On the effect of Ga and In substitutions in the Ca11Bi10 and Yb11Bi10 bismuthides crystallizing in the tetragonal Ho11Ge10 structure type.

The Ga- and In-substituted bismuthides Ca11GaxBi10-x, Ca11InxBi10-x, Yb11GaxBi10-x, and Yb11InxBi10-x (x < 2) can be readily synthesized employing molten Ga or In metals as fluxes. They crystallize in the tetragonal space group I4/mmm and adopt the Ho11Ge10 structure type (Pearson code tI84; Wyckoff sequence n2 m j h2 e2 d). The structural response to the substitution of Bi with smaller and electron-poorer In or Ga has been studied by single-crystal X-ray diffraction methods for the case of Ca11InxBi10-x [x = 1.73 (2); octabismuth undecacalcium diindium]. The refinements show that the In atoms substitute Bi only at the 8h site. The refined interatomic distances show an unconventional - for this structure type - bond-length distribution within the anionic sublattice. The latter can be viewed as consisting of isolated Bi3- anions and [In4Bi820-] clusters for the idealized Ca11In2Bi8 model. Formal electron counting and first-principle calculations show that the peculiar bonding in this compound drives the system toward an electron-precise state, thereby stabilizing the observed bond-length pattern.