Synthesis and crystal structure of M11X10 compounds, M = Sr, Ba and X = Bi, Sb.: Electronic requirements and chemical bonding

The intermetallic compounds Sr11Bi10, Ba11Bi10, and (Sr5Ba6)Sb-10) have been obtained from melts of mixtures of the elements. They crystallize in the tetragonal system, space group I4/mmm, Ho11Ge10 structure type, t184 Pearson symbol, Z = 4, with cell parameters a= 12.765(3), 13.230(3), 12.748(2) Angstrom and c = 18.407(3), 19.365(3), 18.761(2) A, respectively. The structures were solved from single-crystal X-ray data and refined by full-matrix least-squares to R1 = 6.71, 5.44, and 5.73%. The structure of M11X10 contains three discrete anionic moieties: square rings X4-, dumbbells X4-, and isolated X3-. Using formal charges the unit cell of M11X10 may be described as containing 44 M (2+), 2X(4-), 8X(4-), and 16X(3-) ions. This structure is discussed in comparison with other Bi or Sb pnictide compounds. Bonding is analyzed therein using molecular orbital (EHMO) calculations for the anions (dumbbell and square units) and also the periodic tight-binding method. Lone pair repulsions inside and between the anionic units are evidenced; they are compensated by strong bonding cation-to-anion interactions. Interatomic distances along the series appear to be more dependent on packing than on electronic effects. (C) 2002 Elsevier Science (USA).