Prediction of ambient pressure conventional superconductivity above 80 K in hydride compounds

The quest for high-temperature superconductivity (HTS) has seen significant advancements, notably with the discovery of HTS in cuprates. However, achieving HTS at ambient pressure remains a major challenge. Conventional superconductors rely on electron-phonon coupling, where the interaction between electrons and lattice vibrations drives superconductivity. Reducing the mass of constituent elements, as in metallic hydrides, is a potential strategy to maximize phonon frequencies and thus achieve higher transition temperatures. Initial predictions focused on metallic hydrogen, but its metallization requires extremely high pressures. This led researchers to explore hydride compounds, where chemical pre-compression can achieve metallicity at lower pressures. High-temperature superconductivity has been demonstrated in several hydrogen-rich hydrides under extreme pressures, such as H₃S, LaH₁₁, and CaH₉, but the critical temperatures are still obtained under extreme pressures. The current challenge is to achieve this at or near ambient pressure. Previous attempts to predict ambient-pressure hydride superconductors have yielded promising results such as AlH₃ (up to 54 K) and (Be₂H₆)ₚ (72–84 K), but thermodynamic instability hinders experimental realization. This research proposes a new family of compounds to address this challenge.

The literature review focuses on previous attempts to achieve high-temperature superconductivity in hydrides. Early work centered on metallic hydrogen, but the extreme pressures required for metallization proved difficult. The focus shifted to hydride compounds, where chemical pre-compression lowers the pressure needed for metallicity. Several high-temperature superconductors were discovered under extreme pressure (H₃S, LaH₁₁, CaH₉ etc.). The paper mentions previous predictions of hydrides exhibiting superconductivity, such as AlH₃ and (Be₂H₆)ₚ. However, these materials are thermodynamically unstable, making their experimental synthesis challenging. The work highlights the scarcity of successful ambient pressure high-temperature hydride superconductors which motivated the current study.

The study employed a machine-learning accelerated high-throughput workflow to screen over one million compounds from the Alexandria database. This database contains experimental and hypothetical compounds, focusing on those near the convex hull of thermodynamic stability, making them more likely to be experimentally synthesizable. Geometry optimizations and total energy calculations were performed using VASP (Vienna Ab initio Simulation Package) with the Perdew-Burke-Ernzerhof (PBE) functional. Phonon calculations were conducted using density functional perturbation theory (DFPT) within the Quantum ESPRESSO package. Anharmonic effects were investigated using the stochastic self-consistent harmonic approximation (SSCHA) to assess dynamic stability. Superconductivity was evaluated using three methods: isotropic and anisotropic superconducting density-functional theory (SCDFT) and isotropic Eliashberg theory. These methods were used to calculate the electron-phonon coupling constant (λ), logarithmic average phonon frequency (⟨ωln⟩), density of states at the Fermi level (Nf), transition temperature (Tc), and superconducting gap (Δ).

The research identified a family of Mg₂XH₈ (X = Rh, Ir, Pd, or Pt) compounds as promising candidates for ambient pressure HTS. The compounds exhibit metallic behavior with a peculiar electronic structure near the Fermi level, characterized by Van Hove singularities. Phonon calculations revealed stable phonon modes with significant contributions from hydrogen modes. The electron-phonon coupling is dominated by the low and mid-frequency phonon spectrum, particularly hydrogen modes, leading to strong coupling. Anharmonic calculations using the SSCHA method showed that anharmonicity only slightly affects the phonon spectrum. Superconductivity calculations using three distinct methods (isotropic SCDFT, anisotropic SCDFT, and isotropic Eliashberg) yielded consistent results. Predicted Tc values range from 45–80 K, with the Pt compound potentially reaching above 100 K with appropriate doping. The compounds are predicted to be thermodynamically stable, making them suitable for experimental synthesis and characterization. The electronic structure suggests that these systems are sensitive to doping, potentially enabling further Tc enhancement.

The findings address the long-standing challenge of achieving HTS at ambient pressure in hydrides. The predicted high Tc values and thermodynamic stability of the Mg₂XH₈ family make them promising candidates for experimental investigation. The strong electron-phonon coupling driven by hydrogen modes is key to the high Tc. The sensitivity to doping provides avenues for further improvement. The consistency of results across different computational methods strengthens the prediction. The comparison with a recent preprint (Dolui et al.) highlighting a discrepancy in Tc prediction for Mg₂PtH₈ is discussed, attributing the difference to variations in electron-phonon coupling estimations and treatment of Coulomb interactions. This highlights the sensitivity of these systems to computational details and the need for further refinement in these calculations.

This study predicts a family of thermodynamically stable hydrides (Mg₂XH₈, X=Rh, Ir, Pd, Pt) exhibiting conventional high-temperature superconductivity at ambient pressure. The predicted Tc values are significant, potentially reaching above 100 K with doping. These findings suggest that ambient-pressure HTS in hydrides, while rare, is attainable with careful material selection and design. Further experimental investigation is crucial to validate these predictions and explore the potential of these materials for technological applications. Future research could focus on experimental synthesis and characterization of these compounds, investigation of doping strategies for further Tc enhancement, and detailed studies of the electronic and vibrational properties under various conditions.

The study relies on density functional theory (DFT) calculations, which have inherent limitations, especially in accurately describing strongly correlated systems. While the SSCHA method accounts for anharmonicity, it might not capture all relevant anharmonic effects completely. The accuracy of predicted Tc values is contingent upon the accuracy of DFT-based estimations of the electron-phonon coupling, which is sensitive to computational parameters. The theoretical error bars associated with the various computational methods used need to be considered when interpreting the results. Experimental verification is necessary to confirm the theoretical predictions.