A Metastable CaSH3 Phase Composed of HS Honeycomb Sheets that is Superconducting Under Pressure

Prediction of high-Tc superconductivity in H6SX below megabar pressure

The discovery of high critical temperature in cubic H3S under high pressures attracts significant attention. Of particular recent interest is to search for superconductors stabilized at low pressure in order to approach practical applications. To examine the doping effect on the stability and superconductivity of H3S, we construct H6SX by replacing half of the S atoms with an X atom (X = Si, Ge, Sn, P, As, Sb, Te, Cl, Br and I). In addition to previously reported compounds, we identified seven new superconductors that are dynamically stable below 200 GPa, e.g., Fd3̄m H6SAs, H6SSb, H6SI, H6SSn, H6STe, Cmmm H6SCl, H6SBr. Among these, H6SSb exhibits Tc of 91 K at 200 GPa, and increases to 115 K as pressure decreases to 40 GPa. Electron-phonon analysis indicates that the coupling between the electron of Sb atoms and the associated low-frequency phonons is responsible for the enhancement of the superconductivity. Additionally, the superconductivity of H6SSX is closely related to the electronegativity of X in the same main group, where X with a larger electronegativity results in a strong H-X bond, resulting in a higher superconductivity.

Why Mg2IrH6 Is Predicted to Be a High-Temperature Superconductor, But Ca2IrH6 Is Not

The X2MH6 family, consisting of an electropositive cation Xn+ and a main group metal M octahedrally coordinated by hydrogen, have been identified as promising templates for high-temperature conventional superconductivity. Herein, we analyze the electronic structure of two members of this family, Mg2IrH6 and Ca2IrH6, showing why the former may possess superconducting properties rivaling those of the cuprates, whereas the latter does not. Within Mg2IrH6 the vibrations of the anions IrH6 4- anions are key for the superconducting mechanism, and they induce coupling in thee g * ${e_g^{\ast} }$ set of orbitals, which are antibonding between the H 1s and the Ird x 2 - y 2 ${d_{x^2 - y^2 } }$ ord z 2 ${d_{z^2 } }$ orbitals. Because calcium possesses low-lying d-orbitals,e g * ${e_g^{\ast} }$ →Ca d back-donation is preferred, quenching the superconductivity. Our analysis explains why high critical temperatures were only predicted for second or third row X metal atoms, and may provide rules for identifying likely high-temperature superconductors in other systems where the antibonding anionic states are filled.

A systematic study on the phase diagram and superconductivity of ternary clathrate Ca-Sc-H at high pressures

This work explores potential high-temperature superconductor materials in hydrogen-rich systems. Here, the crystal structure stabilities of ternary Ca-Sc-H systems under high-pressure (P = 100-250 GPa) and their superconductivities are investigated using the particle swarm optimization methodology combined with first-principles calculations. For the predicted candidate structures of Ca-Sc-H systems, the pressure-dependent phase diagram and thermodynamic convex hull were investigated across a wide range of compositions; the electronic properties of all the predicted phases were analyzed in detail to study the bonding behavior of these stable phases. We identified the crystal structures of four thermodynamically stable compounds: R3̄m-CaScH6, Immm-CaSc2H9,C2/m-Ca2ScH10, and R3̄m-CaScH12. Among them, R3̄m-CaScH12 was predicted to have the highest Tc value (i.e., 173 K) at 200 GPa. The discovery of this previously unreported pressure-induced decomposition of Ca-Sc-H systems will pave the way for investigations on the nature of hydrogen-metal interactions.

Prediction for high superconducting ternary hydrides below megabar pressure

The recent findings of high-temperature hydrides ushered a new era of superconductivity research under high pressure. However, the stable pressure for these remarkable hydrides remains extremely high. In this work, we performed the extensive simulations on a series of hydrides with the prototype structure of UH₈and UH₇. Our results indicate several compounds possess superconducting critical temperature (T_c) above liquid nitrogen temperature below 100 GPa, such as CeBeH₈and ThBeH₈that are dynamical stable with aT_cof 201 K at 30 GPa and aT_cof 98 K at 10 GPa, respectively. Further formation enthalpy calculations suggest that thermodynamical stable pressure of CeBeH₈and ThBeH₈compounds is above 50 GPa and 88 GPa with respect to binary compounds and solid elements. Moreover, we also found that ThBeH₇could be dynamically stable down to 20 GPa with aT_cof 70 K. Our further simulations suggested this newly predicted ThBeH₇is thermodynamically stable above pressure of 33 GPa with respect to binary compounds and solid elements. The present results shed light on future design and discovery of high-temperature superconductor at moderate pressure.

Study on superconducting Li-Se-H hydrides

The structures, stabilities and superconducting properties of LiSeH_n (n = 4-10) hydrides at 150-300 GPa were studied by the genetic algorithm (GA) and DFT calculation method. Three stable stoichiometries of LiSeH₄, LiSeH₆ and LiSeH₁₀ were uncovered under high pressure. Four other metastable stoichiometries of LiSeH₅, LiSeH₇, LiSeH₈, and LiSeH₉ were also studied. By analyzing the electronic band structure and electronic density of states, C2 LiSeH₄, Pmm2 LiSeH₆ and C2 LiSeH₁₀ were all found to be metal phases above 150 GPa. Electron-phonon coupling calculations showed that C2 LiSeH₄ and Pmm2 LiSeH₆ were promising superconductors. The predicted T_c values of C2 LiSeH₄ and Pmm2 LiSeH₆ were 77 K at 200 GPa and 111 K at 250 GPa, respectively.

The 2021 room-temperature superconductivity roadmap

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms.In memoriam, to Neil Ashcroft, who inspired us all.

Role of van der Waals forces in the metal-insulator transition of transition metal oxides

Transition metal oxides (TMOs) exhibit great potential in technological applications due to their ability to undergo a rapid metal-insulator transition (MIT). However, the phase stability of TMOs, which models the on/off voltages of electronic devices, remains controversial due to the incomplete knowledge of the determinants of its stability. Herein, we study the effect of van der Waals (vdW) interactions on the phase stability of TMOs by employing the pairwise and screened vdW methods. Our calculations manifest that the vdW interactions are crucial to the TMOs' phase stability and tend to stabilize the insulating phase. Furthermore, the long-range electrodynamic screening interactions correct the TMOs' phase stability by revising the vdW term.

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

Tetrahydrides crystallizing in the ThCr₂ Si₂ structure type have been predicted to become stable for a plethora of metals under pressure, and some have recently been synthesized. Through detailed first-principles investigations we show that the metal atoms within these I 4 / m m m symmetry MH₄ compounds may be divalent, trivalent or tetravalent. The valence of the metal atom and its radius govern the bonding and electronic structure of these phases, and their evolution under pressure. The factors important for enhancing superconductivity include a large number of hydrogenic states at the Fermi level, and the presence of quasi-molecular H 2 δ - units whose bonds have been stretched and weakened (but not broken) via electron transfer from the electropositive metal, and via a Kubas-like interaction with the metal. Analysis of the microscopic mechanism of superconductivity in MgH₄ , ScH₄ and ZrH₄ reveals that phonon modes involving a coupled libration and stretch of the H 2 δ - units leading to the formation of more complex hydrogenic motifs are important contributors towards the electron phonon coupling mechanism. In the divalent hydride MgH₄ , modes associated with motions of the hydridic hydrogen atoms are also key contributors, and soften substantially at lower pressures.

The Li-F-H ternary system at high pressures

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li-F-H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF₃H₂, LiF₂H, Li₃F₄H, LiF₄H₄, Li₂F₃H, and LiF₃H lying within 50 meV/atom of the 0 K convex hull. All of these phases contain H_nF_n+1 ^- (n = 1, 2) anions and Li⁺ cations. Other structural motifs such as LiF slabs, H₃ ⁺ molecules, and F^δ- ions are present in some of the low enthalpy Li-F-H structures. The bonding within the H_nF_n+1 ^- molecules, which may be bent or linear, symmetric or asymmetric, is analyzed. The five phases closest to the hull are insulators, while LiF₃H is metallic and predicted to have a vanishingly small superconducting critical temperature. Li₃F₄H is predicted to be stable at zero pressure. This study lays the foundation for future investigations of the role of temperature and anharmonicity on the stability and properties of compounds and alloys in the Li-F-H ternary system.