New members of the A₂M′M₂^″ structure family (A=Ca, Sr, Yb, La; M′=In,Sn,Pb; M″=Si,Ge)

The new mixed tetrelides Sr₂PbGe₂ and Yb₂SnGe₂, several mixed Ca/Sr (A^II) germanides A₂^II(Sn,Pb)Ge₂ and two polymorphs of La₂InSi₂ represent new members of the general structure family of ternary alkaline-earth/lanthanoid main group silicides/germanides A₂M′M₂^″(M′=In,Sn,Pb;M″=Si,Ge). All compounds were synthesized from melts of the elements and their crystal structures have been determined by means of single crystal X-ray diffraction. Sr₂PbGe₂ (Cmmm, a=402.36(11), b=1542.3(4), c=463.27(10) pm) crystallizes with the Mn₂AlB₂-type structure. In exhibiting infinite planar Ge zig-zag chains, it represents one border of the compound series. The other borderline case, where only [Ge₂] dumbbells are left as Ge building units, is represented by the Ca/Yb tin germanides Ca₂SnGe₂ and Yb₂SnGe₂ (Mo₂FeB₂-type; P4/mbm, a=748.58(13)/740.27(7), c=445.59(8)/435.26(5) pm). In between these two border structures compounds with variable Si/Ge chain lengths could be obtained by varying the averaged size of the A^II cations: Ca_0.45Sr_1.55PbGe₂ (new structure type; Pbam, a=791.64(5), b=2311.2(2), c=458.53(3) pm) contains planar six-membered chain segments [Ge₆]. Tetrameric pieces [Ge₄] are the conspicuous structure elements in Ca_1.16Sr_0.84SnGe₂ and La₂InSi₂ (La₂InNi₂-type; Pbam, a=781.01(2)/762.01(13), b=1477.95(3)/1494.38(6), c=457.004(9)/442.1(3) pm). The tetragonal form of 'La₂InSi₂^′ (exact composition: La₂In_1.07Si_1.93, P4/mbm, a=1309.11(12), c=443.32(4) pm) also crystallizes in a new structure type, containing only [Si₃] trimers as cutouts of the planar chains. In all structures the Si/Ge zig-zag chains/chain segments are connected by In/Sn/Pb atoms to form planar M layers, which are separated by pure A layers. Band structure calculations within the FP-LAPW DFT approach together with the Zintl formalism, extended by the presence of hypervalent bonding of the heavier M′ elements, give insight into the chemical bonding of this series of p-block metallides. An analysis of the band structure for the border phases Sr₂PbGe₂ and Ca₂SnGe₂ shows the considerable π bonding contributions within the Ge building units, which also become apparent from the short Ge–Ge bond lengths. - Graphical abstract: Example of one of the mixed metallides A₂(In/Sn/Pb)(Si/Ge)₂ with planar Si/Ge zig-zag chain segments of variable lengths. - Highlights: • Mixed metallides A₂(In/Sn/Pb)(Si/Ge)₂ were prepared for A=Ca, Sr, Yb, La. • The structures exhibit planar Si/Ge zig-zag chain segments of variable lengths. • In, Sn and Pb atoms are connecting the Si/Ge anions to planar nets. • Atomic size effects are investigated by the synthesis of mixed Ca/Sr germanides. • Bandstructure calculations indicate Si/Ge–Si/Ge π bonding contributions