Recent diffraction studies on layered double hydroxides have shown that the threelayer polytypes that were thought to crystallize in rhombohedral symmetry are actually one-layer polytypes of monoclinic symmetry. However, the prevailing Bookin and Drits (1993) scheme of polytypism, which is based on the widely accepted cation-disordered structure model, fails to predict the occurrence of low symmetry (monoclinic and orthorhombic) polytypes among the layered double hydroxides. In this work, a cation-ordered metal hydroxide layer (layer group p3̅ 12/m or c12/m1) is chosen as the basic building block. Application of the structural synthon approach enables the description of the complete universe of polytypes comprising 1H, 1M 1−7 , 2H, 2O, 3R, 3H, 6H, and 6R among others (M: monoclinic; O: orthorhombic). These polytypes are characterized by their unique stacking vectors. The polytypes of large unit cell volume are obtained by a combination of two or more stacking vectors. This work has relevance to the understanding of several mineral structures, especially those with large unit cells.
The imbibition of divalent cations (Zn2+, Ni2+) into half the cation vacancies in Al(OH)3 results in layered double hydroxides comprising positively charged layers of the composition [M□Al4(OH)12]2+ (□: cation vacancy). Nitrate ions were intercalated for charge neutrality together with water molecules. Structure refinement by the Rietveld method showed that the metal hydroxide crystallized in monoclinic symmetry (space group P121/n1). Upon heating to T = 160 °C, a new phase was obtained which was indexed to a two layer cell of orthorhombic symmetry, suggesting an interpolytype transformation. Using the idealized metal hydroxide layer (layer group p121/a1) as a structural synthon, the different ways of stacking these layers were predicted to arrive at the complete universe of polytypes in this class of compounds. Some of the predicted stacking sequences generated new 2-fold axes, not found in the layer group and resulted in one-layer, two-layer, and three different four-layer polytypes (1O, 2O, and 4O1–3; O: orthorhombic) of orthorhombic symmetry. The high temperature phase can be described as a two-layer polytype (2O) (space group Pn2n) obtained by the rigid translation of the metal hydroxide layers by (1/4, 1/2) relative to one another. Cooling and rehydration restored the as-prepared phase of monoclinic symmetry.
Layered double hydroxides are obtained by partial isomorphous substitution of divalent metal ions by trivalent metal ions in the structure of mineral brucite, Mg(OH)2. The widely reported three-layer polytype of rhombohedral symmetry, designated as polytype 3R1, is actually a one-layer polytype of monoclinic symmetry (space group C2/m, a = 5.401 Å, b = 9.355 Å, c = 11.02 Å, β = 98.89°). This structure has a cation-ordered metal hydroxide layer defined by a supercell a = √3 × a0; b = 3 × a0 (a0 = cell parameter of the cation-disordered rhombohedral cell). Successive layers are translated by (1/3, 0, 1) relative to one another. When successive metal hydroxide layers are translated by (2/3, 0, 1) relative to one another, the resultant crystal, also of monoclinic symmetry, generates a powder pattern corresponding to the polytype hitherto designated as 3R2. This structure model not only removes all the anomalies intrinsic to the widely accepted cation-disordered structure but also abides by Pauling's rule that forbids trivalent cations from occupying neighboring sites and suggests that it is unnecessary to invoke rhombohedral symmetry when the metal hydroxide layer is cation ordered. These results have profound implications for the correct description of polytypism in this family of layered compounds.
Garnet solid-electrolyte-based Li-metal batteries can be used in energy storage devices with high energy densities and thermal stability. However, the tendency of garnets to form lithium hydroxide and carbonate on the surface in an ambient atmosphere poses significant processing challenges. In this work, the decomposition of surface layers under various gas environments is studied by using two surface-sensitive techniques, near-ambient-pressure X-ray photoelectron spectroscopy and grazing incidence X-ray diffraction. It is found that heating to 500 °C under an oxygen atmosphere (of 1 mbar and above) leads to a clean garnet surface, whereas low oxygen partial pressures (i.e., in argon or vacuum) lead to additional graphitic carbon deposits. The clean surface of garnets reacts directly with moisture and carbon dioxide below 400 and 500 °C, respectively. This suggests that additional CO2 concentration controls are needed for the handling of garnets. By heating under O2 along with avoiding H2O and CO2, symmetric cells with less than 10 Ωcm2 interface resistance are prepared without the use of any interlayers; plating currents of >1 mA cm–2 without dendrite initiation are demonstrated.
Rietveld refinement of the structures of the [LixAl(OH)3]Xx·yH2O (X = Br, Cl; y ≈ 1/2, 2/3) layered double hydroxides (LDHs) shows that the halide ion occupies two distinct positions in each phase. One position (2a), promoted by Coulombic attraction, is proximal to the Li+ ion, which is the seat of the positive charge in the metal hydroxide layer. The other position (6h) is within a close hydrogen‐bonding distance from the hydroxy group of the metal hydroxide layer as well as the intercalated water molecule. The LDH with intercalated Br– ions crystallizes in two different structures, one of which is isostructural with the Cl– analogue. In the other structure obtained at a higher relative humidity (≥76 %; y ≈ 2/3), the Br– ion occupies a site proximal to the Al3+ ion (2b), and intercalated water molecules are proximal to the Li+ ion. In this structure, the hydration of the Li+ ion in a distant coordination shell overcomes Coulombic interactions with Br– ions. On partial dehydration (ambient humidity 29 %; y ≈ 1/2), this Br– ion reversibly reverts back to the site (2a) proximal to the Li+ ion, which results in a humidity‐induced structure change without any variation in the basal spacing. Thereby, the position of the halide ion in the interlayer gallery is determined by the interplay of Coulombic and hydrogen‐bonding interactions.
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