Neue gemischte Zinn-reiche Erdalkalimetall-Stannide – Synthese, Strukturchemie und chemische Bindung / New Mixed Tin-rich Alkaline Earth Stannides – Synthesis, Structural Chemistry and Chemical Bonding

Abstract

Ternary mixed Ca/Ba-Sr pentastannides A II Sn 5 (A II = 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 ASn 5 (A = Sr, Ba) were determined by means of single-crystal X-ray data. The structures of both Sr 0.94 Ba 0.06 Sn 5 (monoclinic, space group C2/m, a = 1762.8(11), b = 704.1(3), c = 1986(2) pm, β = 100.31(6) • , Z = 14, R1 = 0.0996) and Sr 0.89 Ba 0.11 Sn 5 (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 A II Sn 3 in forming dimers and trimers of facesharing Sn 6 -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 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.