| Abstract
| - The bonding in the related structures of La12Ga4Sb23 and La13Ga8Sb21 has been analyzedfollowing a retrotheoretical approach, with the use of extended Hückel calculations. Oncethe La3+ cations have been removed, the [Ga4Sb23]36- and [Ga8Sb21]39- metalloid networkscan be decomposed into two noninteracting substructures: [(GaSb3)2(Ga2Sb17)]36- (in La12Ga4Sb23) and [(GaSb3)2(Ga6Sb15)]39- (in La13Ga8Sb21). Trigonal planar GaSb3 units are stackedin a one-dimensional array in La12Ga4Sb23. A pairing distortion results in slight pyramidalization of the GaSb3 units and weak interunit Ga−Ga bonding in the [GaSb3] substructureof La13Ga8Sb21. The one-dimensional [GaSb3] stacks are enclosed by networks of Ga-linkedsquare Sb ribbons, [Ga2Sb17] and [Ga6Sb15]. These networks can be related to a prototypicalsquare sheet of Sb atoms and the oxidation states assigned accordingly. The puckering ofthe unusual Ga6 ring that occurs in the [Ga6Sb15] network of La13Ga8Sb21 is attributed tothe stabilization of bands derived from the ring π system by strengthened Ga−Sb bondingto half of the adjacent Sb atoms. When the La12Ga4Sb23 and La13Ga8Sb21 structures arereassembled, the Fermi levels fall in a region of moderate density of states, consistent withmetallic behavior. Resistivity measurements for the complete RE12Ga4Sb23 (RE = La−Nd,Sm) series confirm that all are metallic. La13Ga8Sb21 exhibits metallic behavior at hightemperatures, but undergoes a metal−superconductor transition at Tc = 2.4 K. Magneticmeasurements corroborate this result.
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