The Li-F-H Ternary System at High Pressures

TL;DR

This study uses crystal structure prediction to explore the high-pressure Li-F-H system, discovering metastable phases with unique bonding and electronic properties, including potential superconductivity, at 300 GPa.

Contribution

First comprehensive prediction of stable and metastable phases in the Li-F-H system at high pressure, revealing novel structural motifs and electronic behaviors.

Findings

Multiple metastable phases within 50 meV/atom of the convex hull.

Presence of H$_n$F$_{n+1}^-$ anions and diverse structural motifs.

LiF$_3$H is metallic with a very low superconducting temperature.

Abstract

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li-F-H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF$_3$H$_2$, LiF$_2$H, Li$_3$F$_4$H, LiF$_4$H$_4$, Li$_2$F$_3$H and LiF$_3$H lying within 50 meV/atom of the 0 K convex hull. All of these phases contain H$_n$F$_{n+1}^-$ ($n$ = 1; 2) anions, and Li$^+$ cations. Other structural motifs such as LiF slabs, H$_3^+$ molecules, and F$^{\delta -}$ ions are present in some of the low enthalpy Li-F-H structures. The bonding within the H$_n$F$_{n+1}^-$ molecules, which may be bent or linear, symmetric or asymmetric, is analyzed. The five phases closest to the hull are insulators, while LiF$_3$H is metallic and predicted to have a vanishingly small superconducting critical temperature. This study lays the foundation for future investigations of the role of temperature and anharmonicity on the stability and properties of compounds and alloys in the Li-F-H ternary system.