Design of a 60.8 K superconducting hydride LiMgZr2H12 at ambient pressure via Lithium doping

High-pressure hydrogen-rich compounds have long been regarded as promising room-temperature superconductor candidates; however, their practical applications are limited by their reliance on extreme compression. This study explores hydrogen-rich superconductors that may be stable at ambient pressures. Inspired by recent investigations of the MgZrH2n family, the LiMgZr2H12 structure with a Pmmm symmetry was constructed, and its thermodynamic, mechanical, and dynamical stability were evaluated using first-principles calculations. Electron-phonon coupling (EPC) analysis suggests that LiMgZr2H12 reaches a superconducting critical temperature (Tc) of 60.8 K at ambient pressure. Compared with MgZrH6, Li doping significantly increases the contribution of hydrogen atoms to the electron density of states near the Fermi level (EF) and enhances the EPC constant of the LiMgZr2H12 structure. LiMgZr2H12 exhibits a superconducting figure of merit of 1.56, which is significantly greater than that of MgZrH6, demonstrating its outstanding potential for practical applications. This work guides ambient-pressure design of high-Tc hydrides.

💡 Research Summary

The paper addresses the longstanding challenge of realizing high‑temperature superconductivity in hydrogen‑rich materials without the need for extreme pressures. Building on the Mg–Zr–H family, the authors construct a quaternary hydride, LiMgZr₂H₁₂, by substituting one Mg atom with Li in a 1 × 1 × 2 supercell of the known MgZrH₆ (Pm‑3) structure, resulting in a Pmmm‑symmetry crystal with lattice parameters a = 3.7833 Å, b = 3.7853 Å, c = 7.5389 Å.

First‑principles calculations using DFT‑PBE (VASP) with a 600 eV plane‑wave cutoff and dense k‑point sampling confirm that the formation energy is negative and that all six independent elastic constants satisfy the Born stability criteria, establishing thermodynamic and mechanical stability. Phonon dispersion computed via PHONOPY shows no imaginary frequencies, indicating dynamical stability at ambient pressure.

Electronic structure analysis reveals a high density of states at the Fermi level (N(E_F) ≈ 2.34 states eV⁻¹ atom⁻¹), dominated by hydrogen and zirconium contributions. The ELF and Bader charge analyses demonstrate predominantly ionic metal–hydrogen bonding, with Li, Mg, and Zr donating roughly 0.86 e, 1.63 e, and 1.66 e respectively, while each H atom gains 0.41–0.55 e. COHP calculations show strong Zr–H bonding (deep negative peaks) and significant Li–H and Mg–H bonding, whereas H–H interactions are negligible, confirming monatomic hydrogen.

Electron‑phonon coupling (EPC) calculations performed with Quantum ESPRESSO (24 × 24 × 12 k‑mesh, 6 × 6 × 3 q‑mesh) yield an EPC constant λ = 2.22 and a logarithmic average phonon frequency ω_log ≈ 396 K. The low‑frequency (0–9 THz) phonons, mainly involving Li, Mg, and Zr, contribute 48 % of λ, while the intermediate‑to‑high‑frequency (9–40 THz) hydrogen‑dominated modes contribute the remaining 52 %. Using the Allen‑Dynes modified McMillan formula with a Coulomb pseudopotential μ* = 0.10, the estimated superconducting critical temperature is Tc ≈ 60.8 K at ambient pressure. This Tc is comparable to the 61 K predicted for MgZrH₆ at 36 GPa, but the pressure requirement is eliminated.

The superconducting figure of merit S = (Tc/Tc(MgB₂)) + (Pc/100 GPa) is calculated as 1.56 for LiMgZr₂H₁₂, about 34 % higher than that of MgZrH₆ (1.16) and surpassing many well‑known high‑pressure hydrides such as H₃S (1.27), YH₉ (1.19), and LaH₁₀ (1.43).

Band‑structure analysis shows van Hove singularities near the Fermi level at Y, S, and X points, enhancing the electronic DOS. Four bands cross the Fermi level; notably, the third band forms a rhombic closed surface with flat facets that are nearly parallel to those of the fourth band, creating nesting channels. This nesting can induce Kohn anomalies, further softening phonons and strengthening EPC, which rationalizes the high λ and Tc.

Regarding synthesis, the authors reference prior high‑pressure experiments that produced a related Mg–Zr–Li–H quaternary hydride (Fm‑3m) at 8 GPa and 873 K via 6 MgH₂ + ZrH₂ + n LiH. They propose a possible ambient‑pressure route: LiMgZr₂H₁₂ → MgH₂ + LiH + 2 ZrH₂ + 5/2 H₂, noting that the left‑hand side is about 170 meV/atom higher, indicating metastability. They suggest that kinetic barriers could be overcome with catalysts, rapid quenching, sealed ampoules, or inert‑gas handling.

In summary, lithium doping of the MgZrH₆ framework yields a quaternary hydride that is thermodynamically, mechanically, and dynamically stable at ambient pressure, exhibits a high EPC constant, and achieves a predicted Tc of 60.8 K with a superior figure of merit. The work demonstrates that strategic elemental substitution can shift the Fermi level into bonding regions, increase hydrogen‑derived DOS, and promote favorable phonon characteristics, thereby offering a viable pathway toward practical, high‑temperature superconductors without extreme compression.