Ternary Na-P-H superconductor under high pressure

Most of the H-rich superconductors have so far been discovered to be clathrate structures. Several clathrate hydrides, including H₃S, LaH₁₀, and CaH₆, were predicted to be excellent superconductors through structural searches and were later confirmed through experimental synthesis. Here, we conduct extensive crystal structure searches and first-principles calculations for ternary Na-P-H hydrides under high pressure. Two stoichiometries of NaPH₆ and NaPH₈ in ternary Na-P-H hydrides are found to be stable under 200 GPa. The P6̄m2 phase of the NaPH₆ hydride is identified as a novel ternary layered H-rich superconductor, exhibiting a high critical temperature Tc of 112.2 K under 200 GPa. Our calculations indicate that the high Tc of the NaPH₆ hydride under high pressure is attributed to the high density of hydrogen s states present at the Fermi level. Furthermore, a metastable Pm3̄ phase of the clathrate NaPH₆ hydride is uncovered under 350 GPa and displays an exceptional Tc of 201.4 K. These results suggest that ternary alkali metal Na-based hydrides hold promise as high-Tc superconductors under high pressure and offer critical insights into the design and synthesis of novel category high-temperature superconductors.