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The new intermetallic type‐I clathrate Na8Zn4Ge42 was obtained by direct reaction of the elements, and its crystal structure was determined by means of single‐crystal X‐ray diffraction methods. The structure is described in space group Pm3n (no. 223) with a = 10.6982(1) Å. A mixed Zn/Ge occupancy is found for one of the network sites (6c). Na8Zn4Ge42 represents an air‐stable electron‐precise Zintl phase with exclusively Na cations located in the cavities of a polyanionic host network of four‐connected Zn and Ge atoms. [ABSTRACT FROM AUTHOR]

The new intermetallic type‐I clathrate Na8Zn4Ge42 was obtained by direct reaction of the elements, and its crystal structure was determined by means of single‐crystal X‐ray diffraction methods. The structure is described in space group Pm3n (no. 223) with a = 10.6982(1) Å. A mixed Zn/Ge occupancy is found for one of the network sites (6c). Na8Zn4Ge42 represents an air‐stable electron‐precise Zintl phase with exclusively Na cations located in the cavities of a polyanionic host network of four‐connected Zn and Ge atoms. [ABSTRACT FROM AUTHOR]

In a solid-state Li ion battery, the solid-state electrolyte exists principally in regions of high externally applied potentials, and this varies rapidly at the interfaces with electrodes because of the formation of electrochemical double layers. We investigate the implications of these for a model solid-state Li ion Li|Li3OCl|C battery, where C is simply a metallic intercalation cathode. We use density functional theory to calculate the potential dependence of the formation energies of the Li+ charge carriers in superionic Li3OCl. We find that Li+ vacancies are the dominant species at the cathode while Li+ interstitials dominate at the anode. With typical Mg aliovalent doping of Li3OCl, Li+ vacancies dominate the bulk of the electrolyte, as well, with freely mobile vacancies that are only ~10-4 of the Mg doping density at room temperature. We study the repulsive interaction between Li+ vacancies and find that this is extremely short-range, typically only one lattice constant because of local structural relaxation around the vacancy, and this is significantly shorter than pure electrostatic screening. We model a Li3OCl-cathode interface by treating the cathode as a nearly ideal metal using a polarizable continuum model with an εr of 1000. There is a large interface segregation free energy of approximately -1 eV per Li+ vacancy. Combined with the short range for repulsive interactions of the vacancies, this means that very large vacancy concentrations will build up in a single layer of Li3OCl at the cathode interface to form a compact double layer. The calculated potential drop across the interface is ~3 V for a nearly full concentration of vacancies at the surface. This suggests that nearly all the cathode potential drop in Li3OCl occurs at the Helmholtz plane rather than in a diffuse space-charge region. We suggest that our conclusions can be generalized to other superionic conductors, as well. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 Na12Ni1-xSn17 and K13-xCo1-xSn17, 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 K13-xCo1-xSn17 and K5-xCo1-xSn9, and consequently, all compounds correspond to diamagnetic valence compounds. Owing to their diamagnetism, K13-xCo1-xSn17, and K5-xCo1-xSn9, as well as the d-block metal free binary compounds K12Sn17 and K4Sn9, were characterized for the first time by 119Sn solid-state NMR spectroscopy. [ABSTRACT FROM AUTHOR]

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 Na5Zn2+xSn10-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. [ABSTRACT FROM AUTHOR]

In view of the search for alternative structures for Li ion battery materials and electron-poor framework semiconductors for thermo-electric applications, the systems Li--Zn--Tt with Tt = Ge or Sn were investigated. lJ3Zn2Sn4 and Li?ZnGe3 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 IJ5Zn2Sn4 was analyzed by means «f 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 P63/mmc (No. 194) with lattice parameters a = 4.528(1) A and c = 22.119(2) A. 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 P63/mmc (No. 194) with lattice parameters a = 4.167(1) Á and c = 6.754(1) À. 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. [ABSTRACT FROM AUTHOR]

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 A14ZnGe16 (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 [Ge4]4- Zintl anions, and one anionic heterometallic [(Ge4)Zn(Ge4)]6- cluster per formula unit. The Zn-Ge cluster comprises two (Ge4) tetrahedra which are linked by a Zn atom, with one (Ge4) tetrahedron coordinating with a triangular face (?3) and the other one with an edge (?2). [(?3-Ge4)Zn(?2-Ge4)]6- is a new isomer of the [(Ge4)Zn(Ge4)]6- anion in Cs6ZnGe8. 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 [E4]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 K14ZnGe16 with MesCu (Mes = 2,4,6-(CH3)3C6H2) in the presence of [18]-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) yielded crystals of the salt [K([18]-crown-6)]2K2[(MesCu)2Ge4](NH3)7.5 with the polyanion [(MesCu)2Ge4]4-. This MesCu-stabilized tetrahedral [Ge4]4- cluster also completes the series of [(MesCu)2Si4-xGex]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 [(Ge4)Zn(Ge4)]6- and [(MesCu)2Ge4]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 A14ZnGe16 phases (A = K, Rb). [ABSTRACT FROM AUTHOR]

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 A12Cu12Sn21 show discrete 33-atom intermetalloid Cu-Sn clusters {Sn@Cu12@Sn20}, which are composed of {Sn20} pentagonal dodecahedra surrounding {Cu12} icosahedra with single Sn atoms at the center. Na12Cu12Sn21 and K12Cu12Sn21 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 A12Cu12Sn21 phases crystallize in the cubic space group Pnēm (No. 224), with the Cu-Sn clusters adopting a face centered cubic arrangement. A formal charge of 12- can be assigned to the {Sn@Cu12@Sn20} cluster unit, and the interpretation of the title compounds as salt-like intermetallic phases featuring discrete anionic intermetalloid [Sn@Cu12@Sn20]12- clusters separated by alkali metal cations is supported by electronic structure calculations. For both Na12Cu12Sn21 and K12Cu12Sn21, DFT band structure calculations (TB-LMTO-ASA) reveal a band gap. The discrete [Sn@Cu12@Sn20]12- cluster is analyzed in consideration of the molecular orbitals obtained from hybrid DFT calculations (Gaussian 09) for the cluster anion. The [Sn@Cu12@Sn20]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. [ABSTRACT FROM AUTHOR]