Novel barium triel/tetrelides with the Pu3Pd5 structure type

A series of ternary barium triel(MIII)/tetrel(MIV)-ides Ba3MxIIIM5−xIV(MIII=Ga,In;MIV=Si,Ge,Sn;x=0−0.7) has been prepared from melts of the elements. They all crystallize with the Pu3Pd5 type structure (orthorhombic, space group Cmcm) exhibiting isolated M5 clusters of slightly distorted nido shape (square pyramids). For the silicides, where the binary border compound Ba3Si5 does not exist, the Pu3Pd5 type is stabilized by substituting 0.7 Si atoms per formula unit against Ga (Ba3Ga0.7Si4.3:a=1024.82(1),b=856.58(1),c=1024.18(1) pm, R1=0.0220) or by a very small substitution of In (Ba3In0.1Si4.9:a=1017.8(2),b=852.5(2),c=1020.1(3) pm, R1=0.0406). A comparable situation is found for the corresponding germanides, where 0.7 atoms of In (Ba3In0.7Ge4.3:a=1051.3(2),b=864.05(14),c=1054.7(3) pm, R1=0.0248) or Ga (Ba3GaxGe5−x:a=1035.1(1),b=861.5(1),c=1036.8(1) pm, R1=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 Ba3Sn5 itself forms the Pu3Pd5 type. In this case, the structure type remains stable up to a Ga content of 0.5 Ga/f.u. (Ba3Ga0.5Sn4.5:a=1100.41(14),b=896.19(11),c=1111.82(14) pm, R1=0.0169) and also with a substantial In content (Ba3InxSn5−x:x≈0.9(1);a=1110.5(2),b=900.0(2),c=1120.7(2) pm, R1=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 Ba3Sn5 (26 v.e./f.u.) that even the maximal amount of triel substitution 0.7 MIII/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.