Building up complexity from strips and sheets:: The electronic structure of the La12Mn2Sb30 alloy

The bonding in the relatively complex La12Mn2Sb30 alloy structure is analyzed with a retrotheoretical/building up process, implemented through a molecular orbital analysis of the various sublattices and the composite structure, In the antimony part of La12Mn2Sb30 there are three relatively noninteracting networks: a three-dimensional Sb-20 sublattice, an Sb-6 strip, and a one-dimensional array of isolated Sb atoms (of Sb-4 stoichiometry). A Zintl-type approach, modified for the clearly hypervalent nature of locally linear and square-planar Sb environments, leads to an initial partitioning of the electrons among the Sb sublattices; this electron counting eventually turns out to be in reasonable agreement with extended Huckel calculations, The electronic structure of the three-dimensional Sb-20 sublattice in La12Mn2Sb30 is derived theoretically from a two-dimensional square Sb sheet through first kinking the square sheet at every fifth diagonal line and then stacking the sheets, with Sb-Sb bond formation, into the third dimension. For the Sb-6 strips a second-order Peierls-type distortion of symmetrical vertex-sharing rhombi leads to the slightly asymmetrical strip structure observed. The d-block splitting of the Mn ions (in an unusual bicapped tetrahedral Sb environment) is described by a molecular model; arguments are given for localized bonding at Mn. There are significant La-Sb network interactions, The ability of the Sb networks in this structure to act as electron reservoirs is supported by our calculations. (C) 1998 Academic Press.