New Condensed Cluster Structure with Triple Metal Layers: La₂INi₂ and La₂ICu₂· Synthesis, Structure, and Bonding

Syntheses of the title compounds result from appropriate reactions of the elements and LaT₃in sealed Nb or Ta containers at 800 °C. These exhibit a new structural type with an unusual metal-rich layered structure, detailed for La₂INi₂ by single-crystal X-ray diffraction: hexagonal P6^-m2, Z = 1, a = 4.1387 Å, c = 8.814 Å (23 °C). This is related to the Gd₂IFe₂-type structure known for La₂IZ₂ for Z = Fe, Co, and others. Both structures contain AlB₂-like slabs, namely, graphite-like Z₂ (Z = Ni, Fe) layers between eclipsed pairs of La layers, but these are instead separated by single iodine layers. The La₂INi₂ structure differs in that the iodine positions along c^→ project onto only one Z position instead of both and the cell is half as large. An additional set of very weak reflections and streaks revealed in widely varying amounts by precession photographs suggests a superstructure in c or intergrowths by small amounts of disordered components, but a reasonable model could not be derived beyond some possible short-range disordering in the iodine layers. Comparisons of La₂INi₂ with La₂IFe₂ in terms of electronic band structures show that filling of antibonding states occurs in the Ni layers. Integrated overlap populations in the Fe vs Ni compounds show corresponding decreases for Z-Z and La-Z bonding and a smaller strengthening of La-La bonding in the layers, while the a lattice parameter increases and the trigonal prismatic La₆Z units decrease in height. These conflicts are reflected in the Z-layer puckering and the doubled c dimension found in La₂IFe₂ and other phases with the Gd₂IFe₂-type structure.