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Silicon is the most important material for solar cells. However, its low conversion rate/quantum efficiency requires a rather high material consumption. Thus, researchers undertake enormous experimental and theoretical efforts to find alternative Si allotropes with a better efficiency and in the ideal case a direct electronic band gap. In recent years and months many new allotropic Si structures have been reported; however, they are often described incoherently and without context to existing structures. Our approach allows a classification of many of these allotropes and a relation of their structures to substructures of, for example, known Zintl phases. For this so-called "chemi-inspired" search for promising Si structures we present a "construction kit" as a guide to introducing a large number of tetrahedral Si allotropes, all derived by a modification of the pristine cubic diamond structure. In addition, this approach allows us to realize structural (topological) relationships between experimentally accessible Zintl phases of different composition, such as open tetrahedral frameworks, to a yet unknown extent, including even known phase transitions of specific Zintl phases. In topological, structural, and computational analyses (on a DFT-PBE0/SVP level of theory) we show the close relationship of ten low-energy Si structures derived from the cubic diamond modification; five of them are new, and some show quasi-direct band gaps. A simple deviation of this approach enables the construction of many more tetrahedral structures., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A systematic approach to the formation of endohedrally filled atom clusters by a high-temperature route instead of the more frequent multistep syntheses in solution is presented. Zintl phases Na12Ni(1-x)Sn17 and K(13-x)Co(1-x)Sn17, containing endohedrally filled intermetalloid clusters [Ni@Sn9](4-) or [Co@Sn9](5-) beside [Sn4](4-), are obtained from high-temperature reactions. The arrangement of [Ni@Sn9](4-) or [Co@Sn9](5-) and [Sn4](4-) clusters, which are present in the ratio 1:2, can be regarded as a hierarchical replacement variant of the hexagonal Laves phase MgZn2 on the Mg and Zn positions, respectively. The alkali-metal positions are considered for the first time in the hierarchical relationship, which leads to a comprehensive topological parallel and a better understanding of the composition of these compounds. The positions of the alkali-metal atoms in the title compounds are related to the known inclusion of hydrogen atoms in the voids of Laves phases. The inclusion of Co atoms in the {Sn9} cages correlates strongly with the number of K vacancies in K(13-x)Co(1-x)Sn17 and K(5-x)Co(1-x)Sn9, and consequently, all compounds correspond to diamagnetic valence compounds. Owing to their diamagnetism, K(13-x)Co(1-x)Sn17, and K(5-x)Co(1-x)Sn9, as well as the d-block metal free binary compounds K12Sn17 and K4Sn9, were characterized for the first time by (119)Sn solid-state NMR spectroscopy., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two modifications of the new Zintl compound Na2ZnSn5 were synthesized by direct reactions of the elements. hP-Na2ZnSn5, which is metastable under standard conditions, is obtained by fast cooling of a melt of stoichiometric composition. Slow cooling of such a melt or tempering of hP-Na2ZnSn5 (e.g., at 300 °C) leads to the thermodynamically stable tI-Na2ZnSn5. Both phases show an open framework structure of four-bonded Zn and Sn atoms exhibiting hexagonal helical channels in which the Na atoms are situated with disorder. Whereas the Zn-Sn network of hP-Na2ZnSn5 is analogous to known Tr-Sn networks (Tr = Ga, In), tI-Na2ZnSn5 features a closely related novel framework with a different channel structure. In the structure model for hP-Na2ZnSn5 there is only one, Zn/Sn mixed occupied, site for the framework atoms, whereas Zn and Sn are fully ordered on three sites in the case of tI-Na2ZnSn5. The phase transition from hP-Na2ZnSn5 to tI-Na2ZnSn5 was studied using high-temperature powder and single-crystal X-ray diffraction methods. Na2ZnSn5 is stable up to about 350 °C and does not melt congruently but decomposes to form Na5Zn(2+x)Sn(10-x). DFT band structure calculations (TB-LMTO-ASA) were performed with ordered model structures which were deduced from a conceivable pathway for the interconversion of the two polymorphic structures of Na2ZnSn5. A band gap at the Fermi level, as expected for a Zintl phase, is found for the ordered structure of tI-Na2ZnSn5. On the basis of an analysis of the relationship between the network structures of the Sn-rich Na-Zn-Sn phases, a general perspective for novel open framework structures with exclusively four-bonded atoms is given.

In view of the search for alternative structures for Li ion battery materials and electron-poor framework semiconductors for thermoelectric applications, the systems Li-Zn-Tt with Tt = Ge or Sn were investigated. Li3Zn2Sn4 and Li2ZnGe3 were obtained by high-temperature syntheses from the elements. The crystal structures of both phases were determined with single-crystal X-ray diffraction methods and the electronic structure of Li3Zn2Sn4 was analyzed by means of DFT calculations (TB-LMTO-ASA). Both phases show diamond polytype analogous Zn-Tt networks with tetrahedrally four-coordinated Zn and Tt atoms. The new phase Li3Zn2Sn4 crystallizes in space group P6(3)/mmc (No. 194) with lattice parameters a = 4.528(1) Å and c = 22.119(2) Å. Zn and Sn atoms are fully ordered on three sites that constitute a 6H diamond polytype like network. Li2ZnGe3 is also described in space group P6(3)/mmc (No. 194) with lattice parameters a = 4.167(1) Å and c = 6.754(1) Å. The Zn-Ge substructure is a hexagonal diamond (2H polytype) like network. The existence of such a Ge-rich Li-Zn-Ge phase has already been reported, but a full structure determination has not yet been published. No indication for an ordering of Zn and Ge atoms on different sites could be deduced from the X-ray diffraction data. Band structure calculations for Li3Zn2Sn4 indicate that the phase is metallic, with the Fermi level at the flank of a pseudogap in the density of states curve. The topological analysis of the electron localization function (ELF) shows covalent Sn-Sn bonding and lone pair like valence basins for the Sn atoms. Concerning the appearance of the lone pair like ELF basins, a strong influence of the basis set for Li that is employed in the calculations is found.

The number of Zintl phases containing polyhedral clusters of tetrel elements that are accessible for chemical reactions of the main-group element clusters is rather limited. The synthesis and structural characterization of two novel ternary intermetallic phases A(14)ZnGe(16) (A = K, Rb) are presented, and their chemical reactivity is investigated. The compounds can be rationalized as Zintl phases with 14 alkali metal cations A(+) (A = K, Rb), two tetrahedral [Ge(4)](4-) Zintl anions, and one anionic heterometallic [(Ge(4))Zn(Ge(4))](6-) cluster per formula unit. The Zn-Ge cluster comprises two (Ge(4)) tetrahedra which are linked by a Zn atom, with one (Ge(4)) tetrahedron coordinating with a triangular face (η(3)) and the other one with an edge (η(2)). [(η(3)-Ge(4))Zn(η(2)-Ge(4))](6-) is a new isomer of the [(Ge(4))Zn(Ge(4))](6-) anion in Cs(6)ZnGe(8). The phases dissolve in liquid ammonia and thus represent rare examples of soluble Zintl compounds with deltahedral units of group 14 element atoms. Compounds with tetrahedral [E(4)](4-) species have previously been isolated from solution for E = Si, Sn, and Pb, and the current investigation provides the "missing link" for E = Ge. Reaction of an ammonia solution of K(14)ZnGe(16) with MesCu (Mes = 2,4,6-(CH(3))(3)C(6)H(2)) in the presence of [18]-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) yielded crystals of the salt [K([18]-crown-6)](2)K(2)[(MesCu)(2)Ge(4)](NH(3))(7.5) with the polyanion [(MesCu)(2)Ge(4)](4-). This MesCu-stabilized tetrahedral [Ge(4)](4-) cluster also completes the series of [(MesCu)(2)Si(4-x)Ge(x)](4-) clusters, which have previously been isolated from solution for x = 0 and 0.7, as the end member with x = 4. The electronic structures of [(Ge(4))Zn(Ge(4))](6-) and [(MesCu)(2)Ge(4)](4-) were investigated in terms of a molecular orbital description and analyses of the electron localization functions. The results are compared with band structure calculations for the A(14)ZnGe(16) phases (A = K, Rb).

Not quasi-crystalline: approximate five-fold symmetry appears in bronze-type Cu-Sn rods that are separated by Na atoms in Na(2.8)Cu(5)Sn(5.6). The rods are compared to the [Sn@Cu(12)@Sn(20)](12-) cluster in the recently characterized A(12)Cu(12)Sn(21) phases (A=Na-Cs), and a saltlike description in analogy to Zintl phases of p-block metals is introduced for these new polar intermetallic phases with a high d-block metal content., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis and crystal structure of the first ternary A-Cu-Sn intermetallic phases for the heavier alkali metals A = Na to Cs is reported. The title compounds A(12)Cu(12)Sn(21) show discrete 33-atom intermetalloid Cu-Sn clusters {Sn@Cu(12)@Sn(20)}, which are composed of {Sn(20)} pentagonal dodecahedra surrounding {Cu(12)} icosahedra with single Sn atoms at the center. Na(12)Cu(12)Sn(21) and K(12)Cu(12)Sn(21) were characterized by single-crystal XRD studies, and the successful synthesis of analogous A-Cu-Sn compounds with A = Rb and Cs is deduced from powder XRD data. The isotypic A(12)Cu(12)Sn(21) phases crystallize in the cubic space group Pn ̅3m (No. 224), with the Cu-Sn clusters adopting a face centered cubic arrangement. A formal charge of 12- can be assigned to the {Sn@Cu(12)@Sn(20)} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [Sn@Cu(12)@Sn(20)](12-) clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na(12)Cu(12)Sn(21) and K(12)Cu(12)Sn(21), DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu(12)@Sn(20)](12-) cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [Sn@Cu(12)@Sn(20)](12-) cluster MOs can be classified with labels indicating the numbers of radial and angular nodes, in the style of spherical shell models of cluster bonding., (© 2011 American Chemical Society)

For a long time, Zintl ions of Group 14 and 15 elements were considered to be remarkable species domiciled in solid-state chemistry that have unexpected stoichiometries and fascinating structures, but were of limited relevance. The revival of Zintl ions was heralded by the observation that these species, preformed in solid-state Zintl phases, can be extracted from the lattice of the solids and dissolved in appropriate solvents, and thus become available as reactants and building blocks in solution chemistry. The recent upsurge of research activity in this fast-growing field has now provided a rich plethora of new compounds, for example by substitution of these Zintl ions with organic groups and organometallic fragments, by oxidative coupling reactions leading to dimers, oligomers, or polymers, or by the inclusion of metal atoms under formation of endohedral cluster species and intermetalloid compounds; some of these species have good prospects in applications in materials science. This Review presents the enormous progress that has been made in Zintl ion chemistry with an emphasis on syntheses, properties, structures, and theoretical treatments., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The endohedral stannaspherene cluster anion [Ir@Sn(12)](3-) was synthesized in two steps. The reaction of K(4)Sn(9) with [IrCl(cod)](2) (cod: 1,5-cyclooctadienyl) in ethylenediamine (en) solution first yielded the [K(2,2,2-crypt)](+) salt (2,2,2-crypt: 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) of the capped cluster anion [Sn(9)Ir(cod)](3-). Subsequently, crystals of this compound were dissolved in en, followed by the addition of triphenylphosphine or 1,2-bis(diphenylphosphino)ethane and treatment at elevated temperatures. [Ir@Sn(12)](3-) was obtained and characterized as the [K(2,2,2-crypt)](+) salt. The isolation of [Sn(9)Ir(cod)](3-) as an intermediate product establishes that the formation of the stannaspherene [Ir@Sn(12)](3-) occurs through the oxidation of [Sn(9)Ir(cod)](3-). Among the structurally characterized tetrel cluster anions, [Ir@Sn(12)](3-) is a unique example of a stannaspherene, and one of the rare spherical clusters encapsulating a metal atom that is not a member of Group 10. Single-crystal structure determination shows that the novel Zintl ion cluster has nearly perfect icosahedral I(h) point symmetry.

Inorganic pentaprismane: The unusual structure of the anion [Co@Ge(10)](3-), which was obtained by the reaction of K(4)Ge(9) with [Co(C(8)H(12))(C(8)H(13))] in ethylenediamine, raises questions about chemical bonding in the anion. The Zintl ion cluster has virtual D(5h) symmetry and is a unique example of a ligand-free cluster that is not a deltahedron. The delocalized chemical bonding is represented in the picture by one of the bonding orbitals of the anion.