Prediction of high- Tc superconductivity in ternary actinium beryllium hydrides at low pressure

Hydrogen-rich superconductors are promising candidates to achieve room-temperature superconductivity. However, the extreme pressures needed to stabilize these structures significantly limit their practical applications. An effective strategy to reduce the external pressure is to add a light element M that binds with H to form ${\mathrm{MH}}_{x}$ units, acting as a chemical precompressor. We exemplify this idea by performing ab initio calculations of the Ac--Be--H phase diagram, proving that the metallization pressure of Ac--H binaries, for which critical temperatures as high as 200 K were predicted at 200 GPa, can be significantly reduced via beryllium incorporation. We identify three thermodynamically stable (${\mathrm{AcBe}}_{2}{\mathrm{H}}_{10}, {\mathrm{AcBeH}}_{8}$, and ${\mathrm{AcBe}}_{2}{\mathrm{H}}_{14}$) and four metastable compounds (fcc ${\mathrm{AcBeH}}_{8}, {\mathrm{AcBeH}}_{10}, {\mathrm{AcBeH}}_{12}$ and ${\mathrm{AcBe}}_{2}{\mathrm{H}}_{16}$). All of them are superconductors. In particular, fcc ${\mathrm{AcBeH}}_{8}$ remains dynamically stable down to 10 GPa, where it exhibits a superconducting-transition temperature ${T}_{\text{c}}$ of 181 K. The Be--H bonds are responsible for the exceptional properties of these ternary compounds and allow them to remain dynamically stable close to ambient pressure. Our results suggest that high-${T}_{\text{c}}$ superconductivity in hydrides is achievable at low pressure and may stimulate experimental synthesis of ternary hydrides.