Recent discovery of superconductive rare earth/actinide superhydrides has ushered in a new era of superconductivity research at high pressures. This distinct type of clathrate metal hydrides was first proposed for alkaline-earth-metal hydride CaH6 that, however, has long eluded experimental synthesis, impeding an understanding of pertinent physics. Here, we report successful synthesis of CaH6 and its measured superconducting critical temperature Tc of 215 K at 172 GPa, which is evidenced by a sharp drop of resistivity to zero and a characteristic decrease of Tc under a magnetic field up to 9 T.An estimate based on the Werthamer-Helfand-Hohenberg model gives a giant zero-temperature upper critical magnetic field of 203 T. These remarkable benchmark superconducting properties place CaH6 among the most outstanding high-Tc superhydrides, marking it as the hitherto only clathrate metal hydride outside the family of rare earth/actinide hydrides. This exceptional case raises great prospects of expanding the extraordinary class of high-Tc superhydrides to a broader variety of compounds that possess more diverse material features and physics characteristics.
The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-$T_{\rm c}$ superconductivity at high pressure. Recently, cerium hydride CeH$_9$ composed of Ce-encapsulated clathrate H cages was synthesized at much lower pressures of 80$-$100 GPa, compared to other experimentally synthesized rare-earth hydrides such as LaH$_{10}$ and YH$_6$. Based on density-functional theory calculations, we find that the Ce 5$p$ semicore and 4$f$/5$d$ valence states strongly hybridize with the H 1$s$ state, while a transfer of electrons occurs from Ce to H …
We report ab-initio calculations of the superconducting properties of two high-Tc sodalite-like clathrate yttrium hydrides, YH6 and YH10, within the fully anisotropic ME theory, including Coulomb corrections. For both compounds we find almost isotropic superconducting gaps, resulting from a uniform distribution of the electron-phonon coupling over phonon modes and electronic states of mixed Y and H character. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential mu*= 0:11, at odds with claims of unusually large Tc in lanthanum hydrides. The corresponding critical …
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 system has gradually made this dream a reality. But the extreme pressures required for the metallization of hydrogen-based superconductors limit their applications. 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 hydrides. In particular, the critical temperatures of Y3LuH24, YLuH12 and YLu3H24 are 283 K, 275 K an…
Recently, the experimental discovery of high-$T_c$ superconductivity in compressed hydrides H$_3$S and LaH$_{10}$ at megabar pressures has triggered searches for various superconducting superhydrides. It was experimentally observed that thorium hydrides, ThH$_{10}$ and ThH$_9$, are stabilized at much lower pressures compared to LaH$_{10}$. Based on first-principles density-functional theory calculations, we reveal that the isolated Th frameworks of ThH$_{10}$ and ThH$_9$ have relatively more excess electrons in interstitial regions than the La framework of LaH$_{10}$. Such interstitial excess …
Recently, the thorium hydride ThH9 possessing an H-rich clathrate structure has been experimentally synthesized to exhibit a superconducting transition temperature Tc of 146 K at 170-175 GPa, while the more H-rich clathrate thorium hydride ThH18 was theoretically predicted to reach a Tc of 296 K at 400 GPa. Using first-principles calculations, we find that ThH9 has a more ionic character between Th atoms and H cages than ThH18 and that the latter has a more substantial hybridization of the Th 6p semicore and H 1s states than the former. These different bonding characteristics of ThH9 and ThH18…
Lanthanum hydride LaH$_{10}$ with a sodalitelike clathrate structure was experimentally realized to exhibit a room-temperature superconductivity under megabar pressures. Based on first-principles calculations, we reveal that the metal framework of La atoms has the excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Further, the interaction between La atoms and H cages induces a delocalizatio…
The high critical superconducting temperatures ($T_c$s) of metal hydride phases with clathrate-like hydrogen networks have generated great interest. Herein, we employ the Density Functional Theory-Chemical Pressure (DFT-CP) method to explain why certain electropositive elements adopt these structure types, whereas others distort the hydrogenic lattice, thereby decreasing the $T_c$. The progressive opening of the H$_{24}$ polyhedra in MH$_6$ phases is shown to arise from internal pressures exerted by large metal atoms, some of which may favor an even higher hydrogen content that loosens the met…
Over the past decade, a combination of crystal structure prediction techniques and experimental synthetic work has thoroughly explored the phase diagrams of binary hydrides under pressure. The fruitfulness of this dual approach is demonstrated in the recent identification of several superconducting hydrides with $T_c$s approaching room temperature. We start with an overview of the computational procedures for predicting stable structures and estimating their propensity for superconductivity. A survey of phases with high $T_c$ reveals some common structural features that appear conducive to the…
Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & rare-earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell combined with in-situ laser heating technique. The superconducti…
Recently, an alkaline earth hydride CaH$_6$ having a sodalitelike clathrate structure has been experimentally synthesized at megabar pressures with a maximum $T_c$ of 215 K, comparable to that of a rare earth hydride LaH$_{10}$. Here, based on first-principles calculations, we find that CaH$_6$ exhibits a huge peak in the Eliashberg spectral function $α^{2}F$ around the low-frequency region of H-derived phonon modes, in contrast to LaH$_{10}$ having a widely spreading spectrum of $α^{2}F$ over the whole frequencies of H-derived phonon modes. It is revealed that the huge peak of $α^{2}F$ in CaH…
Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor, can reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low press…
X-ray Diffraction and Electrical Transport Imaging of Superconducting Superhydride (La,Y)H10
University of Illinois Chicago（伊利诺伊大学芝加哥分校）；Argonne National Laboratory（阿贡国家实验室）；University of Chicago（芝加哥大学）；Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）
Motivated by the recent discovery of near-room temperature superconductivity in high-pressure superhydrides, we investigate from first-principles the high-pressure superconducting phase diagram of the ternary Ca-B-H system, using ab-initio evolutionary crystal structure prediction, and Density Functional Perturbation Theory. We find that below 100 GPa all stable and weakly metastable phases are insulating. This pressure marks the appearance of several new chemically-forbidden phases on the hull of stability, and the first onset of metalization in CaBH$_5$. Metallization is then gradually achie…
The discovery of superconductivity at 203 K in H3S brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Dens…
Lanthanum, yttrium, and cerium hydrides are the three most well-known superconducting binary hydrides (La-H, Y-H, and Ce-H systems), which have gained great attention in both theoretical and experimental studies. Recent studies have shown that ternary hydrides composed of lanthanum and yttrium can achieve high superconductivity around 253 K. In this study we employ the evolutionary-algorithm-based crystal structure prediction (CSP) method and first-principles calculations to investigate the stability and superconductivity of ternary hydrides composed of (Y, Ce) and (La, Ce) under high pressure…
High Tc superconductivity in heavy Rare Earth Hydrides: correlation between the presence of the f states on the Fermi surface, nesting and the value of Tc
The discovery of high-temperature conventional superconductivity in H3S with a critical temperature of Tc=203 K was followed by the recent record of Tc ~250 K in the face-centered cubic (fcc) lanthanum hydride LaH10 compound. It was realized in a new class of hydrogen-dominated compounds having a clathrate-like crystal structure in which hydrogen atoms form a 3D framework and surround a host atom of rare earth elements. Yttrium hydrides are predicted to have even higher Tc exceeding room temperature. In this paper, we synthesized and refined the crystal structure of new hydrides: YH4, YH6, and…
The theoretical exploration of the phase diagrams of binary hydrides under pressure using \emph{ab initio} crystal structure prediction techniques coupled with first-principles calculations has led to the \emph{in silico} discovery of numerous novel superconducting materials. This Perspective article focuses on the alkaline earth and rare earth polyhydrides whose superconducting critical temperature, $T_c$, was predicted to be above the boiling point of liquid nitrogen. After providing a brief overview of the computational protocol used to predict the structures of stable and metastable hydrid…
Clathrate hydride \emph{Fm}\={3}\emph{m}-LaH$_{10}$ has been proven as the most extraordinary superconductor with the critical temperature $T_c$ above 250 K upon compression of hundreds of GPa in recent years. A general hope is to reduce the stabilization pressure and maintain the high $T_c$ value of the specific phase in LaH$_{10}$. However, strong structural instability distorts \emph{Fm}\={3}\emph{m} structure and leads to a rapid decrease of $T_c$ at low pressures. Here, we investigate the phase stability and superconducting behaviors of \emph{Fm}\={3}\emph{m}-LaH$_{10}$ with enhanced chem…
Revisiting Phase Stability and Superconductivity in Ca-H Superhydrides with Anharmonic Effects
College of Physics, Jilin University（吉林大学物理学院）；International Center of Future Science, Jilin University（吉林大学未来科学国际中心）；State Key Laboratory of Superhard Materials, College of Physics, Jilin University（吉林大学物理学院超硬材料全国重点实验室）
Utilizing the high stability of calcium and rare earth hydrides, CaFeAsF1-xHx (x = 0.0-1.0) and SmFeAsO1-xHx (x = 0.0-0.47) have been first synthesized using high pressure to form hydrogen-substituted 1111 type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H- ions occupying F- sites in the blocking layer of CaFeAsF. The resulting CaFeAsF1-xHx is non-superconducting, whereas SmFeAsO1-xHx is a superconductor, with an optimal Tc = 55 K at x 0.2. It was found that up to 40% of the O2- ions can be repl…
Motivated by recent discovery of yttrium-based high-temperature ternary superconducting hydrides (e.g., CaYH$_{12}$, LaYH$_{12}$, and ScYH$_{6}$), we have employed evolutionary algorithm and first-principles calculations to comprehensively examine the structural stability and superconductivity of the YMgH$_{x}$ system at high pressure. The hydrogen content $x$ and the pressure are both important factors in the stability of these candidate structures. We find that the stability of hydrogen-rich materials frequently necessitates higher pressure. For instance, the pressures to stabilize $P4/mmm$-…
