Abstract
Although it was proposed many years ago that compressed hydrogen should be a high-temperature superconductor, the goal of room-temperature superconductivity has so far remained out of reach. However, the successful synthesis of the theoretically predicted hydrides H3S and LaH10 with high superconducting transition temperatures TC provides clear guidance for achieving this goal. The existence of these superconducting hydrides also confirms the utility of theoretical predictions in finding high-TC superconductors. To date, numerous hydrides have been studied theoretically or experimentally, especially binary hydrides. Interestingly, some of them exhibit superconductivity above 200 K. To gain insight into these high-TC hydrides (>200 K) and facilitate further research, we summarize their crystal structures, bonding features, and electronic properties, as well as their superconducting mechanism. Based on hydrogen structural motifs, covalent H3S with isolated hydrogen and several clathrate superhydrides (LaH10, YH9, and CaH6) are highlighted. Other predicted hydrides with various H-cages and two-dimensional H motifs are also discussed. Finally, we present a systematic discussion of the common features, current problems, and future challenges of these high-TC hydrides.
Highlights
The highest superconducting transition temperature TC that can be achieved by cuprates, the most representative class of unconventional superconductors, is 135 K at ambient pressure,2 and even at high pressure only reaches as high as 160 K.3
Hydrides are predicted to be able to achieve high-TC superconductivity at relatively low pressure owing to chemical precompressions,12 and this has led to an upsurge in research on compressed hydrides
From the corresponding phonon density of states (PHDOS) and Eliashberg spectral function a2F(ω) [Fig. 1(d)], such a high TC mainly originates from the large electron–phonon coupling (EPC) contribution (82.6%) of a high-frequency hydrogen vibrational mode (>20 THz)
Summary
The highest superconducting transition temperature TC that can be achieved by cuprates, the most representative class of unconventional superconductors, is 135 K at ambient pressure, and even at high pressure only reaches as high as 160 K.3 On the other hand, it has been proposed that under high pressure, solid hydrogen should achieve metallization and high-TC superconductivity (100–760 K) in either molecular or atomic phases, based on Bardeen–Cooper–Schrieffer (BCS) theory, the pressure required is far beyond what can be experimentally achieved. Excitingly, hydrides are predicted to be able to achieve high-TC superconductivity at relatively low pressure owing to chemical precompressions, and this has led to an upsurge in research on compressed hydrides. As proposed by Ashcroft, hydrides can achieve superconductivity under much lower pressures than metallic hydrogen. Following this principle, numerous hydrides are predicted to have TC values above 200 K,18,39–42 with some of them exhibiting superconductivity near room temperature and even higher.. Numerous hydrides are predicted to have TC values above 200 K,18,39–42 with some of them exhibiting superconductivity near room temperature and even higher.21,22,43 Even more interestingly, these high-TC hydrides (>200 K) contain diverse hydrogen configurations, such as isolated atomic hydrogen in covalent hydrides, two-dimensional (2D) H motifs, and various 3D H cages with covalent H–H bonding character. A comprehensive discussion and conclusions are presented, including a description of current problems and future challenges
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