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.
We employ the chemical fragment formalism to perform a targeted superconductor search in the Nb-Ru-B system, yielding the orthorhombic metal-rich boride NbRuB, which displays BCS-like superconductivity with a Tc = 3.1 K. NbRuB is derived from the chemical fragments Nb3B2 + Ru3B, in which the Nb3B2 fragment contains B-B dimers and the Ru3B fragment contains isolated B atoms. A charge transfer occurs between the fragments. The results indicate that the fragment formalism is a useful chemical tool for the design of new intermetallic superconductors much the same way as the charge reservoir concept has been a useful chemical tool for the design of new copper oxide superconductors.
