High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure

Hydrogen-Vacancy-Induced Stable Superconducting Niobium Hydride at High Pressure

In recent years, the discovery of unconventional polyhydrides under high pressure, including notable instances like CaH6, YH9, and LaH10, with superconducting critical temperature (Tc) above 200 K, has ignited considerable interest in the quest for high-temperature superconductivity in hydrogen-based materials. Recent studies have suggested the highly probable existence of hydrogen vacancies in these high-Tc superconducting hydrides, although there is no conclusive evidence. In this study, taking niobium (Nb) hydride as a model, we showcase the observation of nonstoichiometric face-centered cubic (fcc) NbH4-δ (δ∼0.23-0.51) at pressures ranging from 113 to 175 GPa, employing in situ high-pressure X-ray diffraction experiments in conjunction with first-principles calculations. Remarkably, our further analyses indicate that the hydrogen vacancies, along with the resulting configurational entropy, play crucial roles in stabilizing this nonstoichiometric fcc NbH4-δ. Electrical transport measurements confirmed the superconductivity, as evidenced by zero resistance as well as suppression of Tc with applying magnetic fields, with a Tc reaching up to 34 K. Our current results not only confirm the presence of hydrogen vacancies in high-Tc hydrides, but also provide key insights into the understanding of hydrogen-vacancy-induced stability for nonstoichiometric hydrides under high pressure.

Emerging superconductivity rules in rare-earth and alkaline-earth metal hydrides

Hydrides of alkaline-earth and rare-earth metals have garnered significant interest in high-temperature superconductor research due to their excellent electron-phonon coupling and high T c upon pressurization. This study explores the electronic structures and electron-phonon coupling of metal hydrides XHn (n = 4,6), where X includes Ca, Mg, Sc, and Y. The involvement of d-orbital electrons alters the Fermi surface, leading to saddle-point nesting and a charge density wave (CDW) phase transition, which opens the superconducting gap. For instance, in YH6, the exchange coupling between Y-4d and H-1s holes in the phonon softening region results in T c values up to 230 K. The study suggests that factors, such as the origin of the CDW order, hydrogen concentration, and d-orbital contributions are crucial to superconductivity. This work proposes a new rule for high T c superconductors, emphasizing the importance of double gaps and electron-phonon interactions at exchange coupling sites, and predicts potential high-quality superconductors among rare-earth hydrides.

Enhancing hydrogen positions in X-ray structures of transition metal hydride complexes with dynamic quantum crystallography

Hirshfeld atom refinement (HAR) is a method which enables the user to obtain more accurate positions of hydrogen atoms bonded to light chemical elements using X-ray data. When data quality permits, this method can be extended to hydrogen-bonded transition metals (TMs), as in hydride complexes. However, addressing hydrogen thermal motions with HAR, particularly in TM hydrides, presents a challenge. At the same time, proper description of thermal vibrations can be vital for determining hydrogen positions correctly. In this study, we employ tools such as SHADE3 and Normal Mode Refinement (NoMoRe) to estimate anisotropic displacement parameters (ADPs) for hydrogen atoms during HAR and IAM refinements performed for seven structures of TM (Fe, Ni, Cr, Nb, Rh and Os) and metalloid (Sb) hydride complexes for which both the neutron and the X-ray structures have been determined. A direct comparison between neutron and HAR/SHADE3/NoMoRe ADPs reveals that the similarity between neutron hydrogen ADPs and those estimated with NoMoRe or SHADE3 is significantly higher than when hydrogen ADPs are refined with HAR. Regarding TM-H bond lengths, traditional HAR exhibits a slight advantage over the other methods. However, combining NoMoRe/SHADE3 with HAR results in a minor decrease in agreement with neutron TM-H bond lengths. For the Cr complex, for which high-resolution X-ray data were collected, an investigation of resolution-related effects was possible.

On the use of Monkhorst-Pack scheme to evaluate superconductivity and the issue of umklapp electron-phonon interactions

The Monkhorst-Pack scheme is a method to save time in the days of slow computers. It excludes umklapp phonons with significant consequences. Its widespread application to evaluate superconductivity arises from the desire to reduce phonon contributions to solve a historical difficulty of the BCS theory. An alternative method turns out to be more accurate in Pb and Pd.

Hirshfeld Atom Refinement of Metal-Organic Complexes: Treatment of Hydrogen Atoms Bonded to Transition Metals

Hydrogen positions in hydrides play a key role in hydrogen storage materials and high-temperature superconductors. Our recently published study of five crystal structures of transition-metal-bound hydride complexes showed that using aspherical atomic scattering factors for Hirshfeld atom refinement (HAR) resulted in a systematic elongation of metal-hydrogen bonds compared to using spherical scattering factors with the Independent Atom Model (IAM). Even though only standard-resolution X-ray data was used, for the highest-quality data, we obtained excellent agreement between the X-ray and the neutron-derived bond lengths. We present an extended version of this study including 10 crystal structures of metal-organic complexes containing hydrogen atoms bonded to transition-metal atoms for which both X-ray and neutron data are available. The neutron structures were used as a benchmark, and the X-ray structures were refined by applying Hirshfeld atom refinement using various basis sets and DFT functionals in order to investigate the influence of the technical aspects on the length of metal-hydrogen bonds. The result of including relativistic effects in the Hamiltonian and using a cluster of multipoles simulating interactions with a crystal environment during wave function calculations was examined. The effect of the data quality on the final result was also evaluated. The study confirms that a high quality of experimental data is the key factor allowing us to obtain significant improvement in transition metal (TM)-hydrogen bond lengths from HAR in comparison with the IAM. Individual adjustments and better choices of the basis set can improve hydrogen positions. Average differences between TM-H bond lengths obtained with various DFT functionals upon including relativistic effects or between double-ζ and triple-ζ basis sets were not statistically significant. However, if all bonds formed by H atoms were considered, significant differences caused by different refinement strategies were observed. Finally, we examined the refinement of atomic thermal motions. Anisotropic refinement of hydrogen thermal motions with HAR was feasible only in some cases, and isotropically refined hydrogen thermal motions were in similar agreement with neutron values whether obtained with HAR or with the IAM.

Room-Temperature Superconductivity in Yb/Lu Substituted Clathrate Hexahydrides under Moderate Pressure

Room temperature superconductivity is a dream that mankind has been chasing for a century. In recent years, the synthesis of H₃S, LaH₁₀, and C-S-H compounds under high pressures has gradually made that dream become a reality. But the extreme high pressure required for stabilization of hydrogen-based superconductors limit their applications. So, the next challenge is to achieve room-temperature superconductivity at significantly low pressures, even ambient pressure. In this work, we design a series of high temperature superconductors that can be stable at moderate pressures by incorporating heavy rare earth elements Yb/Lu into sodalite-like clathrate hexahydrides. In particular, the critical temperatures (T_c) of Y₃LuH₂₄, YLuH₁₂, and YLu₃H₂₄ can reach 283 K at 120 GPa, 275 K at 140 GPa, and 288 K at 110 GPa, respectively. Their critical temperatures are close to or have reached room temperature, and minimum stable pressures are significantly lower than that of reported room temperature superconductors. Our work provides an effective method for the rational design of low-pressure stabilized hydrogen-based superconductors with room-temperature superconductivity simultaneously and will stimulate further experimental exploration.

Compressed superhydrides: the road to room temperature superconductivity

Room-temperature superconductivity has been a long-held dream and an area of intensive research. The discovery of H₃S and LaH₁₀under high pressure, with superconducting critical temperatures (T_c) above 200 K, sparked a race to find room temperature superconductors in compressed superhydrides. In recent groundbreaking work, room-temperature superconductivity of 288 K was achieved in carbonaceous sulfur hydride at 267 GPa. Here, we describe the important attempts of hydrides in the process of achieving room temperature superconductivity in decades, summarize the main characteristics of high-temperature hydrogen-based superconductors, such as hydrogen structural motifs, bonding features, electronic structure as well as electron-phonon coupling etc. This work aims to provide an up-to-date summary of several type hydrogen-based superconductors based on the hydrogen structural motifs, including covalent superhydrides, clathrate superhydrides, layered superhydrides, and hydrides containing isolated H atom, H₂and H₃molecular units.

Reply to the 'Comment on "High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure"' by X. Zheng and J. Zheng, Phys. Chem. Chem. Phys., 2022, , DOI: 10.1039/D1CP01474A

Our paper is concerned with the specific hydrogen compound MoH₁₁. The authors of the Comment advocate investigating the role of umklapp processes (UP). For the hydrogen compounds, the main contribution to the strength of the pairing interaction is provided not by acoustic, but by optical phonons. This key factor leads to a diminishing role of the UP for the compound of interest.

Comment on “High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure” by M. Du, Z. Zhang, H. Song, H. Yu, T. Cui, V. Z. Kresinc and D. Duan, Phys. Chem. Chem. Phys., 2021, , 6717

In superconductors, scattered electrons cover the entire surface of the Fermi sphere (circle in the figure, valency = 3). In the MP scheme in the article concerned, the shaded wedge confines coverage, causing errors in results.

Pressure-induced superconducting CS2H10 with an H3S framework

The discovery of the high-temperature superconducting state in the compounds of hydrogen, carbon and sulfur with a critical temperature (T_c) of 288 K at high pressure is an important milestone towards room-temperature superconductors. Here, we have extensively investigated the high-pressure phases of CS₂H₁₀, and found four phases Cmc2₁, P3m1, P3̄m1 and Pm. Among them, P3m1 can be dynamically stable at a pressure as low as 50 GPa, and Cmc2₁ has a high T_c of 155 K at 150 GPa. Both Cmc2₁ and P3m1 are host-guest hydrides, in which CH₄ molecules are inserted into Im3̄m-H₃S and R3m-H₃S sublattices, respectively. Their T_c is dominated by the H₃S lattice inside. The insertion of CH₄ molecules greatly reduces the pressure required for the stability of the original H₃S lattice, but it has a negative impact on superconductivity which cannot be ignored. By studying the effect of CH₄ insertion in the H₃S lattice, we can design hydrides with a T_c close to that of H₃S and a greatly reduced pressure required for stability.