Simple hexagonal structured gold with eight-coordination formed with ordered structural vacancies

Phase engineering of nanomaterials attracts increasing research interest, as crystal structures of metals often determine their physical and chemical properties. Metals tend to form close-packed crystal structures to maximize their space filling and atomic coordination numbers due to the isotropic metallic bonding. Hitherto no metals yet crystallize with the simple hexagonal packing which has low coordination number and packing fraction. Here, we fabricate a novel simple hexagonal structured gold through in situ selective etching on ordered Cu-Au intermetallic compound, where the new crystal structure has only 8 nearest neighbors for each atom and a low packing fraction of 0.60. The simple hexagonal structured Au demonstrates an electrical conductivity of 389 S/m, approximately 105 times lower than that of face-centered cubic structured gold, according to first principles calculation, associated with localized valence electrons. The discovery of simple hexagonal structure suggests that metals can form more diverse crystal structures by changing effective atomic volume, which provides a pathway to engineer new structures of metals for novel physical properties and applications.