Theoretical Design Of Hydrogen-rich High-temperature Superconductors Under High Pressure

Superconductive materials,characterized by zero resistance and from which magnetic flux fields are absolutely expelled,have been the focus of condensed matter physics.Hydrogen-rich metal hydrides are considered as the most favorable candidates for high-temperature superconductors,however,most of hydrides at atmospheric pressure are insulators.High pressure,as an extreme condition,can significantly change the physical and chemical properties of materials,and metalize the insulator hydride to be superconductor.Theoretical and experimental foundings of high-temperature superconductivity in hydrogen sulfide?H₃S,T_c²03 K?and clathrate hydride(LaH₁₀,Tc²50 K)are important steps towards room-temperature superconductivity.However,the room-temperature superconductivity has not been actually achieved,and the ultra-high-pressure condition needed for high-superconducting hydrides also prevent them from being synthesized by high-pressure experiments.There is an urgent need to design hydride superconductors with higher superconducting temperature and lower superconducting pressure to approach the true sense of room-temperature superconductors.As the number of elements increases,the number of thermodynamically stable superconducting compounds in complex hydrides rapidly increases,make it easier to find high-temperature superconductivity in ternary hydrides than in binary hydrides.The author proposes routes to hydrogen-rich high-temperature superconductors.Following these routes and taking a series of ternary hydride systems as examples,theoretical design has been carried out for hydrogen-rich high-temperature superconductors under high pressure,and the following innovative results have been obtained:1.The theoretically-driven founding of the high-temperature superconductivity in clathrate LaH₁₀?T_c²50-260 K?at high-pressure is an important step towards room-temperature superconductors,and there in only a small step away from room-temperature?300 K?superconductivity.Searching for hydrides with superconductivity higher than room-temperature is a key issue in condensed matter physics and needs to be addressed urgently.The author suggests to improve the superconductivity by introducing metal elements into the parent hydride,which containing a large amount of H₂ molecules,to design new ternary high-temperature superconductors.The key idea of this suggestion is that the metal doping dissociates H₂ molecules into atomic H,which increases the density of electron states at the Fermi level and further increases the superconducting transition temperature.Following this suggestion and taking Li doped Mg-H compounds as an example,high-pressure phase diagram of the Li-Mg-H system at 300 GPa has been studied using the CALYPSO crystal structure prediction method and software.It is found that Li and Mg metals provide extra electrons for H₂molecules and promote H₂ molecules dissociate into atomic hydrogen.The number of extra electrons received by each H₂ molecule is linearly related to the spacing of hydrogen atoms.A cubic clathrate structure of Li₂MgH₁₆,with all H₂ molecules have been dissociated,is proposed to be a potential room-temperature superconductor or even a"hot"superconductor,with superconducting temperature of up to 473 K at 250GPa.Results of this study prove that our route to high-temperature superconductivity of introducing electrons by metal-doping to break the hydrogen molecular is reasonable and effective,which can be used to experimentally or theoretically design more hydride superconductors.2.The theoretically founding of superconductivity as high as 473 K in Li₂MgH₁₆proves that our route to high-temperature superconductivity of introducing electrons by metal-doping to break the hydrogen molecular is reasonable and effective.Although predicted to be a"hot"superconductor,Li₂MgH₁₆ is a metastable phase at high-pressure,meaning that Li₂MgH₁₆ is hard to be experimentally synthesized.Therefore,searching for thermodynamically stable hydrides with high superconductivity is still a key issue in condensed matter physics and needs to be addressed urgently.The author followed the suggestion proposed by herself of introducing electrons by metal-doping to break the hydrogen molecular,by replacing the Mg element with a more electron-valenced rare earth element Y,studied the high-pressure phase diagram of Li-Y-H system using the CALYPSO crystal structure prediction method and software.A thermodynamically stable clathrate structure of Li₂YH₁₇ was proposed to be a high-temperature superconductor with superconducting temperature up to 112 K under high pressure,where the clathrate network is exactly the same as a zeolite network.Some binary hydrides(e.g.LaH₁₀,ThH₁₀,and YH₆)have similar zeolite-type networks that have been synthesized experimentally at high pressure,indicating that the thermodynamically stable Li₂YH₁₇ may also be synthesized under high-pressure.The author proposes that the clathrate Li₂YH₁₇ can be used as a prototype structure,and superconductivity might be improved by replacing Li and Y with other metal elements.It is also suggested that the zeolite-type network,the archimedes stereo and the laves phase might be used as prototype structures to design the high superconducting clathrate hydrides.These suggestions are expected to inject new life into the research of hydrogen-rich superconductors.3.Theoretical and experimental founding of high-temperature superconductivity as high as 203 K in H₃S has triggered a wave of searching hydrogen-rich materials at high pressure for high-temperature superconductors.Theoretical researchers have found many hydrogen-rich compounds with superconducting transition temperatures above 200 K.However,these hydrides superconducting only at ultra-high pressures,which prevent them from being synthesized experimentally.It is urgent to try to reduce the superconducting pressure while ensuring high superconductivity.Snider et al.experimentally found that C-S-H compounds undergo an insulator-metal phase transition at pressures higher than 55 GPa.Inspired by this research,the author studied the high-pressure phase diagram of the C-S-H system at 100 GPa by using the CALYPSO crystal structure prediction method and software,and predicted a metastable phase of CSH₇ with a structure similar to that of H₃S compound,whose superconducting temperature is predicted to be as high as 181 K at 100 GPa.CSH₇ can be regarded as the host-guest structure of methane molecule?CH₄?doping H₃S,where the host-guest ionic interaction chemically pre-presses the[SH₃]host sublattice,allowing CSH₇ to be dynamically stable down to 100 GPa,which is much lower than that of cubic H₃S compounds?H₃S is dynamically stable down to 170 GPa?.The results show that the special host-guest interaction of CSH₇ maintains the high superconductivity of the parent H₃S,while reducing the superconducting pressure,which inspired the author that one might reduce the superconducting pressure by introducing the host-guest interaction by doping molecules into superconductors.It is hoped that it can be used to design more host-guest hydride high-temperature superconductors at low pressure.