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.Comment: 32 pages, 8 Figure
