Thermodynamic stabilities of ternary metal borides: An ab initio guide for synthesizing layered superconductors

TL;DR

This study uses density functional theory to identify stable layered ternary metal borides, especially Li$_2$AlB$_4$, which shows potential as a superconductor with a critical temperature around 4 K.

Contribution

The paper introduces a computational approach to predict stable ternary metal borides and assesses their potential as layered superconductors based on electron-phonon coupling.

Findings

Li$_2$AlB$_4$ is a promising superconductor candidate.

Alloying lithium monoboride with metal diborides enhances stability.

In-plane boron phonon modes correlate with bond length and metal valence.

Abstract

Density functional theory calculations have been used to identify stable layered Li-$M$-B crystal structure phases derived from a recently proposed binary metal-sandwich (MS) lithium monoboride superconductor. We show that the MS lithium monoboride gains in stability when alloyed with electron-rich metal diborides; the resulting ordered Li$_{2(1-x)}M_x$B$_2$ ternary phases may form under normal synthesis conditions in a wide concentration range of $x$ for a number of group-III-V metals $M$. In an effort to pre-select compounds with the strongest electron-phonon coupling we examine the softening of the in-plane boron phonon mode at $\Gamma$ in a large class of metal borides. Our results reveal interesting general trends for the frequency of the in-plane boron phonon modes as a function of the boron-boron bond length and the valence of the metal. One of the candidates with a promise to be an MgB$_2$-type superconductor, Li$_2$AlB$_4$, has been examined in more detail: according to our {\it ab initio} calculations of the phonon dispersion and the electron-phonon coupling $\lambda$, the compound should have a critical temperature of $\sim4$ K.