Sr(Hg1–x Sn x )4: variations of the EuIn4-type structure

Abstract

Abstract Starting from the new compound SrHg2Sn2, which is isoelectronic and also isotypic to the indide SrIn4, the successive substitution of Sn against the electron poor Hg has been investigated in a combined synthetic, crystallographic, and bond-theoretical study. Along the 1:4 section Sr(Hg1–x Sn x )4 a series of compounds with Sn contents x between 0.5 and 0.2 were synthesized from stoichiometric ratios of the elements. Their crystal structures, which represent three different variants of the EuIn4-type structure, have been determined using single crystal X-ray data. The most electron rich compound SrHg2Sn2 crystallizes in the original EuIn4-type [monoclinic, C2/m, a = 1257.9(14), b = 490.1(4), c = 997.8(12) pm, β = 117.60(6)°, Z = 4, R1 = 0.0838], with a fully ordered Hg and Sn distribution. The four atom sites form two different folded ladders with an alternating Hg/Sn distribution. Like in the KHg2-type, the ladders are connected via six-membered rings. In between, double tubes with an internal Sn–Sn bond are connected via further Sn–Sn bonds to form sheets similar to those observed in SiAs. The most electron-poor phase SrHg3.2Sn0.8 crystallizes in a strongly distorted variant of this structure [a = 1172.8(4), b = 497.9(2), c = 1010.0(4) pm, β = 118.860(7)°, Z = 4, R1 = 0.0549]. Herein, additional Hg–Hg bonds are formed, and the open tubes are distorted into rods of edge-sharing rhombohedra resembling the structure motifs of elemental Hg. At an intermediate valence electron (v.e.) number, i.e., in SrHg2.5Sn1.5, an isomorphous tripled superstructure (a = 2704.4(5), b = 493.87(7), c = 1197.1(2) pm, β = 90.838(14)°, Z = 12, R1 = 0.0475) occurs, where the building blocks of the two variants of the EuIn4-type structure alternate in a 1:2 ratio. The bonding situation and the “coloring,” i.e., the Hg/Sn distribution in the polyanionic network, are discussed considering the different sizes of the atoms and the charge distribution (Bader AIM charges), which has been calculated within the framework of the FP-LAPW density functional theory for SrHg2Sn2 and the model compounds “SrHg3Sn” and “SrHg4.”