Stabilizing a hydrogen-rich superconductor at 1 GPa by charge transfer modulated virtual high-pressure effect

Abstract

Applying pressure around megabar is indispensable in the synthesis of high-temperature superconducting hydrides, such as ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$. Stabilizing the high-pressure phase of hydride around ambient condition is a severe challenge. Based on the density-functional theory calculations, we give the first example that the structure of hydride ${\mathrm{CaBH}}_{5}$ predicted above 280 GPa can maintain its dynamical stability with pressure down to 1 GPa, by modulating the charge transfer from metal atoms to hydrogen atoms via the replacement of Ca with alkali metal atoms, e.g., Cs, in which the $[{\mathrm{BH}}_{5}{]}^{2\ensuremath{-}}$ anion shrinks along $c$ axis and expands in the $ab$ plane, experiencing an anisotropic virtual high pressure. This mechanism, namely charge transfer modulated virtual high-pressure effect, plays a vital role in enhancing the structural stability and leading to the reemergence of ambient-pressure-forbidden $[{\mathrm{BH}}_{5}{]}^{2\ensuremath{-}}$ anion around 1 GPa in ${\mathrm{CsBH}}_{5}$. Moreover, we find that ${\mathrm{CsBH}}_{5}$ is a strongly coupled superconductor, with transition temperature as high as 98 K, well above the liquid-nitrogen temperature. Our findings provide a novel mechanism to reduce the critical pressure required by hydrogen-rich compound without changing its crystal structure, and also shed light on searching ambient-pressure high-temperature superconductivity in metal borohydrides.