Abstract
Ternary mixed Ca/Ba-Sr pentastannides AIISn5 (AII = Ca, Sr, Ba) have been synthesized from stoichiometric mixtures of the elements or from tin-rich melts. The crystal structures of two new compounds of overall composition ASn5 (A = Sr, Ba) were determined by means of single-crystal X-ray data. The structures of both Sr0.94Ba0.06Sn5 (monoclinic, space group C2/m, a = 1762.8(11), b = 704.1(3), c = 1986(2) pm, β = 100.31(6)º, Z = 14, R1 = 0.0996) and Sr0.89Ba0.11Sn5 (orthorhombic, space group Cmcm, a = 708.1(2), b = 1770.4(8), c = 2781.6(11) pm, Z = 20 , R1 = 0.0821) are closely related and can be described by different stacking sequences of comparable nets. They both resemble the structural features of the tristannides AIISn3 in forming dimers and trimers of facesharing Sn6-octahedra, which are further connected via common corners. According to the higher tin content, the rods formed of the octahedra are interspersed by additional Sn atoms, which themselves show a bonding situation resembling the structure of elementary tin. The complex tin network formed by the strong Sn-Sn bonds alone can be regarded as a cutout of the hexagonal diamond structure. In this view, the similarities of the title compounds to the known binary stannides BaSn5 and SrSn4 become apparent. The phase widths of the latter have been investigated and shown to reach up to Sr0.37Ba0.63Sn5 (BaSn5 type, hexagonal, space group P6/mmm, a = 536.8(2), c = 695.2(3) pm, R1 = 0.0312) and Sr0.79Ca0.21Sn4 (SrSn4 type, orthorhombic, space group Cmcm, a = 461.7(3), b = 1714.1(14), c = 706.7(4) pm, Z = 4, R1 = 0.0861), respectively. The total density of states calculated for the orthorhombic pentastannide within the FP-LAPW DFT band structure approach shows a broad minimum at the Fermi level, which can be explained using the Zintl and the Wade/Jemmis electron counting rules.
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