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Thomas F Fässler

Publications and source records attributed to Thomas F Fässler.

16 recordsLinked to original sources
PubMed2006

Condensation of [Si3O9]6- anions in the solid state to the dimeric cyclotrisilicate anion [Si6O17]10-.

The structure of the silicate Rb10[Si6O17] containing a novel dimeric cyclotrisilicate anion is reported. The compound is formed by the reaction of a mixture of SiO2 and Rb at temperatures above 700 degrees C. Systematic investigations by means of differential thermal analysis and temperature-dependent powder X-ray diffraction experiments revealed that the new compound evolved from Rb6[Si3O9], which occurred as an intermediate product. Thus, the dimeric anion [Si6O17]10- is formed by condensation of the monomeric cyclotrisilicate [Si3O9]6-. For both silicates, [Si6O17]10- and [Si3O9]6-, the characteristic ring vibration modes were observed in the IR spectrum. The structure of Rb10[Si6O17] was solved and refined from single-crystal X-ray diffraction data in the orthorhombic space group Pbca (No. 61). Synthesis and structure determination of Rb10[Si6O17] bridge the gap to show that the recently reported structures of Rb14[Si4][Si6O17] and Rb14[Ge4][Si6O17] are indeed fascinating intergrowth structures of the stable oxide Rb10[Si6O17] and the Zintl phases RbSi (Rb4Si4) and RbGe (Rb4Ge4), respectively.

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PubMed2006

Localized and delocalized chemical bonding in the compounds CaNiGe2, SrNiGe2, and SrNiSn2.

The new compounds CaNiGe2, SrNiGe2 and SrNiSn2 have been synthesized from the elements by arc melting techniques with subsequent annealing of the sample at 1270 K, and their structures have been determined by single-crystal X-ray diffraction methods. They crystallize in the CeNiSi2 structure (space group Cmcm). For CaNiGe2: a = 4.2213(7) A, b = 17.375(4) A, c = 4.0514(7) A, R(1) = 0.033 (all data); for SrNiGe2: a = 4.429(1) A, b = 17.420(4) A, c = 4.200(1) A, R(1) = 0.041 (all data); and for SrNiSn2: a = 4.5924(7) A, b = 18.710(3) A, c = 4.5228(6) A, R(1) = 0.021 (all data). The main structural motifs are two-dimensionally condensed Ni-centered Ge5 or Sn5 square pyramids. The crystal chemistry and chemical bonding are discussed. Analyses of the electronic structures of CaNiGe2, SrNiGe2, and SrNiSn2, with the help of the electron localization function (ELF), indicate the coexistence of localized covalent and delocalized bonding between the metal atoms involved.

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PubMed2006

Synthesis, crystal structure, and catalytic properties of MgCo6Ge6.

The ternary compound MgCo6Ge6 represents a novel member of the RM6X6 phases, which contains a graphite-type Ge network, Kagomé nets of Co atoms, and Ge2 dumbbells with an unexpected short Ge-Ge contact in the range of a localized Ge-Ge single bond. The title compound shows a large variety of chemical bonding, which ranges from metallic to multicenter and covalent bonding. The role of polar intermetallic alloys as promising candidates for the application as catalysts for the selective hydrogenation of alpha,beta-unsaturated aldehydes is discussed. MgCo6Ge6 possesses a remarkable activity and selectivity for the hydrogenation of cis/trans-citral to geraniol and nerol.

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PubMed2005

KBi(2-x)Pbx (0 < x <or= 1): a Zintl phase evolving from a distortion of the cubic Laves-phase structure.

The quasibinary system KBi(2-x)Pbx has been investigated, both experimentally and theoretically. Phases with compositions 0 < or = x < or = 1.2 were synthesized and structurally characterized by X-ray diffraction experiments. For low values of x (0 < or = x < 0.6), KBi(2-x)Pbx adopts the cubic Laves-phase structure MgCu2 (space group Fdm), which contains a rigid framework of corner-condensed symmetry-equivalent tetrahedra formed by randomly distributed Bi and Pb atoms. For compositions x > or = 0.6, these tetrahedra become alternately elongated and contracted. The distortion of the framework lowers the space-group symmetry to F43m (KBi(1.2)Pb(0.8), F43m, Z = 8, a = 9.572(1) A). Magnetometer measurements show that KBi2 (x = 0) is metallic and goes through a superconducting transition below 3.5 K. First principles calculations reveal that the Fd3m --> F43m distortion is largest for KBiPb (x = 1.0), which at the same time turns into a semiconductor. Thus, F43m KBiPb corresponds to a proper charge-balanced Zintl phase, K+[BiPb]-, with separated polyanionic tetrahedra, (Bi2Pb2)2-. However, it was not possible to prepare F43m KBiPb. Syntheses attempting to increase the Pb content in KBi(2-x)Pbx above x = 0.8 yielded additional, not yet characterized, ternary phases.

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PubMed2005

Ordering of vacancies in type-I tin clathrate: superstructure of Rb8Sn44 square2.

One important step toward the understanding of the mechanisms of thermoelectric properties is the knowledge about local distortions and vacancy ordering in clathrates. The type-I clathrate Rb8Sn44 shows in single-crystal and powder diffraction patterns a 2 x 2 x 2 supercell of the primitive cubic unit cell (Pmn), which originates from an ordering of the partially occupied site. The latter is distributed around a 41 screw axis, and the vacancies lead to a relaxation of the tin framework, thereby creating a local distortion of the tetrakaidecahedron.

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PubMed2005

NaSn2: a novel binary Zintl phase with 2D polyanions of Realgar-type units [Sn8]4-.

NaSn2, obtained from the reaction of stoichiometric amounts of the elements, crystallizes in a new structure type, in which two-dimensional polyanions are separated by Na+ cations. The tin framework is made up of Realgar-type Sn8 units. The density of states obtained from the TB-LMTO-ASA calculations with the implementation of the electron localization function (ELF) reveals that NaSn2 has all characteristics of a Zintl phase. Thus, the title compound can be written as (Na+)4(2infinity)[Sn8(4-)]. NaSn2 dissolves readily in ethylenediamine, under the formation of a reddish brown solution indicative of the presence of [Sn9](4-) anions.

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PubMed2004

Endohedral zintl ions: intermetalloid clusters.

Tetrels can be regarded as most promising candidates for the construction of larger clusters. Recent examples have shown that larger clusters are particularly stable if they contain interstitial atoms (e.g. [Pt@Pb12]2-). Many salts of the polyhedral anions are soluble, but a number of examples-usually those with higher charges-occur only as quasi-discrete units in saltlike crystals (Zintl phases) or as building blocks in intermetallic phases. In this Minireview, the chemistry of intermetalloid clusters is reviewed with reference to the endohedral Zintl ions, Zintl phases, and polyhedral building blocks of intermetallic compounds, including heteroatomic species in the gas phase. We focus on selected examples and discuss the new findings in the context of recent advances in the field of metalloid clusters and (endohedral) fullerenes and fullerides.

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PubMed2004

Nonclassical bonding in the novel structure of Ba2Bi3 and unexpected site preference in the coloring variant Ba2BiSb2.

The new phase Ba(2)Bi(3) crystallizes in the W(2)CoB(2) structure type. Its structure contains rigorously planar anionic layers of (4.6.4.6)(4.6(2))(2) nets with three- and four-bonded Bi, that are separated by Ba atoms. An unexpected site preference is observed in the coloring variant Ba(2)BiSb(2) with Sb occupying only the three-bonded sites. The nonclassical bonding in the anionic network can be rationalized from a reformulation of the Zintl concept as (Ba(2+))(2)[Bi(3)](3)(-)(e(-)). Bonding distances suggest that the extra electron fills Bi-Bi antibonding states. The densities of states obtained from TB-LMTO-ASA calculations show metallic character for both compounds.

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PubMed2004

Synthesis, characterization, and electronic structure of Ba5In4Bi5: an acentric and one-electron deficient phase.

The new ternary phase Ba(5)In(4)Bi(5) was synthesized by direct reaction of the corresponding elements at high temperature. It crystallizes in a noncentrosymmetric space group and represents a new structure type (tetragonal, P4nc with a=10.620(2) and c=9.009(2) A, Z=2). The structure is built of interconnected heteroatomic clusters of In(4)Bi(5), square pyramids with In(4)-bases and four exo-bonded bismuth atoms (bond to the In atoms). According to Wade's rule the compound is electron-deficient with one electron per cluster, that is, [In(4)Bi(5)](10-) instead of the expected [In(4)Bi(5)](11-) for a closed-shell species. The clusters are discussed also in light of the known heteroatomic deltahedral clusters with the same composition but different charge, [In(4)Bi(5)](3-). Band structure calculations on the new compound suggest substantial participation of barium in the overall bonding of the structure that "accounts" for the electron shortage

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PubMed2003

SrSn4: a superconducting stannide with localized and delocalized bond character.

The title compound is the tin-richest phase in the system Sr-Sn and is obtained by stoichiometric combination of the elements. SrSn(4) peritecticly decomposes under formation of SrSn(3) and Sn at 340 degrees C. The structure determined from a single crystal shows a new structure type with a novel structure motive in tin chemistry. It can be described by a corrugated, distorted quadratic net of tin atoms as the only building unit. The nets intersect at common Sn atoms, and the resulting channels host the Sr atoms. The structure can alternatively be described as an intergrowth structure of the AlB(2)-type and W-type. The atoms that are connected by the two shortest Sn-Sn distances (2.900 and 3.044 A) form a two-dimensional net consisting of hexagons of tin atoms. The hexagons have boat conformation in contrast to the rather similar alpha-As structure type, where hexagons have a chair conformation. Further tin atoms connect the two-dimensional net of Sn hexagons. Temperature-dependent magnetic susceptibility measurements show that SrSn(4) is superconducting with T(c) = 4.8 K at 10 G. LMTO band structure and density of states calculations verify the metallic behavior of SrSn(4). An analysis of the electronic structure with the help of the electron localization function (ELF) shows that localized covalent bonds beside delocalized bonds coexist in SrSn(4).

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PubMed2003

Na7Sn12: a binary Zintl phase with a two-dimensional covalently bonded tin framework.

Na(7)Sn(12) was synthesized by quenching of stoichiometric amounts of the elements (700 degrees C) in a sealed niobium ampule and further thermal treatment at 270 degrees C for 40 days. Single crystals of Na(7)Sn(12) were obtained from a mixture with the composition Na(6)SrSn(16). The structure of Na(7)Sn(12) consists of two-dimensional polyanions 2 (infinity) [Sn(12)(7-)], which are separated by Na atoms. Bonding Sn-Sn contacts in the polyanion vary between 2.827(2) and 3.088(2) A. Crystal data: monoclinic, P2/n, Z = 4, a = 13.375(3) A, b = 9.239(2) A, c = 17.976(4) A, gamma = 90.15(3) degrees, V = 2243.0(8) A(3), mu = 13.22 mm(-1), d(calc) = 4.694 g cm(-3), R1(F) = 6.1% (for all reflections). Extended-Hückel tight-binding calculations with the implementation the electron localization function (ELF) reveal that Na(7)Sn(12) can be viewed as an intermetallic compound with exclusively localized bonding and nonbonding regions as expected from the 8 - N rule. Thus Na(7)Sn(12) is a Zintl phase with the formula (Na(+))(7)[(2b)Sn(2)(-)](1)[(3b)Sn(-)](5)[(4b)Sn(0)](6).

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PubMed2003

The role of non-bonding electron pairs in intermetallic compounds.

The Electron Localisation Function, ELF pictorially visualises chemists' intuitive ideas of single and multiple bonds as well as non-bonding electron pairs in molecules. The power of the representation of chemical bonds via ELF is that on the one hand covalent, polar, and ionic bonds are distinguishable, and that on the other hand ELF can be calculated for molecules and solids. This enables us to transfer the ideas of chemical bonding from molecular to intermetallic compounds. Localised two-electron-two-centre bonds and lone pairs are present in solid-state valence compounds (Zintl phases) as expected by the 8-N rule. In solids, lone pairs are generally more contracted than in molecules due to 'lone-pair repulsion'. In intermetallic compounds localised electrons predominantly occur in the form of lone pairs. Lattice vibrations influence the strength of lone pair interactions and non-bonded interactions lead to an exchange of delocalised and localised electrons. Such a mechanism of local electron pair formation gives rise to ideas of a chemical view of the phenomenon of superconductivity in intermetallic compounds.

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