Structural Design Of High-temperature Superconductors With Light Elements,Including Hydrogen,Boron,and Carbon,under High Pressure

Since the discovery of superconductivity by the Dutch physicist H.K.Onnes in1911,the search for high-temperature or even room-temperature superconductors has been one of the central topics in the field of condensed matter physics.Bardeen-Cooper-Schrieffer（BCS）theory predicted that light-element compounds（such as hydrides,borides,carbides,etc.）could be high-temperature superconductors because of their high Debye frequencies.In recent years,pressure-stabilized hydrogen-rich superconductors have attracted more attention and scientists have made a series of significant research progress in this field.The recent discoveries of high-pressure hydrides H₃S and La H₁₀ with superconducting critical temperatures（T_c）of 203 and250-260 K,respectively,made them tantalizingly close to realizing room-temperature superconductivity.However,these materials are not only difficult to be synthesized but also unstable at ambient pressure,which greatly limits their wide application.It is worth noting that a class of light-element compounds without hydrogen exhibit excellent superconducting properties at lower pressures or even at ambient pressure due to their strong covalent bonds.Among them,Mg B₂ is the highest-temperature superconductor known at ambient pressure with T_c=39 K.Further improving the superconducting temperature of traditional superconductors at ambient pressure is also the interest of scientists.Therefore,we have carried out a series of theoretical studies on the light-element compounds with the design goal of finding high-temperature superconductors at high pressure and even at ambient pressure.By using the first-principles methods in combination with CALYPSO structure prediction methods and other technologies,we have carried out structural design and superconductivity research on ternary hydrogen-rich materials M-Si-H（M=Li and La）,multielement hydrogen-based and boron-carbon-based cage materials.The following innovative results are obtained:1.In 2008,the first high-pressure experimental study of hydrogen-rich superconductors found that Si H₄ has a superconducting temperature of 17 K at 96 and120 GPa.However,this observation was highly controversial,as subsequent studies suggested that the Si H₄ underwent chemical decomposition under high pressure.In terms of theoretical studies,numerous superconducting candidate structures for Si H_xhave been proposed,but only P-3-Si H₄ is predicted to be thermodynamically stable at higher pressures,and the T_c value of this structure is only 35 K at 300 GPa.In this work,by introducing alkali metal Li into the Si-H system,we found that metal Li as an electron donor can not only stabilize the Si-H system,but also effectively manipulate the electronic properties of the material.Based on the CALYPSO structure prediction methods,we have systematically explored the phase stability of Li-Si-H ternary compounds at a pressure range of 50-350 GPa and uncovered several thermodynamically stable stoichiometries.Among them,Li₂Si H₆ is stabe in the pressure range of 50-350 GPa;Li Si₂H₉ has a T_c of 54 K at its lowest thermodynamically stable pressure of 172 GPa,which is higher than that of Si H₄（T_c=35 K）.Additionally,by replacing the Li with La element which has a larger atomic radius and more abundant valence electrons,a hydrogen-rich thermodynamically stable superconductive phase of La Si₂H₁₄ was predicted.The further electron-phonon coupling（EPC）calculation indicate that La Si₂H₁₄ is a phonon-mediated superconductor with an estimated T_c of 88K at 300 GPa,which is higher than that of Li Si₂H₉.Actually,the high-temperature superconductivity is mainly attributed to the strong EPC and the H-dominated density of states near the Fermi level.These results highlight the role of metal doping in chemically tuning crystal structures,which induce the significant change of electronic properties and improve the superconductivity for the Si-H system,shedding light on the exploration and discovery of high-T_c superconductivity in a variety of ternary or quaternary hydrogen-rich compounds associated with metal incorporation.2.Recently,based on the hydrogen-rich clathrates,researchers have utilized two types of metal elements to synergistically optimize the T_c of binary hydrides to a certain extent.If more kinds of metal elements are introduced into the clathrate,the T_c of binary hydrides may be also increased.In fact,after multi-elements mixing,the lattice distortion caused by the differences in atomic radius and mass can lead to the softening of phonon frequencies,which can increase the EPC and thus enhance the superconducting transition temperature.Moreover,the contribution of configuration entropy under high-temperature conditions is significant after multi-element mixing,which is more favorable for experimental synthesis.Therefore,based on the Ca H₆clathrate structure,density functional theory（DFT）calculations are performed on members of this family containing up to 4 different metal atoms,where the 16 candidate metal elements originate from theⅠA,ⅡA,ⅢB,ⅣB,lanthanides,and actinides in the periodic table.After extensive first-principles calculations,we finally designed 19possible 4-metal substitution alloy structures that could be stable under high pressure,of which nearly 10 exhibit superconductivity exceeding 200 K.In addition,the estimated T_c of（Na,Y,Hf,Zr）H₆ is 279 K;and that of（Na,Zr,Hf,Sc）H₆ is 253 K,significantly higher than the highest T_c（Sc H₆,T_c=209 K）among the four clathrate binary hydrides.Further energy calculations show that both configuration entropy and phonon vibration energy play important roles in stabilizing these substitution alloy structures.These results may guide further experimental studies in this field.3.In light of the tremendous potential for practical applications of high-temperature superconducting materials under ambient conditions,the search for such materials has long been a subject of intense research interest.Recently,experimental scientists have successfully synthesized the host-guest cage-type Sr B₃C₃ structure at relatively low pressure.This structure can be regarded as a hole-doped conductor in which the guest cation Sr²⁺is embedded into the framework of the host（B₃C₃）^3-cage.Previous calculations indicate that Sr B₃C₃ is a phonon-mediated superconductor with an estimated T_c of 43 K at ambient pressure,and subsequent electrical measurements further confirm the superconductivity of this material.Given that Rb⁺has similar atomic radius and electronegativity to Sr²⁺,it is suitable to be incorporated to the synthesized clathrate Sr B₃C₃ to manipulate electronic properties and result in the significant modification of superconductivity.Therefore,we carried out theoretical research on Rb-substituted Sr B₃C₃.The compositional space of Rb_xSr_1-xB₃C₃ at 0 and50 GPa was systematically explored using first-principles calculations in combination with cluster expansion and CALYPSO structure prediction methods.Through this,we predicted a series of dynamically stable Rb_xSr_1-xB₃C₃（x≤0.5）compounds,among which Rb_0.5Sr_0.5B₃C₃ is a potential high-temperature superconducting candidate material.Based on Migdal-Eliashberg theory,it is found that Rb_0.5Sr_0.5B₃C₃ is a two-gap anisotropic superconductor and the estimated T_c could reach 88 K,which is higher than the 43 K of the parent Sr B₃C₃ structure,and also surpasses the boiling point of liquid nitrogen.These results provide theoretical guidance for the design of high-T_csuperconducting materials in lightweight-element compounds at ambient pressure.