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

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 compounds. LaAuSb, CeAuSb, PrAuSb, NdAuSb, and SmAuSb crystallize in the YPtAs-type structure, and have a doubled unit cell compared to other LnAuZ phases as a result of the buckling of the Au–Sb honeycomb layers to create interlayer Au–Au dimers. The dimers accommodate the one excess electron per Au and thus these new phases can be considered Ln23+(Au–Au)0Sb23–. Band structure, density of states, 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 (Z = 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.