Mixed Si/Ge Nine-Atom Zintl Clusters: ESI Mass Spectrometric Investigations and Single-Crystal Structure Determination of Paramagnetic [Si_9–<i>x</i>Ge_<i>x</i>]^3–

Mixed Si/Ge compounds are of special interest as potential materials for photovoltaic applications. In order to evaluate the usage of soluble precursor compounds, we investigated the synthesis of heteroatomic nine-atom clusters that consist of Si and Ge atoms through dissolution of the ternary Zintl phases K₁₂Si_{17–<i>x</i>}Ge_<i>x</i> (<i>x</i> = 9, 12) and Rb₁₂Si_{17–<i>x</i>}Ge_<i>x</i> (<i>x</i> = 9). Electrospray ionization (ESI) mass spectrometry demonstrates the presence of mixed Si_9–<i>x</i>Ge_<i>x</i> clusters in acetonitrile solution. From ammonia solutions of the ternary phases, four compounds that contain 3-fold negatively charged [Si_9–<i>x</i>Ge_<i>x</i>]^3– clusters are obtained. The paramagnetic behavior is confirmed by EPR spectroscopy. [E₉]^3– Zintl clusters are considered as intermediate structures in the stepwise oxidation of [E₉]^4– clusters to novel element allotropes (E = Si–Pb). The structure of Rb­[Rb-crypt]₂[Si_2.3(1)Ge_6.7(1)]­(NH₃)₇ and the isostructural structures of [Rb-crypt]₃[Si_2.2(1)Ge_6.8(1)]­(NH₃)₈, [K-crypt]₃[Si_2.4(1)Ge_6.6(1)]­(NH₃)_8.5, and [K-crypt]₃[Si_4.6(1)Ge_4.4(1)]­(NH₃)_8.5 are investigated by single-crystal X-ray diffraction (crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane). The Si/Ge ratio of the products correlates with the composition of the ternary precursor phases

Mixed Si/Ge Nine-Atom Zintl Clusters: ESI Mass Spectrometric Investigations and Single-Crystal Structure Determination of Paramagnetic [Si_9–<i>x</i>Ge_<i>x</i>]^3–

Mixed Si/Ge compounds are of special interest as potential materials for photovoltaic applications. In order to evaluate the usage of soluble precursor compounds, we investigated the synthesis of heteroatomic nine-atom clusters that consist of Si and Ge atoms through dissolution of the ternary Zintl phases K₁₂Si_{17–<i>x</i>}Ge_<i>x</i> (<i>x</i> = 9, 12) and Rb₁₂Si_{17–<i>x</i>}Ge_<i>x</i> (<i>x</i> = 9). Electrospray ionization (ESI) mass spectrometry demonstrates the presence of mixed Si_9–<i>x</i>Ge_<i>x</i> clusters in acetonitrile solution. From ammonia solutions of the ternary phases, four compounds that contain 3-fold negatively charged [Si_9–<i>x</i>Ge_<i>x</i>]^3– clusters are obtained. The paramagnetic behavior is confirmed by EPR spectroscopy. [E₉]^3– Zintl clusters are considered as intermediate structures in the stepwise oxidation of [E₉]^4– clusters to novel element allotropes (E = Si–Pb). The structure of Rb­[Rb-crypt]₂[Si_2.3(1)Ge_6.7(1)]­(NH₃)₇ and the isostructural structures of [Rb-crypt]₃[Si_2.2(1)Ge_6.8(1)]­(NH₃)₈, [K-crypt]₃[Si_2.4(1)Ge_6.6(1)]­(NH₃)_8.5, and [K-crypt]₃[Si_4.6(1)Ge_4.4(1)]­(NH₃)_8.5 are investigated by single-crystal X-ray diffraction (crypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane). The Si/Ge ratio of the products correlates with the composition of the ternary precursor phases

Switching the Structure Type upon Ag Substitution: Synthesis and Crystal as well as Electronic Structures of Li₁₂AgGe₄

Li-rich compounds of metals and semimetals are interesting candidates for anode materials for rechargeable batteries. The investigation of the Li-rich part of the Li–Ag–Ge phase diagram led to the discovery of the new compound Li₁₂AgGe₄, which represents the Li-richest phase in the ternary phase system. The phase-pure compound is synthesized by high-temperature reaction of Li with stoichiometric amounts of premelted reguli of Ag and Ge. The structure was determined by single-crystal X-ray diffraction. Li₁₂AgGe₄ crystallizes in the Li₁₃Si₄ structure type in the space group <i>Pbam</i> (no. 55) with lattice parameters of <i>a</i> = 8.0420(2) Å, <i>b</i> = 15.1061(4) Å, and <i>c</i> = 4.4867(1) Å and exhibits the unique Zintl anion [AgGe₂]^7–iso(valence) electronic to the CO₂ moleculeand Ge₂ dumbbells. Li₁₂AgGe₄ adopts the atom packing of the lighter homologue Li₁₃Si₄ and not that of Li₁₃Ge₄ by the selective substitution of one out of seven Li positions by Ag. The calculation of the electronic structure indicates metallic property and the presence of strong covalent bonds between Ag and Ge in the linear triatomic Ge–Ag–Ge unit as well as π character between the Ge atoms of the dumbbells. The Ag–Ge bond order of the linear AgGe₂ unit reaches its maximum at <i>E</i>_F of Li₁₂AgGe₄ with full occupancy of all atomic positions (in contrast to the related Li₁₂Ag_1–<i>x</i>Si₄), indicating that the formation of covalent Ag–Ge bonds is the driving force for the formation of the structure type

Fully and Partially Li-Stuffed Diamond Polytypes with Ag–Ge Structures: Li₂AgGe and Li_2.53AgGe₂

In view of the search for and understanding of new materials for energy storage, the Li–Ag–Ge phase diagram has been investigated. High-temperature syntheses of Li with reguli of premelted Ag and Ge led to the two new compounds Li₂AgGe and Li_{2.80–<i>x</i>}AgGe₂ (<i>x</i> = 0.27). The compounds were characterized by single-crystal X-ray diffraction. Both compounds show diamond-polytype-like polyanionic substructures with tetrahedrally coordinated Ag and Ge atoms. The Li ions are located in the channels provided by the network. The compound Li₂AgGe crystallizes in the space group <i>R</i>3̅<i>m</i> (No. 166) with lattice parameters of <i>a</i> = 4.4424(6) Å and <i>c</i> = 42.7104(6) Å. All atomic positions are fully occupied and ordered. Li_{2.80–<i>x</i>}AgGe₂ crystallizes in the space group <i>I</i>4₁/<i>a</i> (No. 88) with lattice parameters of <i>a</i> = 9.7606(2) Å and <i>c</i> = 18.4399(8) Å. The Ge substructure consists of unique ¹_∞[Ge₁₀] chains that are interconnected by Ag atoms to build a three-dimensional network. In the channels of this diamond-like network, not all of the possible positions are occupied by Li ions. Li atoms in the neighborhood of the vacancies show considerably enlarged displacement vectors. The occurrence of the vacancy is traced back to short Li–Li distances in the case of the occupation of the vacancy with Li. Both compounds are not electron-precise Zintl phases. The density of states, band structure, and crystal orbital Hamilton population analyses of Li_{2.80–<i>x</i>}AgGe₂ reveal metallic properties, whereas a full occupation of all Li sites leads to an electron-precise Zintl compound within a rigid-band model. Li₂AgGe reveals metallic character in the ab plane and is a semiconductor with a small band gap along the <i>c</i> direction

First-Order Phase Transition in BaNi₂Ge₂ and the Influence of the Valence Electron Count on Distortion of the ThCr₂Si₂ Structure Type

Structural instability has a strong influence on the understanding of superconductivity in iron-containing 122 phases. Similar to the 122 iron-based high-temperature superconductors, the intermetallic compound BaNi₂Ge₂ undergoes an orthorhombic-to-tetragonal structural phase transition. The compound was prepared by arc-melting mixtures of the elements under an argon atmosphere. Single crystals were obtained by a special heat treatment in a welded tantalum ampule. The crystal structure of the compound was investigated by powder and single-crystal X-ray diffraction. Differential thermal analysis of BaNi₂Ge₂ showed a reversible phase transition at ca. 480 °C. In situ temperature-dependent synchrotron powder X-ray diffraction studies revealed that below 480 °C the crystal structure of BaNi₂Ge₂ is orthorhombic [own structure type, space group <i>Pnma</i>, <i>a</i> = 8.3852(4) Å, <i>b</i> = 11.3174(8) Å, and <i>c</i> = 4.2902(9) Å at 30 °C] and the high-temperature phase above 510 °C belongs to the tetragonal ThCr₂Si₂-type structure [space group <i>I</i>4/<i>mmm</i>, <i>a</i> = 4.2664(1) Å, and <i>c</i> = 11.2537(3) Å at 510 °C]. The reversible first-order low-temperature ↔ high-temperature phase transition around 480 °C is associated with distortion of the [Ni₂Ge₂] layer of low-temperature modification. The anisotropy of thermal expansion of the unit cell in BaNi₂Ge₂ was analyzed. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function. In related compounds, the level of distortion of the uncollapsed tetragonal ThCr₂Si₂-type structure depends on the valence electron count (VEC)

First-Order Phase Transition in BaNi₂Ge₂ and the Influence of the Valence Electron Count on Distortion of the ThCr₂Si₂ Structure Type

Structural instability has a strong influence on the understanding of superconductivity in iron-containing 122 phases. Similar to the 122 iron-based high-temperature superconductors, the intermetallic compound BaNi₂Ge₂ undergoes an orthorhombic-to-tetragonal structural phase transition. The compound was prepared by arc-melting mixtures of the elements under an argon atmosphere. Single crystals were obtained by a special heat treatment in a welded tantalum ampule. The crystal structure of the compound was investigated by powder and single-crystal X-ray diffraction. Differential thermal analysis of BaNi₂Ge₂ showed a reversible phase transition at ca. 480 °C. In situ temperature-dependent synchrotron powder X-ray diffraction studies revealed that below 480 °C the crystal structure of BaNi₂Ge₂ is orthorhombic [own structure type, space group <i>Pnma</i>, <i>a</i> = 8.3852(4) Å, <i>b</i> = 11.3174(8) Å, and <i>c</i> = 4.2902(9) Å at 30 °C] and the high-temperature phase above 510 °C belongs to the tetragonal ThCr₂Si₂-type structure [space group <i>I</i>4/<i>mmm</i>, <i>a</i> = 4.2664(1) Å, and <i>c</i> = 11.2537(3) Å at 510 °C]. The reversible first-order low-temperature ↔ high-temperature phase transition around 480 °C is associated with distortion of the [Ni₂Ge₂] layer of low-temperature modification. The anisotropy of thermal expansion of the unit cell in BaNi₂Ge₂ was analyzed. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function. In related compounds, the level of distortion of the uncollapsed tetragonal ThCr₂Si₂-type structure depends on the valence electron count (VEC)

Li₁₈Na₂Ge₁₇A Compound Demonstrating Cation Effects on Cluster Shapes and Crystal Packing in Ternary Zintl Phases

The novel ternary Zintl phase Li₁₈Na₂Ge₁₇ was synthesized from a stoichiometric melt and characterized crystallographically. It crystallizes in the trigonal space group <i>P</i>31<i>m</i> (No. 157) with <i>a</i> = 17.0905(4) Å, <i>c</i> = 8.0783(2) Å, and <i>V</i> = 2043.43(8) ų (final <i>R</i> indices R1 = 0.0212 and wR2 = 0.0420 for all data). The structure contains three different Zintl anions in a 1:1:1 ratio: isolated anions Ge^4–, tetrahedra [Ge₄]^4–, and truncated, Li-centered tetrahedra [Li@Ge₁₂]^11–, whose hexagonal faces are capped by four Li cations, resulting in the Friauf polyhedra [Li@Li₄Ge₁₂]^7–. According to the Zintl–Klemm concept, Li₁₈Na₂Ge₁₇ is an electronically balanced Zintl phase, as experimentally verified by its diamagnetism. The compound is structurally related to Li₇RbGe₈, which also contains [Ge₄]^4– and [Li@Li₄Ge₁₂]^7– in its anionic substructure. However, exchanging the heavier alkali metal cation Rb for Na in the mixed-cation germanides leads to drastic changes in stoichiometry and crystal packing, demonstrating the great effects that cations exert on such Zintl phases through optimized cluster sheathing and space filling

Derivatization of Phosphine Ligands with Bulky Deltahedral <i>Zintl</i> ClustersSynthesis of Charge Neutral Zwitterionic Tetrel Cluster Compounds [(Ge₉{Si(TMS)₃}₂)^<i>t</i>Bu₂P]M(NHC^Dipp) (M: Cu, Ag, Au)

Reactions of silylated clusters [Ge₉{Si­(TMS)₃}₃]⁻ or [Ge₉{Si­(TMS)₃}₂]²⁻ with dialkylhalophosphines R₂PCl (Cy, ^<i>i</i>Pr, ^<i>t</i>Bu) at ambient temperature yield the first tetrel <i>Zintl</i> cluster compounds bearing phosphine moieties. Varying reactivity of the dialkylhalophosphines toward the silylated clusters is observed depending on the bulkiness of the phosphine’s alkyl substituents and on the number of hypersilyl groups at the tetrel cluster. Reactions between phosphines with small cyclohexyl- (Cy) or isopropyl- (^<i>i</i>Pr) groups and the tris-silylated cluster [Ge₉{Si­(TMS)₃}₃]⁻ yield the novel neutral cluster compounds [Ge₉{Si­(TMS)₃}₃PR₂] (R: Cy (<b>1</b>), ^<i>i</i>Pr (<b>2</b>)) with discrete Ge–P <i>exo</i> bonds. By contrast, the bulkier phosphine ^<i>t</i>Bu₂PCl does not react with [Ge₉{Si­(TMS)₃}₃]⁻ due to steric crowding. However, the reaction with the bis-silylated cluster [Ge₉{Si­(TMS)₃}₂]²⁻ yields the novel cluster compound [Ge₉{Si­(TMS)₃}₂P^<i>t</i>Bu₂]⁻ (<b>3</b>). Subsequent reactions of compound <b>3</b> with NHC^DippMCl (M: Cu, Ag, Au) yield the charge neutral zwitterionic compounds [(Ge₉{Si­(TMS)₃}₂)^<i>t</i>Bu₂P]­M­(NHC^Dipp) (M: Cu, Ag, Au) (<b>4</b>–<b>6</b>), in which compound <b>3</b> acts as a phosphine ligand bearing a bulky tetrel <i>Zintl</i> cluster moiety. Compounds <b>4</b>–<b>6</b> also represent the first uncharged examples for 3-fold substituted tetrel <i>Zintl</i> clusters

Derivatization of Phosphine Ligands with Bulky Deltahedral <i>Zintl</i> ClustersSynthesis of Charge Neutral Zwitterionic Tetrel Cluster Compounds [(Ge₉{Si(TMS)₃}₂)^<i>t</i>Bu₂P]M(NHC^Dipp) (M: Cu, Ag, Au)

Reactions of silylated clusters [Ge₉{Si­(TMS)₃}₃]⁻ or [Ge₉{Si­(TMS)₃}₂]²⁻ with dialkylhalophosphines R₂PCl (Cy, ^<i>i</i>Pr, ^<i>t</i>Bu) at ambient temperature yield the first tetrel <i>Zintl</i> cluster compounds bearing phosphine moieties. Varying reactivity of the dialkylhalophosphines toward the silylated clusters is observed depending on the bulkiness of the phosphine’s alkyl substituents and on the number of hypersilyl groups at the tetrel cluster. Reactions between phosphines with small cyclohexyl- (Cy) or isopropyl- (^<i>i</i>Pr) groups and the tris-silylated cluster [Ge₉{Si­(TMS)₃}₃]⁻ yield the novel neutral cluster compounds [Ge₉{Si­(TMS)₃}₃PR₂] (R: Cy (<b>1</b>), ^<i>i</i>Pr (<b>2</b>)) with discrete Ge–P <i>exo</i> bonds. By contrast, the bulkier phosphine ^<i>t</i>Bu₂PCl does not react with [Ge₉{Si­(TMS)₃}₃]⁻ due to steric crowding. However, the reaction with the bis-silylated cluster [Ge₉{Si­(TMS)₃}₂]²⁻ yields the novel cluster compound [Ge₉{Si­(TMS)₃}₂P^<i>t</i>Bu₂]⁻ (<b>3</b>). Subsequent reactions of compound <b>3</b> with NHC^DippMCl (M: Cu, Ag, Au) yield the charge neutral zwitterionic compounds [(Ge₉{Si­(TMS)₃}₂)^<i>t</i>Bu₂P]­M­(NHC^Dipp) (M: Cu, Ag, Au) (<b>4</b>–<b>6</b>), in which compound <b>3</b> acts as a phosphine ligand bearing a bulky tetrel <i>Zintl</i> cluster moiety. Compounds <b>4</b>–<b>6</b> also represent the first uncharged examples for 3-fold substituted tetrel <i>Zintl</i> clusters

Single Crystal Growth and Thermodynamic Stability of Li₁₇Si₄

Single crystals of Li₁₇Si₄ were synthesized from melts Li_<i>x</i>Si_{100–<i>x</i>} (<i>x</i> > 85) at various temperatures and isolated by isothermal centrifugation. Li₁₇Si₄ crystallizes in the space group <i>F</i>4̅3<i>m</i> (<i>a</i> = 18.7259(1) Å, <i>Z</i> = 20). The highly air and moisture sensitive compound is isotypic with Li₁₇Sn₄. Li₁₇Si₄ represents a new compound and thus the lithium-richest phase in the binary system Li–Si superseding known Li₂₁Si₅ (Li_16.8Si₄). As previously shown Li₂₂Si₅ (Li_17.6Si₄) has been determined incorrectly. The findings are supported by theoretical calculations of the electronic structure, total energies, and structural optimizations using first-principles methods. Results from melt equilibration experiments and differential scanning calorimetry investigations suggest that Li₁₇Si₄ decomposes peritectically at 481 ± 2 °C to “Li₄Si” and melt. In addition a detailed investigation of the Li–Si phase system at the Li-rich side by thermal analysis using differential scanning calorimetry is given