Rb5Hg19: Eine neue,geordnete Defektvariante des BaAl4‐Strukturtyps

Rb₅Hg₁₉: A New Defect Variant of the BaAl₄ Structure Type The hitherto unknown, silver metallic, air sensitive Rb₅Hg₁₉ can be prepared directly from the elements. It crystallizes tetragonally with lattice constants a = 1156.1 pm, c = 1051.0 pm (Z = 2, space group I4/m, No. 87, melting temperature 467 K) in a new defect variant of the BaAl₄ structure type. The structure is based on distorted 4⁴ nets of Hg₄ rectangles and squares forming rectangular and square pyramids together with additional Hg atoms. The vertices of the pyramids point in alternate directions with respect to [001]. Opposite vertices show relatively short contacts d_Hg–Hg = 292.3 pm along [001]. The defect character of the Rb₅Hg₁₉ structure shows up by single Hg atoms substituting each fifth Hg–Hg group in an ordered way and serving as common vertices for two subsequent square pyramids along [001]. The replacement of Hg–Hg groups by single Hg atoms explains the vertical and horizontal distortions of the 4⁴ nets and explains the differences of the Rb coordination with respect to Ba (bicapped tetragonal hexagonprism) in the BaAl₄ type, in which a hypothetical RbHg₄ could eventually crystallize. The symmetry relations of Rb₅Hg₁₉ and α‐La₃Al₁₁ (the only defect variant of BaAl₄ except Rb₅Hg₁₉) to the aristo type BaAl₄ are discussed.