Gold-Gold Bonding: The Key to Stabilizing the 19-Electron Ternary Phases LnAuSb (Ln = La-Nd and Sm) as New Dirac Semimetals

TL;DR

This paper introduces a new family of LnAuSb compounds with 19 valence electrons stabilized by Au-Au bonding, leading to a structure that supports Dirac semimetal behavior, expanding understanding of gold chemistry in stable, topologically interesting materials.

Contribution

It reports the discovery of a new stable phase of LnAuSb compounds with a unique bonding mechanism that enables Dirac semimetal properties, highlighting gold's unconventional chemistry.

Findings

LnAuSb compounds have a 19-electron count stabilized by Au-Au bonds.

The YPtAs-type structure enables symmetry-allowed Dirac band crossings.

LaAuSb exhibits a bulk Dirac cone below the Fermi level.

Abstract

We report a new family of ternary 111 hexagonal LnAuSb (Ln = La-Nd, Sm) compounds that, with a 19 valence electron count, has one extra electron compared to all other known LnAuZ compound. The "19th" electron is accommodated by Au-Au bonding between the layers; this Au-Au interaction drives the phases to crystallize in the YPtAs-type structure rather than the more common LiGaGe-type. This is critical, as the YPtAs structure type has the symmetry-allowed band crossing necessary for the formation of Dirac semimetals. Band structure, density of stats, and crystal orbital calculations confirm this picture, which results in a nearly complete band gap between full and empty electronic states and stable compounds; we can thus present a structural stability phase diagram for the LnAuZ (Ln = Ge, As, Sn, Sb, Pb, Bi) family of phases. Those calculations also show that LaAuSb has a bulk Dirac cone below the Fermi level. The YPtAs-type LnAuSb family reported here is an example of the uniqueness of gold chemistry applied to a rigidly closed shell system in an unconventional way.