Structural Synthon Approach to Predict the Possible Polytypes of Layered Double Hydroxides

The complete universe of possible polytypes of layered double hydroxides (LDH) is predicted on the basis of symmetry arguments. A single [MX₂] (X = OH) layer, also defined as a structural synthon, belongs to the layer group P$\bar{3}$ 2/m1. These layers can be stacked in such a way as to conserve the unique 3‐axis of the layer in the resultant crystal. The different stacking sequences that facilitate symmetry conservation, yield the different possible polytypes of rhombohedral and hexagonal symmetries. More polytypes can be envisaged by including stacking sequences that systematically destroy the principal symmetry elements of the structural synthon. Thereby, stacking sequences that destroy the 3‐axis, while retaining the 2‐axis, yield possible polytypes of monoclinic symmetry. The nitrate‐containing LDH of zinc and aluminum crystallizes in a faulted structure in which, the planar faults are shown to arise on account of stacking sequences whose local symmetry is monoclinic. This approach to polytype prediction expands on an earlier reported method by Bookin and Drits and is very general with important implications for other classes of layered materials.     

Polytype and Stacking Faults in the Li2CoSiO4 Li‐Ion Battery Cathode

Atomic‐resolution imaging of the crystal defects of cathode materials is crucial to understand their formation and the correlation between the structure, electrical properties, and electrode performance in rechargeable batteries. The polytype, a stable form of varied crystal structure with uniform chemical composition, holds promise to engineer electronic band structure in nanoscale homojunctions. 1 – 3 Analyzing the exact sites of atoms and the chemistry of the boundary in polytypes would advance our understanding of their formation and properties. Herein, the polytype and stacking faults in the lithium cobalt silicates are observed directly by aberration‐corrected scanning transmission electron microscopy. The atomic‐scale imaging allows clarification that the polytype is formed by stacking of two different close‐packed crystal planes in three‐dimensional space. The formation of the polytype was induced by Li–Co cation exchange, the transformation of one phase to the other, and their stacking. This finding provides insight into intrinsic structural defects in an important Li₂CoSiO₄ Li‐ion battery cathode.     

Structural relations between weberite and zirconolite polytypes—refinements of doped 3T and 4M Ca2Ta2O7 and 3T CaZrTi2O7

New weberite-type Ca₂Ta₂O₇ and zirconolite-type CaZrTi₂O₇ polytypes have been prepared by doping with Nd/Zr and Th/Al, respectively, and their structures have been refined using single-crystal X-ray diffraction intensity data. The 3T zirconolite polytype, Ca_0.8Ti_1.35Zr_1.3Th_0.15Al_0.4O₇, has a=7.228(1), c=16.805(1) Å. The 3T weberite-type polytype, Ca_1.92Ta_1.92Nd_0.08Zr_0.08O₇, has a=7.356(1), c=18.116(1) Å. Both 3T polytypes have space group P3₁21, Z=6. The 4M Ca₂Ta₂O₇ polytype has the same composition, from electron microprobe analyses, as the 3T polytype, and has cell parameters: a=12.761(1), b=7.358(1), c=24.565(1) Å, β=100.17(1)°, space group C2, Z=16. The structural relationships between the different zirconolite and weberite polytypes are discussed. A consideration of the structures from the viewpoint of anion-centered tetrahedral arrays shows that zirconolite can be considered as an anion-deficient fluorite derivative phase. However, the fluorite-type topology of edge-shared OM₄ tetrahedra is not maintained in the Ca₂Ta₂O₇ weberite-type polytypes, even though they have a fluorite-like fcc packing of metal atoms. One of the oxygen atoms moves from a tetrahedral Ta₃Ca interstice to an adjacent Ta₂Ca₄ octahedral interstice in the weberite polytypes.     

Polytypism in the lithium-aluminum layered double hydroxides: the [LiAl2(OH)6]+ layer as a structural synthon

The [LiAl(2)(OH)(6)](+) layer obtained from gibbsite-Al(OH)(3) belongs to the layer group symmetry P-312/m. This layer satisfies the defining characteristics of a synthon in that it predicts all the polymorphic modifications of the layered double hydroxides of Li and Al. The various possible ways of stacking these layers can be derived by the systematic elimination of the principal symmetry elements comprising the layer group. This approach yields the complete universe of possible structures. When the 3 axis of the layer is conserved in the stacking, the resultant crystal adopts the structure of the 1H, 2H, or 3R polytypes (H, hexagonal; R, rhombohedral). When the 3 axis is destroyed and the 2/m axis is retained, the crystal adopts monoclinic symmetry and crystallizes in the structures of the 1M(1) or 1M(2) (M, monoclinic) polytypes; the two polytypes differ only in their translational component. Experimentally, gibbsite-based precursors yield the 2H polytype, and bayerite-based precursors yield the 1M polytype. Faulted structures incorporating differently oriented 1M(1) motifs or a mixture of 1M(1) and 1M(2) motifs are also obtained. These stacking faults result in cation disorder along the c axis and produce signature effects on the line shapes of select reflections in the powder X-ray diffraction patterns. This symmetry-guided approach is general and can be extended to other classes of layered solids.     

Polytype Transformations in the SO42– Containing Layered Double Hydroxides of Zinc with Aluminum and Chromium: The Metal Hydroxide Layer as a Structural Synthon

Solid–solid inter‐polytype transformations are observed during the thermal dehydration of sulfate‐containing layered double hydroxides (LDHs). The metal hydroxide layer behaves as a “structural synthon” and the interconversion of polytypes of rhombohedral and hexagonal symmetries takes place by rigid translations of successive layers by (± 1/3, ± 2/3) relative to one another in the ab plane. These translations are selected among the many possible, as they preserve the coincidence of the symmetry elements of the individual layers and thereby conserve the threefold symmetry of the crystal across the inter‐polytype conversions. As a result, these transformations are enthalpically not expensive. These translations are facilitated at near ambient temperatures (30–60 °C) by the reversible dehydration of the LDH, which involves the deinsertion/insertion of water molecules within the restricted space of the interlayer region.     

Anion mediated polytype selectivity among the basic salts of Co(II)

Basic salts of Co(II) crystallize in the rhombohedral structure. Two different polytypes, 3R₁ and 3R₂, with distinct stacking sequences of the metal hydroxide slabs, are possible within the rhombohedral structure. These polytypes are generated by simple translation of successive layers by (2/3, 1/3, z) or (1/3, 2/3, z). The symmetry of the anion and the mode of coordination influences polytype selection. Cobalt hydroxynitrate crystallizes in the structure of the 3R₂ polytype while the hydroxytartarate, hydroxychloride and α-cobalt hydroxide crystallize in the structure of the 3R₁ polytype. Cobalt hydroxysulfate is turbostratically disordered. The turbostratic disorder is a direct consequence of the mismatch between the crystallographically defined interlayer sites generated within the crystal and the tetrahedral symmetry of the SO₄²⁻ ions.     

First germanium doped titanium disulfide polytypes: Crystal structure and metal–metal interactions

Single crystals of Ge-doped TiS₂ polytypes, 1T, (4H)₂, 12R, and their corresponding new a√3 × a√3 superstructure were grown by chemical vapor transport method. The crystals were characterized by combining X-ray diffraction and transmission electron microscopy techniques. The structures of these polytypes are all based on close packing layers of sulfur of CdI₂-type structure. Except in the 1T polytype, the germanium atoms are observed to be equally distributed over both partial and complete occupancy layers. A significant distortion of the metal–sulfur distances is observed in the superstructure polytypes, as a consequence of metal–metal corrugated layers. The 12R-a√3 × a√3 superstructure is revealed by both electron diffraction and X-ray diffraction by the presence of satellite reflections. Electron diffraction patterns from the 12R polytype show highly structured diffuse scattering surrounding the main spots. These diffuse segments, which are arranged in triangles sharing vertices, correspond to a 2a* × 2a* superstructure and are attributed to the short-range order of metal atoms in the partially filled layers.     

First experimental evidence of a new D4-AgCoO2 delafossite stacking

A new polytype of AgCoO(2) delafossite has been prepared from the ordered OP4-(Li/Na)CoO(2) layered compound using ion-exchange reaction in molten salts. As expected from the structural model assuming a topotactic process, the lamellar structure of this new polytype is an alternate combination of the already known 2H and 3R delafossite polytypes. It crystallizes in the P6(3)/mmc space group with cell parameters a(hex.) = 2.871(1) ? and c(hex.) = 24.448(1) ?. Thermal stability, morphology characterization, and electrical properties are reported here and compared with those of the 2H and 3R AgCoO(2) polytypes.     

Structural Principles and Thermoelectric Properties of Polytypic Group 14 Clathrate‐II Frameworks

We have investigated the structural principles and thermoelectric properties of polytypic group 14 clathrate‐II frameworks using quantum chemical methods. The experimentally known cubic 3C polytype was found to be the energetically most favorable framework, but the studied hexagonal polytypes (2 H, 4 H, 6 H, 8 H, 10 H) lie energetically close to it. In the case of germanium, the energy difference between the 3C and 6H clathrate‐II polytypes is ten times smaller than the difference between the experimentally known 3C‐Ge (α‐Ge) and 4H‐Ge polytypes. The thermoelectric properties of guest‐occupied clathrate‐II structures were investigated for compositions Na–Rb–Ga–Ge and Ge–As–I. The clathrate‐II structures show promising thermoelectric properties and the highest Seebeck coefficients and thermoelectric power factors were predicted for the 3C polytype. The structural anisotropy of the largest studied hexagonal polytypes affects their thermoelectric power factors by over a factor of two.     

Thermally induced polytype transformations among the layered double hydroxides (LDHs) of Mg and Zn with Al

The hydrotalcite-like layered double hydroxide (LDH) of Mg with Al shows dramatic changes in the peaks arising from the (h0l)/(0kl) family of reflections in its powder X-ray diffraction pattern during thermal treatment. DIFFaX simulations show that these changes arise due to the transformation of the disordered 3R1 polytype into the 1H polytype on dehydration. The 1H polytype is an essential precursor to the decomposition reaction, which results in the formation of an oxide residue with the rock salt structure. In contrast, the LDH of Zn with Al does not undergo any such transformation, retaining the structure of the 3R1 polytype until decomposition into the oxide residue. Given the poor octahedral site preference of the Zn2+ ion, the 1H polytype is neither structurally stable nor is it topochemically necessary for the thermal decomposition of the Zn-Al LDH, the end product of the decomposition reaction being an oxide with the wurtzite structure.     

NMR Crystallography Reveals Carbonate Induced Al-Ordering in ZnAl Layered Double Hydroxide

Layered double hydroxides (LDHs) serve a score of applications in catalysis, drug delivery, and environmental remediation. Smarter crystallography, combining X-ray diffraction and NMR spectroscopy revealed how interplay between carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH. Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH 10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal treatment to avoid extensive recrystallisation. In ideal configuration, all Al cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR revealed two different Al local environments were present in all samples in a ratio dependent on synthesis pH. NMR-crystallography, integrating NMR spectroscopy and X-ray diffraction, succeeded to identify them as Al residing in the highly ordered crystalline phase, next to Al in disordered material. With increasing synthesis pH, crystallinity increased, and the side phase fraction decreased. Using ¹H−¹³C, ¹³C−²⁷Al HETCOR NMR in combination with ²⁷Al MQMAS, ²⁷Al-DQ-SQ measurements and Rietveld refinement on high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs for CO₃²⁻ over HCO₃⁻ was linked to strict Al and CO₃²⁻ordering in the crystalline LDH. Even upon equilibration of the LDH in pure NaHCO₃ solutions, only CO₃²⁻ was adsorbed by the LDH. This reveals the structure directing role of bivalent cations such as CO₃²⁻ during crystallization of [M²⁺₄M³⁺₂(OH)₂]²⁺[A²⁻]₁⋅yH₂O LDH phases.     

Evolutionary exploration of polytypism in lead halide perovskites

The regular ABX₃ cubic perovskite structure is composed of close-packed AX₃ layers stacked along the 〈111〉 axis. An equivalent hexagonal close-packed network can also be formed, in addition to a series of intermediate polytype sequences. Internally, these correspond to combinations of face- and corner-sharing octahedral chains that can dramatically alter the physical properties of the material. Here, we assess the thermodynamics of polytypism in CsPbI₃ and CsPbBr₃. The total energies obtained from density functional theory are used to paramaterize an axial Ising-type model Hamiltonian that includes linear and cubic correlation terms of the pseudo-spin. A genetic algorithm is built to explore the polytype phase space that grows exponentially with the number of layers. The ground-state structures of CsPbX₃ polytypes are analysed to identify features of polytypism such as the distinct arrangements of layers and symmetry forbidden sequences. A number of polytypes with low ordering energies (around thermal energy at room temperature) are predicted, which could form distinct phases or appear as stacking faults within perovskite grains.

Beyond the regular perovskite structure based on cubic-close packing exists a range of possible polytypes that we explore using computational chemistry.     

Metallat‐Ionen [Al(OR)4]— als Chelatliganden für Übergangsmetall‐Kationen

Metalat Ions [Al(OR)₄]^— as Chelating Ligands for Transition Metal Cations Waterfree CoCl₂ can be reacted with [{Li(Diglyme)}{Al(O^tBu)₄}] in THF to the complex [Li(THF)₄][{CoCl₂}{Al(O^tBu)₄}]. Addition of diglyme to the reaction mixtures gives the blue compound [Li(diglyme)₂][{CoCl₂}{Al(O^tBu)₄}] ( 1 ). According to this procedure the Fe^II complex [Li(Diglyme)₂][{FeCl₂}₂{Al(O^tBu)₄}] ( 2 ) was formed by treatment of FeCl₂ with Li[Al(O^tBu)₄]. [{Li(diglyme)}{Al(O^tBu)₄}] in THF/diglyme can be used as alkoxide transfer reagent on TiCl₄ to give the neutral complex [TiCl₂(O^tBu)₂(diglyme)] ( 3 ). The sky‐blue salt [Li(THF)₄]₂[{CoCl₂}₃{Al(OCH₂Ph)₄}₂] ( 4 ) was obtained by reaction of Li[Al(OCH₂Ph)₄] with CoCl₂ in THF. By treatment of 4 with diglyme ligand redistribution was observed giving the sky‐blue compound [Li(Diglyme)₂]₂[{CoCl₂}₃{Al(OCH₂Ph)₄}₂] ( 5 ) and the violet salt [Li(Diglyme)₂]₂[Co₂Cl₅(OCH₂Ph)] ( 6 ). A similar salt can be synthesized also directly from Li[Al(O^tBu)₄] and CoCl₂ in diglyme to give [Li(Diglyme)₂]₂[Co₂Cl₅(O^tBu)] ( 7 ). 1 — 7 were characterized by IR spectroscopy, partly by mass spectrometry and X‐ray analyses. UV‐VIS spectra were recorded from 1 and 5 . According to the X‐ray analyses the M^II ions as well as the Al^III ions are coordinated distorted tedrahedrally. In 1 , 2 , 4 und 5 the unit [Al(OR)₄]^— acts a chelating ligand as desired.     

Syntheses and Crystal Structures of Novel Ternary Alkali Metal Gold Acetylides MIAuC2 (MI = Li,Na, K,Rb, Cs)

By the reaction of AuI with alkali metal hydrogen acetylides M^IC₂H (M^I = Li–Cs) in liquid ammonia and subsequent heating of the remaining residue in refluxing pyridine (M^I = Li, Na, K) or as a solid phase at about 110 °C in vacuum (M^I = Rb, Cs) ternary alkali metal gold acetylides M^IAuC₂ were obtained. Their crystal structures were investigated by the means of X‐ray powder diffraction. [Au(C₂)_2/2^–] chains are the characteristic structural motif which are packed in a hexagonal (LiAgC₂) and tetragonal arrangement (NaAuC₂–CsAuC₂), respectively. Simple calculations based on the close packing of rods and spheres can explain these different arrangements. The existence of C–C triple bonds in the title compounds is confirmed by Raman spectroscopic investigations.     

Neue Hypersilanide der Erdmetalle Aluminium,Gallium und Indium

New Hypersilanides of the Earth Metals Aluminium, Gallium, and Indium The dialkylaluminiumchlorides R₂AlCl (with R = Me, Et; Me = CH₃, Et = C₂H₅) react with base‐free lithium‐tris(trimethylsilyl)silanide (Li–Hsi; Hsi = –Si(SiMe₃)₃), forming the pyrophoric dialkyl aluminiumhypersilanides R₂Al–Hsi. The methyl compound is dimeric in solid state (triclinic space group P1, Z = 1 dimer), as in Al₂Me₆ the association takes place by two Al–Me–Al bridges, forming a centrosymmetric molecule of approximately C_2h point‐symmetry. Contrary to this (Me₂GaCl)₂ and Li–Hsi form a mixture of (MeGa(Hsi)Cl)₂ and [Me₃Ga–Hsi]Li. The monochloride again is a centrosymmetric, chlorine‐bridged dimer (monoclinic space group P2₁/n, Z = 2 dimers). The extremely air sensitive gallate can be prepared from GaMe₃ and Li–Hsi (1 : 1 ratio), as well as the homologous [Me₃Ga–Hsi]Na and [Me₃Ga–Hsi]K from GaMe₃ and the corresponding alkalimetal hypersilanides. The 1 : 1 toluene‐solvat of the sodium salt crystallizes in the orthorhombic space group Pbca (Z = 8) with polymeric zig‐zag‐chains, in which the toluene‐capped Na‐ions act as GaMe…Na…Me₂Ga‐bridges between [Me₃Ga–Hsi]^– anions. The reaction of InCl₃ with Li–Hsi (1 : 3 ratio) mainly gives LiCl, metallic In and the “dihypersilyl” Hsi–Hsi. Ruby‐red (Hsi)₂In–In(Hsi)₂ could also be obtained in low yield and characterized by X‐ray structure elucidation (space group P2₁/c, Z = 4). The ¹H, ¹³C, ²⁹Si and ⁷Li NMR‐ and the vibrational spectra of the hypersilanides have been measured and discussed.     

Studies on Mechanochemical Method to Synthesize LDH Nanoparticles

Magnesium‐ferrum layered double hydroxide (Mg‐Fe‐LDH) and zinic‐aluminum layered double hydroxide (Zn‐Al‐LDH) compounds were prepared through a mechanochemical method. The influence of molar ratio of M²⁺ to M³⁺ (R value) on the property of LDH nanoparticles has been studied and the results showed that R=3:1 is the optimum value for the both samples. Besides pure water, the mixture of water and ethanol with the volume ratio of 3:1 is also used to wash the precipitates and used as suspending agent during the peptization process and our results showed that the addition of ethanol can improve the monodispersity of LDH nanoparticles greatly.     

Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B−B Bond

Deprotonation of the doubly arylene‐bridged diborane(6) derivative 1 H₂ with (Me₃Si)₃CLi or (Me₃Si)₂NK gives the B−B σ‐bonded species M[ 1 H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4′‐di‐tert‐butyl‐2,2′‐biphenylylene). With nBuLi as the base, the yield of Li[ 1 H] drops to 20 % and the 1,1‐bis(9‐borafluorenyl)butane Li[ 2 H] is formed as a side product (30 %). In addition to the 1,1‐butanediyl fragment, the two boron atoms of Li[ 2 H] are linked by a μ‐H bridge. In the closely related molecule Li[ 3 H], the corresponding μ‐H atom can be abstracted with (Me₃Si)₃CLi to afford the B−B‐bonded conjugated base Li₂[ 3 ] (THF, 150 °C; 15 %). Li[ 1 H] and Li[ 2 H] were characterized by NMR spectroscopy and X‐ray crystallography.     

On Demand One-Pot Mild Preparation of Layered Double Hydroxides and Their Hybrid Forms: Advances through the Epoxide Route

Epoxide ring opening driven alkalinization process was explored with the aim of preparing layered double hydroxide (LDH) phases on demand, at room temperature. Employing iodide as nucleophilic agent, the precipitation reaction can be driven under much lower halide concentrations. This scenario favors the selective intercalation of concomitant bulky oxo anions as nitrate or perchlorate in the LDH products, allowing for the one-pot synthesis of an LDH able to delaminate in formamide. Even large dicarboxylic acids, ⁻O₂C-(CH₂)_n-CO₂⁻, with n up to 8, can be quantitively intercalated within the growing LDH phase, providing a versatile one-pot route for hybrid LDHs as well. Under the mild conditions employed, governed by a continuous pH rise from a starting acid condition, a M^II to M^*III ratio of 2 prevails, independently from the overall cationic composition. However, after moderate hydrothermal aging LDH phases bearing a cationic ratio higher than 2 could result. The solubility of a given chloride-containing M^II₂M^*III LDH can be approximated as a linear combination of the solubility of the pure hydroxylated phases of the constitutive cations, M(OH)₂ and M^*(OH)₃.     

The New Homoleptic Tetracyanamidoaluminate LiBa2[Al(CN2)4]

The new tetracyanamidoaluminate LiBa₂[Al(CN₂)₄] was prepared by solid state metathesis reaction in a fused copper ampoule from a mixture of BaF₂, AlF₃, and Li₂(CN₂) at 550 °C. The crystal structure was solved and refined based on single‐crystal X‐ray diffraction data [P2₁2₁2₁, Z = 4, a = 6.843(1) Å, b = 11.828(2) Å, c = 11.857(2) Å]. The compound belongs to the known formula type LiM₂[Al(CN₂)₄] (M = Sr, Eu) containing the homoleptic [Al(CN₂)₄]^5– ion. However, LiBa₂[Al(CN₂)₄] forms a distinct crystal structure, containing a two‐dimensional [(NCN)_2/2Li(NCN)₂Al(NCN)_2/2] network with four‐coordinate Li⁺ and Al³⁺ ions. Two crystallographically independent Ba²⁺ ions are situated in eightfold environment of terminal nitrogen atoms of cyanamide ions.