Novel barium triel/tetrelides with the [formula omitted] structure type

Abstract

A series of ternary barium triel( M III )/tetrel( M IV )-ides Ba 3 M x III M 5 − x IV ( M III = Ga, In; M IV = Si, Ge, Sn; x = 0 − 0.7 ) has been prepared from melts of the elements. They all crystallize with the Pu 3 Pd 5 type structure (orthorhombic, space group Cmcm) exhibiting isolated M 5 clusters of slightly distorted nido shape (square pyramids). For the silicides, where the binary border compound Ba 3 Si 5 does not exist, the Pu 3 Pd 5 type is stabilized by substituting 0.7 Si atoms per formula unit against Ga ( Ba 3 Ga 0.7 Si 4.3 : a = 1024.82 ( 1 ) , b = 856.58 ( 1 ) , c = 1024.18 ( 1 ) pm, R 1 = 0.0220 ) or by a very small substitution of In ( Ba 3 In 0.1 Si 4.9 : a = 1017.8 ( 2 ) , b = 852.5 ( 2 ) , c = 1020.1 ( 3 ) pm, R 1 = 0.0406 ). A comparable situation is found for the corresponding germanides, where 0.7 atoms of In ( Ba 3 In 0.7 Ge 4.3 : a = 1051.3 ( 2 ) , b = 864.05 ( 14 ) , c = 1054.7 ( 3 ) pm, R 1 = 0.0248 ) or Ga ( Ba 3 Ga x Ge 5 − x : a = 1035.1 ( 1 ) , b = 861.5 ( 1 ) , c = 1036.8 ( 1 ) pm, R 1 = 0.0148 ) are necessary to stabilize a Ge-rich compound of this structure type. For the stannides, the situation is somewhat different, because the binary phase Ba 3 Sn 5 itself forms the Pu 3 Pd 5 type. In this case, the structure type remains stable up to a Ga content of 0.5 Ga/f.u. ( Ba 3 Ga 0.5 Sn 4.5 : a = 1100.41 ( 14 ) , b = 896.19 ( 11 ) , c = 1111.82 ( 14 ) pm, R 1 = 0.0169 ) and also with a substantial In content ( Ba 3 In x Sn 5 − x : x ≈ 0.9 ( 1 ) ; a = 1110.5 ( 2 ) , b = 900.0 ( 2 ) , c = 1120.7 ( 2 ) pm, R 1 = 0.0262 ). As the five-atom nido cluster requires only 24 valence electrons per formula unit according to Zintl and Wade, an excess of electrons would be assumed for the binary tetrelides such as Ba 3 Sn 5 (26 v.e./f.u.) that even the maximal amount of triel substitution 0.7 M III /f.u. attained in the title compounds cannot sufficiently compensate for. An assessment of the geometric influence of varied valence electron counts however, coupled with a detailed analysis of the calculated electron densities and the partial densities of states in the energy region above the pseudo band gap at 24 v.e./f.u. clearly shows that these excess electrons populate π -bonding M- p /Ba- d states and actually contribute to a stabilization of the structure. In the range above 25.3 v.e./f.u. predominantly antibonding M- p states are populated, resulting in a widening of the base of the cluster.