Kinetic instability and superconductivity in Li$_2$AuH$_6$ and Li$_2$AgH$_6$ at ambient pressure

TL;DR

This study investigates the kinetic stability and superconducting properties of Li$_2$AuH$_6$ and Li$_2$AgH$_6$ at ambient pressure, revealing instability and a lower superconducting transition temperature than previously predicted.

Contribution

It provides the first detailed analysis of the kinetic stability of these compounds and predicts a reduced Tc due to structural changes.

Findings

Li$_2$AgH$_6$ undergoes lattice collapse

Li$_2$AuH$_6$ remains fluorite-type but with hydrogen dimerization

Superconducting transition temperature of Li$_2$AuH$_6$ is 22 K

Abstract

Li$_2$AuH$_6$ and Li$_2$AgH$_6$ have been proposed as promising candidates for high-temperature superconductors under ambient pressure. While previous studies confirm the dynamic stability of these two thermodynamically unstable systems, their kinetic stability against quantum and thermal fluctuations remains to be verified. In this work, we use path integral molecular dynamics simulations to examine the kinetic stability of Li$_2$AuH$_6$ and Li$_2$AgH$_6$ under ambient pressure. We find both compounds are kinetically unstable. Li$_2$AgH$_6$ undergoes lattice collapse, whereas Li$_2$AuH$_6$ retains a stable fluorite-type Li-Au sublattice, but hydrogen atoms partially dimerize into molecules and diffuse within the host lattice. Using the stochastic path-integral approach, which is a nonperturbative approach applicable to systems with diffusive atoms, we investigate the superconductivity of Li$_2$AuH$_6$ in this state. We predict a superconducting transition temperature of 22 K, well below earlier predictions, due to the low density of states at the Fermi level caused by the collapse of hydrogen sublattice and hydrogen dimerization.