Bonding patterns in new intermetallic antimonides

Bonding in intermetallic compounds has eluded description by a simple overall scheme. Although electron-counting rules have been developed for two limiting classes, the non-polar Hume-Rothery and the polar Zintl compounds, no clear bonding schemes exist between the two extremes. In this research, several new intermetallic antimonides have been synthesized through reaction of the elements, their crystal structures determined by X-ray diffraction, and their electronic structures calculated using the extended Hückel method. They possess structures featuring extensive homoatomic bonding networks that test the limits of current bonding theories.; La₁₃Ga₈Sb₂₁, RE ₁₂Ga₄Sb₂₃ (RE = La-Nd, Sm), and REGaSb₂ (RE = La-Nd, Sm) adopt new structure types containing unusual Ga-Ga and Sb–Sb bonding patterns. Columnar triangular assemblies of face-sharing RE trigonal prisms occur in La ₁₃Ga₈Sb₂₁ and RE₁₂Ga ₄Sb₂₃. These trigonal prisms are centred by Ga or Sb atoms, resulting in isolated trigonal planar GaSb₃ units. Channels outlined by square Sb ribbons linked by either puckered Ga₆ rings (in La ₁₃Ga₈Sb₂₁) or Ga₂ pairs (in RE₁₂Gau4Sb₂₃) enclose the RE columns. In REGaSb₂, square nets of Sb atoms and two-dimensional $2\infty GaSb$ layers, within which zigzag Ga chains occur, are separated by RE atoms.; Networks of Ge-Ge and Sb-Sb bonding are found in the structures of FeGe _1–xSb_x (0 ≤ x ≤ 0.33). Honeycomb (63) Ge nets and kagomé (3636) nets of Fe atoms are stacked alternately, leaving open channels that are filled by isolated Ge atoms and Sb₂ dumbbells. Large voids in these structures can be filled by transition metal (R) atoms to give the new series of compounds R _1–x(R,Fe)₆Ge₄(Ge,Sb)₂ ( R = Ti, Cr, Mn).; Homoatomic main-group element bonding predominates in the new antimonides. Pairs, rings, and chains containing strong covalent bonds coexist with ribbons and nets containing weaker delocalized bonds. By considering the Sb-Sb bonds within the square networks of La₁₃Ga₈Sb₂₁, RE₁₂Ga₄Sb₂₃, and REGaSb ₂ as one-electron bonds, the Zintl concept can be extended to these relatively polar compounds. However, the Zintl concept fails for non-polar FeGe_1–xSb_x and R _1–x(R,Fe)₆Ge₄(Ge,Sb) ₂.