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

Abstract

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.