Metal borohydrides as ambient-pressure high- Tc superconductors

The extreme pressures required to stabilize the recently discovered superhydrides represent a major obstacle to their practical application. In this Letter, we propose and substantiate a route to attain high-temperature superconductivity in hydrides at ambient pressure, by doping commercial metal borohydrides. Using first-principles calculations based on density functional theory and Migdal-Eliashberg theory, we demonstrate that in Ca(BH4)2 a moderate hole doping of 0.03 holes per formula unit, obtained through a partial replacement of Ca with monovalent K, is sufficient to achieve Tc's as high as 110 K. The high Tc arises because of the strong electron-phonon coupling between the B-H sigma molecular orbitals and bond-stretching phonons. Using a random sampling of large supercells to estimate the local effects of doping, we show that the required doping can be achieved without significant disruption of the electronic structure and at moderate energetic cost. Given the wide commercial availability of metal borohydrides, the ideas presented here can find prompt experimental confirmation. If successful, the synthesis of high-Tc doped borohydrides will represent a formidable advancement towards the technological exploitation of conventional superconductors.