Designing High-Tc Quaternary Hydrides Under Moderate Pressures Through Substitutional Doping

In recent years, there has been considerable interest in exploring high-temperature superconductors in hydrides under low-pressure conditions. A series of superconducting ternary LaB/BeH8-type hydrides with Fm-3m symmetry were designed to be stable below one million atmospheres and LaBeH8 was successfully synthesized and confirmed. Inspired by this, a series of quaternary XAZ2H16 compounds (where X and A represent La, Ca, Sr, Ba, Ac, Ce, Th, Na and K; Z represents Be, B, Si or P) were constructed through substitutional doping on LaB/BeH8-type structure. Eight hydrides were found to be thermodynamically stable under pressure, with LaThBe2H16 and AcBaSi2H16 exhibiting the lowest pressures of 92 and 81 GPa, respectively. Importantly, all investigated hydrides displayed dynamic stability at pressures below one million atmospheres, with LaThBe2H16 remaining stable even at pressures as low as 8 GPa. When Z was Be, Z atoms tend to form ionic bonds with H atoms. However, when Z was a non-metallic element (B, Si or P), covalent bonds between Z atoms and H atoms were observed. Electron-phonon coupling calculations indicate that all of these quaternary hydrides have potential as superconductors. Notably, LaThBe2H16 was predicted to have a superconducting transition temperature (Tc) of 126 K at a pressure as low as 10 GPa, while LaYBe2H16 exhibited the highest Tc of 221 K at 75 GPa. It is worth highlighting that LaThBe2H16 exhibited a merit factor (S) of up to 3.13, surpassing that of the ternary Fm-3mXZH8 compounds. These findings provide valuable insights for the search for high-Tc superconductors under lower pressure conditions and encourage further exploration of polynary superconducting hydrides.