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Ternary mixed Ca/Sr-Ba triplumbides of overall composition A II Pb 3  were synthesized from stoichiometric mixtures of the elements. The structures of the compounds have been determined by means of single crystal X-ray data. All structures exhibit close-packed ordered APb 3  layers containing Pb Kagomé nets, which are stacked in different orientations. Depending on the stacking sequences, the resulting lead polyanion resembles the oxygen nets of the hexagonal (face sharing octahedra, h stacking, Ni 3 Sn-type structure) or the cubic perovskites (corner sharing octahedra, c stacking, Cu 3 Au-type structure). The known binary compound BaPb 3 , the structure of which has been redetermined from single crystal data (trigonal, space group R3̄̄m, a = 729.06(2), c = 2564.43(10) pm, Z = 9, R1 = 0.0353), shows a (hhc) 3  stacking (TaCo3-type structure). A small partial substitution of barium against calcium (Ca 0.03 Ba 0.97 Pb 3 : trigonal, space group R3̄̄m, a = 726.0(2), c = 3443(2) pm, Z = 12, R1 = 0.0542) or strontium (Sr 0.11 Ba 0.89 Pb 3 : a = 727.3(2), c = 3421(2) pm, Z = 12, R1 = 0.0424) causes a structural change to the HT-PuGa 3  structure type with a (hhcc) 3  stacking sequence. At an approximate 1 : 1 ratio (35 to 53 % Sr) of strontium and barium (Sr 0.56 Ba 0.44 Pb 3 : trigonal, space group P6 3 /mmc, a = 715.82(2), c = 1717.91(7) pm, Z = 6, R1 = 0.0309) the PuAl 3  structure type [(hcc) 2 -stacking] has a distinct homogenity range. The series is terminated with the pure c stacking of SrPb 3  and CaPb 3 . As already noted from the above series, the stacking of the close-packed layers is influenced by the ratio of the atomic radii of the contributing elements. The electronic stability ranges, which are discussed on the basis of the results of FP-LAPW band structure calculations and in comparison to further compounds known from the literature, can be explained using Zintl/Wade rules. Still, due to the presence of only partially occupied steep Pb-p bands of σ bonding characteristic, the compounds are metals exhibiting pseudo band gaps at or near the Fermi level. Thus this structure family represents an instructive case of transition from polar ionic/covalent towards (inter)metallic chemistry.

zeitschrift für naturforschung b

Gemischte Plumbide (Ca/Sr) x Ba 1 –x Pb 3 . Strukturchemie und chemische Bindung/ Mixed Plumbides (Ca/Sr) x Ba 1−x Pb 3 . Structural Chemistry and Chemical Bonding