrid superconductors at high pressure.