Gemischte Plumbide (Ca/Sr)xBa1 –xPb3. Strukturchemie und chemische Bindung/ Mixed Plumbides (Ca/Sr)xBa1−xPb3. Structural Chemistry and Chemical Bonding

Abstract

Ternary mixed Ca/Sr-Ba triplumbides of overall composition AIIPb3 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 APb3 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, Ni3Sn-type structure) or the cubic perovskites (corner sharing octahedra, c stacking, Cu3Au-type structure). The known binary compound BaPb3, 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 (Ca0.03Ba0.97Pb3: trigonal, space group R3̄̄m, a = 726.0(2), c = 3443(2) pm, Z = 12, R1 = 0.0542) or strontium (Sr0.11Ba0.89Pb3: a = 727.3(2), c = 3421(2) pm, Z = 12, R1 = 0.0424) causes a structural change to the HT-PuGa3 structure type with a (hhcc)3 stacking sequence. At an approximate 1 : 1 ratio (35 to 53 % Sr) of strontium and barium (Sr0.56Ba0.44Pb3: trigonal, space group P63/mmc, a = 715.82(2), c = 1717.91(7) pm, Z = 6, R1 = 0.0309) the PuAl3 structure type [(hcc)2-stacking] has a distinct homogenity range. The series is terminated with the pure c stacking of SrPb3 and CaPb3. 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.