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Keywords:

  • Tin;
  • Stannides;
  • Alkaline earth metals;
  • X-ray diffraction;
  • Band structure calculation

Abstract

Ternary mixed Ca/Sr/Ba tristannides of overall composition ASn3 (A = Ca, Sr, Ba) were synthesized from mixtures of the elements. The structures of the compounds were determined by means of single-crystal X-ray data or Rietveld powder refinements. All structures exhibit close packed ordered ASn3 layers containing Sn Kagomé nets, which are stacked in different orientations. Depending on the stacking sequences, the resulting tin polyanion resembles the oxygen nets of the cubic (corner sharing octahedra, c stacking, Cu3Au-type structure, i.e. CaSn3) or of the hexagonal perovskites (face sharing octahedra, h stacking, Ni3Sn-type structure, i.e. BaSn3). In the binary compound CaSn3 (Cu3Au-type) up to 46(5) % of calcium can be substituted against strontium (cubic, space group Pmequation imagem, a = 479.75(3), Z = 1, RP = 0.0587, RBragg = 0.0343). The known binary phase SrSn3 forms the Mg3In structure type with a (hhcc)3 stacking sequence and exhibits no noticeable phase width. A small partial substitution of strontium, against barium (Sr0.91(2)Ba0.09(2)Sn3 to Sr0.81(2)Ba0.19(2)Sn3: trigonal, space group Requation imagem, a = 698.9(2)/701.21(14), c = 2472.4(10)/2465.0(7) pm, Z = 9, R1 = 0.0637/0.0738) causes the packing to switch to the (hhc)3 stacking of the TaCo3-type structure. At a Ba proportion of 26(2) % (Sr0.74(2)Ba0.26(2)Sn3: hexagonal, space group P63/mmc, a = 704.21(12), c = 2197.6(5) pm, Z = 8, R1 = 0.0705) a further structure change to the BaSn2.57Bi0.43 structure type ((hhhc)2-stacking) occurs. The series is terminated with the pure h stacking of BaSn3, the stability range of which starts at the composition Sr0.22(2)Ba0.78(2)Sn3 (hexagonal, space group P63/mmc; a = 717.92(12), c = 548.15(10) pm, Z = 2, R1 = 0.0297). As easily noted from this series, the stacking sequence depends on the ratio of the atomic radii of the contributing atoms/ions. The observed trend can be rationalized by the decreasing coordination number of tin when going from c to h stacking. Covalent as well as metallic aspects of the chemical bonding in the tristannides are discussed on the basis of the results of FP-LAPW calculations (density of states, band structure and valence electron densities).