Designing ternary superconducting hydrides with A15-type structure at moderate pressures

Ever since hydrogen-based superconductors have been actively considered as candidates for room-temperature superconductors, binary hydrides have been almost completely explored theoretically. Now researchers are turning their interest to ternary hydrides. However, the enormous compositional and structural possibilities in the ternary hydride space limit the search efficiency. Here, using an advanced crystal structure search method combined with first-principles calculations, a series of ternary hydrides AXH₆ (MgPH₆, MgAsH₆, CaSbH₆, CaSnH₆, NaSbH₆ and ScSiH₆) with the A15-type structure are predicted to be stable at around one million atmospheres. Among them, CaSnH₆ presents the lowest thermodynamically stable pressure of 110 GPa. CaSbH₆, CaSnH₆ and ScSiH₆ may be dynamically stable down to 41 (70), 71 (90) and 86 (70) GPa, respectively, without (with) anharmonic effects and temperature effects. Hydrogen atoms in these A15-type structures are found to form weak covalent bonds to X atoms. More interestingly, we find that ternary hydrides with the AXH₆ stoichiometry may assume the A15-type structure under pressure when the sum of the chemical valences of the two non-hydrogen atoms is six or seven. Based on electron-phonon coupling (EPC) calculations, all these ternary hydrides are estimated to be superconductors. CaSbH₆, CaSnH₆ and ScSiH₆ have superconducting transition temperatures (T_cs) of 65, 53 and 116 K at 45, 75 and 90 GPa, respectively, while MgPH₆ has the highest T_c of 148 K at 135 GPa. Our study will provide guidance for designing high T_c ternary hydrides near ambient pressures.