Alkali Metal Silicides and Germanides

Alkali metal silicides and germanides in particular have received much attention as precursors for novel materials. This group of compounds can be classified by the anionic substructure type into three- and two-dimensional networks, cluster-containing Zintl phases, and lithium-rich phases. This section will focus on binary representatives. Their role as precursors for novel materials is described in Chapter 1.2.3. , whereas ternary and quaternary phases will be covered in Chapter 1.3.

Cluster-containing Zintl Phases

The first and most prominent examples of alkali metal silicides and germanides contain cluster anions and are typical Zintl phases. They are synthesized by melting a stoichiometric mixture of the heavier alkali metals sodium to cesium with silicon or germanium. For example, the Zintl phases A4T4 (A = Na–

1.2 ZINTL PHASES

monocapped square antiprism [T9]⁴⁻ which is known in the germanides A4Ge9 (A = K–Cs, Figure 1.6b)^[63,64] as well as in A12T17 (A = K–Cs, T = Si, Ge) where it occurs next to [T4]⁴⁻ tetrahedra.^[64-67]

According to the Wade-Mingos rules for structure prediction in clusters,^[68,69] [T9]⁴⁻ can be described as a nido-cluster with 22 skeletal electrons. Most of these cluster compounds are soluble in polar solvents such as liquid ammonia and ethylenediamine, rendering them the foundation of very prolific research on group IV Zintl anions in solution. Starting from intermetallic phases containing [T4]⁴⁻ and [T9]⁴⁻

clusters, a number of other soluble clusters were obtained and subsequently functionalized.^[70]

Figure 1.6. Structures of selected binary alkali metal silicides and germanides comprising Zintl clusters: a) K4Si4,^[71] b) K4Ge9.^[63]

Lithium-rich Phases

Lithium with its relatively covalent bonding contributions can stabilize more negatively charged tetrelide anions than its heavier homologues. Thus, most lithium tetrelides contain smaller clusters or isolated anions. For instance, Li12T7 (T = Si, Ge) comprises planar five-membered rings as well as planar Y-shaped stars (Figure 1.7a),^[72] and dumbbells constitute the polyanions in Li7T3 (T = Si, Ge, Figure 1.7b).^[73,74] Isolated anions are present in the lithium-richest phases Li13T4 (next to dumbbells),^[75-77]

Li15T4 (Figure 1.7c),^[78,79] high-temperature Li4.1T4,^[80,81] and Li17T4 (T = Si, Ge in all cases).^[81,82] With the exception of metastable Li15Si4, all lithium-rich silicides and germanides can be synthesized directly by melting a stoichiometric mixture of the respective elements.

Figure 1.7. Structures of selected Li-rich alkali metal silicides and germanides: a) Li12Si7,^[83] b) Li7Si3,^[73] c) Li15Si4.^[78]

In contrast to the electron-precise cluster compounds presented before, the lithium-rich tetrelides do not represent classical Zintl phases. For example, the expected composition according to the Zintl

The dumbbells in Li7T3 and Li13T4 have partial multiple bond character, but again they cannot be described in an electron-precise way. NMR spectroscopy proved that the planar five-membered rings in Li12T7 are indeed aromatic,^[84,85] while the Y-shaped stars are commonly viewed as carbonate-like.^[86]

The resulting description of [T5]⁶⁻ and [T4]⁸⁻ clusters in a 2:1 ratio again does not add up with the number of transferrable Li valence electrons, leaving two electrons per formula unit of Li12T7

unaccounted for.

Li15Si4 and Li15Ge4 as the crystalline products of full lithiation of silicon and germanium anodes play a major role in LIB research.^[52,54] Li15Si4 is a metastable phase which slowly decomposes above 170 °C.^[78]

It was first structurally characterized from the product of the electrochemical lithiation of silicon.^[52]

The phase can be prepared by mechanical alloying or in flux syntheses with excess Li as a solvent.^[78,87]

Li15Ge4 is isostructural but thermodynamically stable.^[81]

Three- and Two-dimensional Networks

Extended three- or two-dimensional networks are typically found as the anionic substructure of alkali metal-poor silicides and germanides. Already in 1965, the binary phases Na8Si46 and NaxSi136 (x < 24) were found via thermal decomposition of Na4Si4 (Figure 1.8a, b).^[88-90] Isostructural compounds are obtained by thermal decomposition of A4Si4 (A = K–Cs) and A4Ge4 (A = Na–Rb).^[91] Analysis of X-ray diffraction data soon revealed that the compounds both crystallize in structure types which were already known from gas hydrates: type-I and type-II clathrates.^[92,93] In these remarkably air- and water-stable intermetallic phases, silicon forms a host structure consisting of different cages which are filled with sodium. Later investigations also demonstrated that either clathrate I-type Na8Si46 or clathrate II-type NaxSi136 is formed selectively when carefully choosing appropriate reaction conditions.^[94]

Reactions of Na4Si4 and K4Si4 with gaseous HCl or H2O also yield the corresponding clathrate compounds.^[95]

Thermal decomposition of Na4Ge4 in a dynamic vacuum also yielded zeolite-like Na1−xGe3+z. This binary intermetallic is characterized by large open Ge channels which are filled with Na and disordered Ge atoms (Figure 1.8d).^[96] In syntheses from the elements at 8 GPa binary NaSi6 was found which crystallizes in the Eu4Ga8Ge16 structure type.^[97] In this compound, the silicon atoms form a three-dimensional network with large open channels, which are filled by the Na guest species (Figure 1.8c).

In contrast to the above-mentioned open framework structures which do not represent electronically balanced Zintl phases, the lithium-poor binaries LiT (T = Si, Ge) and Li7Ge12 are electron-precise. In the LiT compounds, the tetrel atoms form a three-dimensional network of three-connected Si and Ge atoms, respectively, with Li⁺ cations distributed throughout (Figure 1.8e).^[98,99] Li7Ge12 comprises two-dimensional polygermanide sheets [_∞²Ge12]⁷⁻ which are separated from each other by layers of Li⁺ cations (Figure 1.8f).^[100,101] While LiGe and Li7Ge12 can be synthesized directly by melting of stoichiometric amounts of the elements, the formation of LiSi under these conditions is kinetically hindered.^[98] LiSi can however be obtained at high pressures or via mechanical alloying.[98,102,103]

Tetragonal and hexagonal high pressure modifications of LiGe are obtained at 4 GPa.^[104]

1.2 ZINTL PHASES

Figure 1.8. Structures of selected binary alkali metal silicides and germanides comprising three- or two-dimensional networks:

a) Na8Si46 (clathrate-I type, large and small cages are highlighted in red and grey, respectively),^[91] b) NaxSi136 (clathrate-II type, large and small cages are highlighted in red and grey, respectively),^[90] c) NaSi6,^[105] d) Na1−xGe3+z,^[96] e) LiGe,^[99] f) Li7Ge12.^[101]