Electronic structures and properties of RE12Ga4Sb23 (RE = La-Nd, Sm) and superconducting La13Ga8Sb21

The bonding in the related structures of La12Ga4Sb23 and La13Ga8Sb21 has been analyzed following a retrotheoretical approach, with the use of extended Huckel calculations. Once the La3+ cations have been removed, the [Ga4Sb23](36-) and [Ga8Sb21](39-) metalloid networks can be decomposed into two noninteracting substructures: [(GaSb3)(2)(Ga2Sb17)](36-) (in La12Ga4Sb23) and [(GaSb3)(2)(Ga6Sb15)](39-) (in La13Ga8Sb21). Trigonal planar GaSb3 units are stacked in a one-dimensional array in La12Ga4Sb23. A pairing distortion results in slight pyramidalization of the GaSbs units and weak interunit G;a-Ga bonding in the [GaSb3] substructure of La13Ga8Sb21. The one-dimensional [GaSb3] stacks are enclosed by networks of Ga-linked square Sb ribbons, [Ga2Sb17] and [Ga6Sb15]. These networks can be related to a prototypical square sheet of Sb atoms and the oxidation states assigned accordingly. The puckering of the unusual Gas ring that occurs in the [Ga6Sb15] network of La13Ga8Sb21 is attributed to the stabilization of bands derived from the ring pi system by strengthened Ga-Sb bonding to half of the adjacent Sb atoms. When the La12Ga4Sb23 and La13Ga8Sb21 structures are reassembled, the Fermi levels fall in a region of moderate density of states, consistent with metallic behavior. Resistivity measurements for the complete RE12Ga4Sb23 (RE = La-Nd, Sm) series confirm that all are metallic. La13Ga8Sb21 exhibits metallic behavior at high temperatures, but undergoes a metal-superconductor transition at T-c = 2.4 K. Magnetic measurements corroborate this result.