In-silico synthesis of lowest-pressure high-$T_\text{c}$ ternary superhydrides

TL;DR

This paper predicts high-temperature superconducting ternary hydrides BaSiH8 and SrSiH8 at relatively low pressures using theoretical methods, and introduces a new approach to estimate the lowest synthesis pressure based on enthalpy barriers.

Contribution

It presents the first theoretical prediction of stable high-$T_c$ ternary superhydrides at low pressures and introduces a novel method to estimate the lowest synthesis pressure using enthalpy barriers.

Findings

BaSiH8 has a $T_c$ around 71 K at 130 GPa.

SrSiH8 has a $T_c$ around 126 K at 174 GPa.

A new method estimates the lowest synthesis pressure based on enthalpy barriers.

Abstract

We report the theoretical prediction of two high-performing hydride superconductors BaSiH$_8$ and SrSiH$_8$. They are thermodynamically stable above pressures of $130$ and $174\,\text{GPa}$, respectively, and metastable below that. Employing anharmonic phonon calculations, we determine the minimum pressures of dynamical stability to be around $3\,\text{GPa}$ for BaSiH$_8$ and $27\,\text{GPa}$ for SrSiH$_8$, and using the fully anisotropic Migdal-Eliashberg theory, we predict $T_\text{c}$'s around $71$ and $126\,\text{K}$, respectively. We also introduce a method to estimate the lowest pressure of synthesis, based on the calculation of the enthalpy barriers protecting the BaSiH$_8$ $Fm\bar{3}m$ structure from decomposition at various pressures. This $kinetic$ pressure threshold is sensibly higher than the one based on $dynamic$ stability, but gives a much more rigorous limit for synthesizability.