A strategy to increase the superconducting transition temperature through covalent bond unsaturation

Numerous studies have shown that hydrogen-rich binary superconductors usually require high pressure to achieve superconductivity. Interestingly, their covalent bonds often lack electrons, resulting in the formation of unsaturated covalent bonds, which play an important role in superconductivity. This provides a new idea for breaking through the high-pressure dilemma faced by binary hydride superconductors and improving dynamical stability and superconductivity by regulating the formation of covalent bonds. This paper selects the typical binary hydride H 3 S as the research object, aiming to explore new binary phosphide superconducting materials by replacing the S atom in H 3 S with P atoms. Through first-principles calculations of 32 binary phosphides, we discovered a dynamically stable SP 3 structure at 20 GPa. Considering that S (3s 2 3p 4 ) and P (3s 2 3p 3 ) are in the same period and have similar electronic structures, a strong covalent bond framework is formed in SP 3 . These covalent bonds lack electrons and exhibit the characteristics of unsaturated bonds. To further reduce the stability pressure, we increased the P content in the structure, aiming to reduce the number of electrons involved in bonding and enhance the degree of unsaturation of the bond, thereby obtaining an SP 7 structure that remains dynamically stable at ambient pressure. Through the analysis of its electronic structure and superconducting properties, we found that compared with SP 3 , the charge transfer amount in SP 7 is reduced, the electron localization function (ELF) value is enhanced, the covalent property is stronger, and the T C increases from 10.8 K to 13.8 K, which reveals the relationship between the superconducting properties and the degree of covalent bond unsaturation.