The intermetallic compounds Sr 11 Bi 10 , Ba 11 Bi 10 , and (Sr 5 Ba 6 )Sb 10 have been obtained from melts of mixtures of the elements. They crystallize in the tetragonal system, space group I4/mmm, Ho 11 Ge 10 structure type, tI84 Pearson symbol, Z ؍ 4, with cell parameters a ؍ 12.765(3), 13.230(3), 12.748(2) A s and c ؍ 18.407(3), 19.365(3), 18.761(2) A > , respectively. The structures were solved from single-crystal X-ray data and re5ned by full-matrix least-squares to R1 ؍ 6.71, 5.44, and 5.73%. The structure of M 11 X 10 contains three discrete anionic moieties: square rings X 4؊ 4 , dumbbells X 4؊ 2 , and isolated X 3؊ . Using formal charges the unit cell of M 11 X 10 may be described as containing 44 M 2؉ , 2X 4؊ 4 , 8X 4؊ 2 , 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 e4ects.
2002 Elsevier Science (USA)