Electronic Structures and Properties of RE12Ga4Sb23 (RE = La−Nd, Sm) and Superconducting La13Ga8Sb21

The bonding in the related structures of La 12 Ga 4 Sb 23 and La 13 Ga 8 Sb 21 has been analyzed following a retrotheoretical approach, with the use of extended Hückel calculations. Once the La 3+ cations have been removed, the [Ga 4 Sb 23 ] 36 - and [Ga 8 Sb 21 ] 39 - metalloid networks can be decomposed into two noninteracting substructures: [(GaSb 3 ) 2 (Ga 2 Sb 17 )] 36 - (in La 12 Ga 4 Sb 23 ) and [(GaSb 3 ) 2 (Ga 6 Sb 15 )] 39 - (in La 13 Ga 8 Sb 21 ). Trigonal planar GaSb 3 units are stacked in a one-dimensional array in La 12 Ga 4 Sb 23 . A pairing distortion results in slight pyramidalization of the GaSb 3 units and weak interunit Ga−Ga bonding in the [GaSb 3 ] substructure of La 13 Ga 8 Sb 21 . The one-dimensional [GaSb 3 ] stacks are enclosed by networks of Ga-linked square Sb ribbons, [Ga 2 Sb 17 ] and [Ga 6 Sb 15 ]. These networks can be related to a prototypical square sheet of Sb atoms and the oxidation states assigned accordingly. The puckering of the unusual Ga 6 ring that occurs in the [Ga 6 Sb 15 ] network of La 13 Ga 8 Sb 21 is attributed to the stabilization of bands derived from the ring π system by strengthened Ga−Sb bonding to half of the adjacent Sb atoms. When the La 12 Ga 4 Sb 23 and La 13 Ga 8 Sb 21 structures are reassembled, the Fermi levels fall in a region of moderate density of states, consistent with metallic behavior. Resistivity measurements for the complete RE 12 Ga 4 Sb 23 (RE = La−Nd, Sm) series confirm that all are metallic. La 13 Ga 8 Sb 21 exhibits metallic behavior at high temperatures, but undergoes a metal−superconductor transition at T c = 2.4 K. Magnetic measurements corroborate this result.