Layered Double Hydroxides: Proposal of a One-Layer Cation-Ordered Structure Model of Monoclinic Symmetry

Abstract

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.

Structure models for the hydrated and dehydrated nitrate-intercalated layered double hydroxide of Li and Al

“…The key to the successful choice of a monoclinic cell is to find the appropriate value for the stacking angle β, in c-stacking. 20 In this we were greatly aided by DIFFaX simulations (Fig. 3).…”

Crystal chemistry of natural layered double hydroxides: 4. Crystal structures and evolution of structural complexity of quintinite polytypes from the Kovdor alkaline-ultrabasic massif, Kola peninsula, Russia

“…We note that the structural model of the 1 M polytype proposed by us for quintinite-1 M was used by Jayanthi et al . (2015) for synthetic LDHs and applied for the Rietveld structure refinement of the synthetic analogue of zaccagnaite, [Zn 4 Al 2 (OH) 12 ][(CO 3 )(H 2 O) 3 ] (Marappa and Kamath, 2015).…”

“…Previously published structural models of quintinite appeared to be helpful for the characterization of hydrotalcite-supergroup members (Lozano et al ., 2012; Mills et al ., 2012 a ; Theiss et al ., 2015); synthetic layered double hydroxides (Zhang and Evans, 2012; Jayanthi and Kamath, 2013, 2016; Radha et al ., 2014; Jayanthi et al ., 2015; Marappa and Kamath, 2015); their crystal structure simulation and analysis (Radha and Kamath, 2012; Richardson, 2013; Ghamami et al ., 2016; Tsukanov and Psakhie, 2016).…”

Electronic‐Structure Calculations of Cation‐Ordered II‐III Layered Double Hydroxides: Origin of the Distortion of the Metal‐Coordination Symmetry

“…The [Cr(OH) 6 ] polyhedron has D 2 h symmetry in the [Cu–Cr] LDH, and C 3 v symmetry in the [Zn–Cr] LDH. In the observed structures, the [M(OH) 6 ] polyhedra have C 3 symmetry when M = Cu 2+ , and D 3 symmetry when M = Zn 2+ . In the computed structures, the coordination symmetry of the [Cu(OH) 6 ] polyhedron varies from C s to C 2 and that of the [Zn(OH) 6 ] polyhedron is D 3 and C 2 .…”

“…6 ] polyhedra have C 3 symmetry when M = Cu 2+ , [22] and D 3 symmetry when M = Zn 2+ . [23] In the computed structures, the coordination symmetry of the [Cu(OH) 6 ] polyhedron varies from C s to C 2 and that of the [Zn(OH) 6 ] polyhedron is D 3 and C 2 . The lowering of the coordination symmetry of the divalent ions from the ideal D 3d is especially puzzling in crystals of high symmetry, and it forms the subject matter of this paper.…”