Birnessite polytype systematics and identification by powder X-ray diffraction

Abstract

The polytypes of birnessite with a periodic stacking along the c* axis of one-, two-, and three-layers are derived in terms of an anion close-packing formalism. Birnessite layers may be stacked so as to build two types of interlayers: P-type in which basal O atoms from adjacent layers coincide in projection along the c* axis, thus forming interlayer prisms; and, O-type in which these O atoms form interlayer octahedra. The polytypes can be categorized into three groups that depend on the type of interlayers: polytypes consisting of homogeneous interlayers of O-or P-type, and polytypes in which both interlayer types alternate. Ideal birnessite layers can be described by a hexagonal unit-cell (a h = b h ≈ 2.85 Å and γ = 120°) or by an orthogonal C-centered cell (a = √3 b, b h ≈ 2.85 Å and γ = 90°); and, hexagonal birnessite polytypes (1H, 2H 1 , 2H 2 , 3R 1 , 3R 2 , 3H 1 , and 3H 2 ) have orthogonal analogues (1O, 2O 1 , 2O 2 , 1M 1 , 1M 2 , 3O 1 , and 3O 2 ).X-ray diffraction (XRD) patterns from different polytypes having the same layer symmetry and the same number of layers per unit cell exhibit hkl reflections at identical 2θ positions. XRD patterns corresponding to such polytypes differ only by their hkl intensity distributions, thus leading to possible ambiguities in polytype identification. In addition, the characteristics of the birnessite XRD patterns depends not only on the layer stacking but also on the presence of vacant layer sites, and on the type, location and local environment of interlayer cations. Several structure models are described for birnessite consisting of orthogonal vacancyfree or of hexagonal vacancy-bearing layers. These models differ by their stacking modes and by their interlayer structures, which contain mono-, di-, or tri-valent cations. Calculated XRD patterns for these models show that the hkl intensity distributions are determined by the polytype, with limited influence of the interlayer structure. Actual structures of

Structure of nanocrystalline phyllomanganates produced by freshwater fungi

“…The ratio of the d-spacings for the 11,20 and 31,02 bands (~1.73) is close to 3 , thus indicating hexagonal layer symmetry. This symmetry is confirmed by the symmetrical shape of the 31,02 band, as the 31 and 02 contributions are split when the layer symmetry is orthogonal (a > 3 × b; Webb et al 2005;Drits et al 2007;Lanson et al 2008). This band is at the same position for IRB20-1 and IRB20-2, but shifted to lower 2θ values for KR21-2, indicative of larger in-plane unit-cell parameters (Fig.…”

“…1; Drits et al 1997Drits et al , 2007Jürgensen et al 2004;Villalobos et al 2006;Lanson et al 2008). Fungal cells have a scattering intensity that rises at 16 °2θ, plateaus at 20-40 °2θ, and decreases steadily afterward.…”

Sorption and desorption of tetracycline on layered manganese dioxide birnessite

“…7), corresponding to the 001 and 002 reflections and the 20, 11 band of a birnessite with turbostratic disorder (Drits et al 2007;Grangeon et al 2010). With an increase in TC loading, a small peak at the low-angle side with a d-spacing of *9.8 Å started to show up.…”

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

“…The AC layer with the generalized composition [MX 2 ] is found in a very diverse range of materials such as metal sulfides MS 2 (M = Ti, Mo, Nb, Ta), [21,22] metal selenides MSe 2 (M = Mo, Ta), [23] divalent halides MX 2 (M = Ca, Mg, Fe, Co, Ni, Cd), [24,25] metal hydroxides M(OH) 2 (M = Mg, Ni, Co), [26,27] birnessite-type oxides MO 2 (M = Mn), [28,29] lithium oxides LiMO 2 [30,31] and basic salts M(OH) 2-x A x (M = Ni, Co; x = 0.66-0.25; A = Cl, NO 3 ). [32][33][34] We therefore call the AC layer a 'structural synthon', the term "structural" is used to describe the extended nature of the AC layer, as opposed to the sub.-molecular synthon originally proposed by Corey [35] and the supramolecular synthon of Desiraju [36,37] which are entities of finite dimension.…”