Cation Ordering in Synthetic Layered Double Hydroxides

Abstract

Abstraet--A combined powder X-ray diffraction (XRD) and X-ray absorption (XAS) study of Fe(III) cation ordering within pyroaurite is described. It is concluded that there is no correlation between Fe(III) cation positions over distances of a few tens of angstroms, but that there is a very high level of local ordering, involving the absence of Fe(III)-Fe(III) neighbors. These observations are rationalized in terms of a significant frequency of lattice defects in the form of cation vacancies or Mg for Fe(III) substitutions. These results are expected to be generalizable to other M(II)/M(III) layered double hydroxides (LDHs), but are in contrast to the long-range cation orderir~g observed in Li/AI LDHs. This raises the interesting possibility of differing properties and stabilities based on the degree of cation ordering.

Ambient-Temperature Synthesis, Evolution, and Characterization of Cobalt-Aluminum Hydrotalcite-Like Solids

“…Clustering of Co and A1 atoms is also consistent with the strain broadening found by Rietveld refnement of the powder XRD pattern. Differences between the a cell parameter from XRD and the cation-cation distance from EXAFS data were also reported for takovite (Scheidegger et al, 1998), the Ni-A1 analog of hydrotalcite, but not for pyroaurite, the Mg-Fe(III) analog (Vucelic et al, 1997). Cation clustering and the associated discrepancy between XRD lattice parameters and EXAFS interatomic distances may be a consequence of strain arising from significant differences in ionic radius between the two cations occupying the hydroxide layers of hydrotalcite-like phases, as for Co(II), r = 0.735 and AI(III), r = 0.530 *); and Ni(II), r = 0.700 ,~, and Al(III); but not Mg(]I), r = 0.720 A, and Fe(IIl), r = 0.645 A (Shannon and Prewitt, 1969).…”

“…CoHT-1 precipitates may have Co-and Al-rich domains, rather than local cation ordering driven by A1 avoidance (Vucelic et al, 1997) or random cation ordering (Cavani et al, 1991). The symmetry of hydrotalcites requires that the a cell parameter be equal to the cation-cation distance, assuming random ordering of the cations.…”

Relative Humidity-Induced Reversible Hydration Of Sulfate-Intercalated Layered Double Hydroxides

“…Evidence in favor of cation ordering is provided by an array of techniques such as extended X-ray absorption fine structure (Vucelic et al, 1997;Bigey et al, 1997;Roussel et al, 2000Roussel et al, , 2001, magic angle spinningnuclear magnetic resonance (Sideris et al, 2008(Sideris et al, , 2012Cadars et al, 2011), and X-ray diffraction (XRD) (Krivovichev et al, 2010;Radha and Kamath, 2013). Cation ordering generates a large unit cell with a = Hn6a o (n = 3 for x = 0.33) (Hofmeister and Platen, 1992).…”

Mechanochemical Preparation and Structural Characterization of Layered Double Hydroxides and their Amino Acid-Intercalated Derivatives

“…The absence of Fe-O-Fe linkage confirms that the Fe 3+ ions do not occupy neighboring sites, which would be unfavorable due to charge repulsion [11] . It is to be noted that EXAFS measurements performed for LDHs are not sensitive to the interlamellar species; the fitted data provide information on the local order around the cations in the layers only [155] .…”

“…linkage is unavoidable [11] . The energetically unfavorable state is due to the electrostatic repulsion between the neighboring trivalent metals in the layers as well as to the repulsion between the charge-compensating anions between the layers.…”