Novel Tin Structure Motives in Superconducting BaSn5 – The Role of Lone Pairs in Intermetallic Compounds [1]

Abstract

BaSn 5 is the tin richest phase in the system Ba/Sn and is obtained by stoichiometric combination of the elements. The compound peritecticly decomposes under formation of BaSn 3 and a Sn±Ba melt at 430°C. The structure shows a novel structure motive in tin chemistry. Tin atoms are arranged in graphite-like layers (honeycombs). Two such layers form hexagonal prisms which are centered by Sn. Consequently the central tin atom has the unusual coordination number 12. The two-dimensional tin slabs which consist of two 3 6 and one 6 3 nets of Sn atoms are separated by 6 3 nets of Ba atoms with Ba above the center of each tin hexagon. The structure of BaSn 5 can be rationalized as a variante of AlB 2 and thus also of the superconducting MgB 2 .Temperature dependent magnetic susceptibility measurements show that BaSn 5 is superconducting with T c = 4.4 K.Reinvestigation of the magnetism of the Ba richer phase BaSn 3 reveals for this compound a T c of 2.4 K. LMTO band structure and density of states calculations verify the metallic behavior of BaSn 5 . The van Hove scenario of high-temperature cuprate superconductors is discussed for this`classical' intermetallic superconductor. An analysis of the electronic structure with the help of fat-band projections and the electron localization function (ELF) shows that the van Hove singularity in the DOS originates from non-bonding (lone) electron pairs in the intermetallic phase BaSn 5 . The role of lone pairs in intermetallic phases is discussed with respect to superconducting properties.

Polyhedral nine-atom clusters of tetrel elements and intermetalloid derivatives

“…The known examples show that such anionic clusters undergo oxidative addition on the one hand, but can also add step-by-step transition metals on the other. Up to now homoatomic clusters, homoatomic complexes and endohedral clusters derived from Zintl tetrel anions reach the size of 36 Ge atoms in [(Ge 9 ) 4 ] 8− , 45 Ge atoms in [Au 3 Ge 45 ] 8− and 18 E atoms in [Pd 2 @E 18 ] 4− (E = Ge and Sn), respectively. Still there is a necessity for a better understanding of the reactivity and the cluster growth mechanisms, since oxidative coupling reactions often proceed even without the addition of an oxidation reagent.…”

“…The isomerization of the resulting closo clusters with h 4 coordination leads to h 5 -E 9 complexes analogous to the above-mentioned [(h 5 -Pb 5 ){Mo(CO) 3 } 2 ] 4− unit (figure 5b). Recently, other complexes involving d-block elements have been isolated: [Sn 9 -Ir(cod)] 3− (cod = cyclooctadiene) (Wang et al in press), [Ge 9 -Cu(P i Pr) 3 ] 3− (Scharfe & Fässler in press), [E 9 -Zn(C 6 H 5 )] 3− (E = Si-Pb) (Sevov & Goicoechea 2006) and [Sn 9 -Cd(Sn n Bu 3 )] 3− (Zhou et al 2009a).…”

“…A combination of the oxidative coupling reaction described in §3(a) and the complexation of the resulting polyanion reported in §3(b) leads to larger clusters reaching the nanometre scale. [Au 3 Ge 45 ] 9− was found as a by-product of the reaction of ethylenediamine solutions of K 4 Ge 9 and Ph 3 PAu(I)Cl (figure 6d; Spiekermann et al 2007a). By changing the reaction conditions, the yield of this fascinating large cluster can be raised.…”

“…Furthermore, five-membered Ge faces resemble a structural motif found in three-dimensional solids, such as alkali metal and alkaline earth metal germanides with clathrate structures. Ge-Au contacts in [Au 3 Ge 45 ] 9− confine the cluster to a more or less spherical shape. Interestingly, [Au 3 Ge 45 ] 9− exhibits no Au-Au contacts, despite the presence of a largely negative cluster charge (Spiekermann et al 2007a).…”

“…Apparently, nine-atom clusters must be oxidized during the reaction, which is ascribed to cod and/or PPh 3 . A comparison of solid-state structures of [M@Pb 12 ] 2− confirms that the smaller the transition metal M is, the more strongly distorted is the Pb skeleton.…”