Strontium Metallides along the Section SrIn<sub>4</sub>

In a combined synthetic, X-ray single-crystal and bond-theoretical study the phase widths of SrIn₄ and the related Sr₃In₁₁ upon a sucessive substitution of indium against the more electron-poor, more electronegative and only slightly smaller element mercury have been studied. Along the 1:4 section Sr(In_1–xHg_x)₄ the monoclinic structure of SrIn₄ (EuIn₄-type structure) is stable up to x = 0.14(1) (SrIn_3.44Hg_0.56: monoclinic, C2/m, a = 1208.3(6), b = 502.5(2), c = 997.7(6) pm, β =115.16(3)°, Z = 4, R1 = 0.0324). Further increased mercury content results in a distinct stability region of the very common BaAl₄ structure type (tetragonal, I4/mmm, Z = 2), which starts at x = 0.241(7) (SrIn_3.04Hg_0.96: a = 469.4(2), c = 1246.4(7) pm, R1 = 0.0374) and reaches up to the fully ordered compound SrIn₂Hg₂ (x = 0.5: a = 456.4(3), c = 1273.5(11) pm, R1 = 0.0572). Unexpectedly, in the small composition range from x = 0.68(1) to 0.758(6) (SrIn_1.16Hg_2.84: a = 1178.5(4), b = 495.20(12), c = 1016.8(4) pm, β =119.67(2)°, R1 = 0.0542; SrIn_0.97Hg_3.03: a = 1167.30(4), b = 495.32(2), c = 1018.50(3) pm, β =119.657(2)°, R1 = 0.0426) the EuIn₄ structure type exhibits a further small stability range. The structures of the two slightly different In- and Hg-rich variants of the EuIn₄ type are described and compared, indicating the similarities to the KHg₂ (layers of folded In/Hg ladders) and the BaAl₄ (flat square pyramids) structure types as well as to the structure of elemental mercury (rhombohedra). From samples of an intermediate In/Hg ratio (e. g. x = 0.55), the new 1:3 compound SrIn_1.2Hg_1.8 (monoclinic, C2/m, a = 1168.5(13), b = 497.0(4), c = 1471(2) pm, β =92.07(10)°, Z = 8, R1 = 0.0818) is formed. It crystallizes in a new structure type, which – according to the compound's composition – can be described as a stacking variant of the EuIn₄ and the KHg₂ structure type. In contrast to the wide stability ranges of the 1:4 compounds, only a very small substitution of In ranging up to only 6 % of Hg is possible in the binary indide Sr₃In₁₁ (Sr₃In_10.32(8)Hg_0.68(8): La₃Al₁₁ structure type, orthorhombic, Immm, a = 487.6(2), b = 1140.2(5), c = 1421.1(7) pm, Z = 2, R1 = 0.0703). The formation, stability ranges and the In/Hg 'coloring' of the polyanions are discussed for all compounds taking into account geometric (radius ratios, molar volumes, Sr coordination polyhedra etc.) and electronic aspects, which have been evaluated using FP-LAPW band structure calculations.