New members of the structure family (A=Ca, Sr, Yb, La; ; )

The new mixed tetrelides and , several mixed Ca/Sr (A super(II)) germanides and two polymorphs of represent new members of the general structure family of ternary alkaline-earth/lanthanoid main group silicides/germanides . All compounds were synthesized from melts of the elements and their crystal structures have been determined by means of single crystal X-ray diffraction. (Cmmm, a=402.36(11), b=1542.3(4), c=463.27(10) pm) crystallizes with the structure. In exhibiting infinite planar Ge zig-zag chains, it represents one border of the compound series. The other borderline case, where only dumbbells are left as Ge building units, is represented by the Ca/Yb tin germanides and ( ; 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 super(II) cations: (new structure type; Pbam, a=791.64(5), b=2311.2(2), c=458.53(3) pm) contains planar six-membered chain segments . Tetrameric pieces are the conspicuous structure elements in and ( ; 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 (exact composition: , P4/mbm, a=1309.11(12), c=443.32(4) pm) also crystallizes in a new structure type, containing only 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 elements, give insight into the chemical bonding of this series of p-block metallides. An analysis of the band structure for the border phases and shows the considerable pi bonding contributions within the Ge building units, which also become apparent from the short Ge-Ge bond lengths. Example of one of the mixed metallides A sub(2)(In/Sn/Pb)(Si/Ge) sub(2) with planar Si/Ge zig-zag chain segments of variable lengths.