First-Principles Investigation of the Phase Diagram and Superconducting Properties of the Sc-Mg-H System under High Pressure

Abstract

Superconductors, characterized by their zero electrical resistance and perfect diamagnetism, are considered to have broad application prospects. In our study, we utilized particle swarm optimization techniques in conjunction with first-principles computational methods to predict structures. This approach led to the discovery of four stable configurations of scandium-magnesium-hydrogen (Sc-Mg-H) compounds under elevated pressure conditions, ranging from 30 to 250 gigapascals (GPa): ScMgH8-P4/mmm(stable from 30 to 250 GPa), ScMgH12-Cmmm (stable from 70 to 250 GPa), Sc2MgH18-P3m1 (stable from 110 to 250 GPa), and ScMg2H18-P3m1 (stable from 200 to 250 GPa). By calculating the formation enthalpy of Sc-Mg-H compounds over the pressure interval of 30–250 GPa, we obtained the pressure-composition phase diagram of the Sc-Mg-H system and investigated the crystal structure stability and superconductivity of these compounds. The electron localization function (ELF) results indicate the presence of covalent and ionic bonds between H-H atoms. Phonon spectrum calculations demonstrate that these four structures are dynamically stable within within the pressure intervals where thermodynamic stability is maintained. Electron-phonon coupling (EPC) calculations show that the superconducting transition temperature exhibits an upward trend with the increment of hydrogen content. This rise is largely ascribed to the heightened contribution from hydrogen atoms residing within the hydrogen-cage structure to the density of states (DOS) at the Fermi level. The calculated Tc for Sc2MgH18 is the highest, reaching 112 K at 150 GPa. At 100 GPa, the Tc values forScMgH8 and ScMgH12 are 56 K and 87 K, respectively. ScMg2H18 requires the highest stabilizing pressure, up to 200 GPa, with a Tc of 98 K. Our research indicates valuable insights for future research on high-temperature superconductors and may offer theoretical guidance for the synthesis of novel materials with superior properties.