The Zintl phase Ba 3 Si 4 has been synthesized from the elements at 1273 K as a single phase. No homogeneity range has been found. The compound decomposes peritectically at 1307(5) K to BaSi 2 and melt. The butterfly-shaped Si 4 6Ϫ Zintl anion in the crystal structure of Ba 3 Si 4 (Pearson symbol tP28, space group P4 2 /mnm, a ϭ 8.5233(3) Å , c ϭ 11.8322(6) Å ) shows only slightly different Si-Si bond lengths of d(SiϪSi) ϭ 2.4183(6) Å (1ϫ) and 2.4254(3) Å (4ϫ). The compound is diamagnetic with χ ഠ Ϫ50 ϫ 10 Ϫ6 cm 3 mol Ϫ1 . DC resistivity measurements show a high electrical resistivity (ρ(300 K) ഠ 1.2 ϫ 10 Ϫ3 Ω m) with positive temperature gradient dρ/dT. The temperature dependence of the isotropic signal shift and the spin-lattice relaxation times in 29 Si 1651 NMR spectroscopy confirms the metallic behavior. The experimental results are in accordance with the calculated electronic band structure, which indicates a metal with a low density of states at the Fermi level. The electron localization function (ELF) is used for analysis of chemical bonding. The reaction of solid Ba 3 Si 4 with gaseous HCl leads to the oxidation of the Si 4 6Ϫ Zintl anion and yields nanoporous silicon.

Two new binary Zintl phases, Sr 3 Sn 5 and Ba 3 Sn 5 were synthesized and structurally characterized. The revised structure of Ba 3 Pb 5 is also reported. All three compounds are isotypic and crystallize with a modified Pu 3 Pd 5 structure type. The anionic substructure is composed of X 5 6± square pyramidal clusters (X = Sn, Pb), which are described as arachno clusters according to the Wade-Mingos electron counting rules. The electronic structure of the pyramidal Zintl anions and the influence of the number of skeletal electrons of these clusters are investigated using the electron localization function (ELF). The structural relationship between Ba 3 Sn 5 and the Zintl phases Ba 3 Si 4 and Ba 3 Ge 4 are analyzed. Additionally, two new Zintl phases Ba 3 Ge 2.82 Sn 2.18 and Ba 3 Ge 3.94 Sn 0.06 , have been synthezised and their structures are reported, which directly show that the exchange of tin against germanium leads to a change from the M 3 X 5 to the M 3 X 4 structure type. This effect is traced back to the maximal charge acquisition property of the Zintl anions of heavier and lighter tetralides.Neue Zintl-Anionen vom arachno-Typ im Sr 3 Sn 5 , Ba 3 Sn 5 und Ba 3 Pb 5 sowie der Ladungseinfluû auf die Zintl-Cluster Inhaltsu È bersicht. Sr 3 Sn 5 , Ba 3 Sn 5 und Ba 3 Pb 5 wurden durch Feststoffsynthese erhalten und strukturell charakterisiert. Die bisher bekannte Struktur von Ba 3 Pb 5 wurde korrigiert. Alle drei Verbindungen sind demnach isotyp und geho È ren einem modifizierten Pu 3 Pd 5 an und enthalten die quadratisch-pyramidalen Zintl-Anionen X 5 6± (X = Sn, Pb). Diese erfu È llen die Elektronenabza È hlregeln von Wade und Mingos fu È r arachno-Cluster. Mit Hilfe der Elektronenlokalisierungsfunktion (ELF) wird die ra È umliche Elektronenstruktur verdeutlicht und ihre Abha È ngigkeit von der Valenzelektronen-zahl untersucht. Die engen strukurellen Beziehungen zwischen Ba 3 Sn 5 , Ba 3 Si 4 und Ba 3 Ge 4 werden aufgezeigt. Des weiteren wird u È ber zwei neue Zintl-Phasen, Ba 3 Ge 2.82 Sn 2.18 und Ba 3 Ge 3.94 Sn 0.06 , berichtet, bei denen der Ûbergang vom M 3 X 5 -in den M 3 X 4 -Typ durch Austausch von Zinn gegen Germanium gezeigt werden kann. Dieser Wechsel wird nicht auf Gro È ûeneffekte sondern auf die maximale Ladungsaufnahmefa È higkeit der Zintl-Anionen der schwereren und der leichteren Tetralide zuru È ckgefu È hrt. Fig. 5 Top: Bandstructure and DOS for Ba 3 Sn 5 calculated using EHMO (left) and LMTO (right) methods. Bottom: Bandstructure and DOS for Sr 3 Sn 5 calculated using EHMO (left) and LMTO (right) methods.

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In the systems Ba/Mg/(Si, Ge), two new ternary Zintl phases with the compositions BaMg(4)Si(3) and BaMg(4)Ge(3) were found and structurally characterized. Both compounds are isotypic and crystallize with a new structure type [BaMg(4)Si(3), P4/mmm, Z = 1, a = 4.6115(9) Å, c = 8.615(2) Å; BaMg(4)Ge(3), P4/mmm, Z = 1, a = 4.6335(8) Å, c = 8.746(2) Å]. The anionic partial structure is built of X(2)(6)(-) dumbbells and isolated X(4)(-) ions (X = Si, Ge), and the structure can be described according to the Zintl-Klemm-Busmann concept with the formulation (Ba(2+))(Mg(2+))(4)[X(2)(6)(-)][X(4)(-)]. The BaMg(4)X(3) structure can also be described as a result of the fusion of the structures of Mg(2)X and BaMg(2)X(2). In this regard, BaMg(4)Si(3) and BaMg(4)Ge(3) are very striking examples of chemical twinning.