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Structure of the Carbonate-Intercalated Layered Double Hydroxides: A Reappraisal Shivanna Marappa* and P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India S Supporting Information *

ABSTRACT: Carbonate-intercalated layered double hydroxides have hitherto been thought to crystallize with rhombohedral symmetry (space group R3̅m). This widely accepted structure model comprises positively charged metal hydroxide layers wherein the cations are disordered. However, spectroscopic studies and simple chemical considerations militate against the possibility of cation disorder. This study shows that the observed powder X-ray diffraction pattern can indeed be fit to a cationordered crystal with monoclinic symmetry. With use of the carbonate-intercalated layered double hydroxide of Zn and Al as an illustration, the structure of the layered double hydroxide is refined by the Rietveld method. The resulting structure (space group C2/m, a = √3 × a0; b = 3 × a0; c ∼ c0/3; β ∼ 103°, where a0 and c0 are cell parameters of the cation-disordered structure) resolves many of the anomalies of the cation-disordered structure.

1. INTRODUCTION Amelioration of atmospheric CO2 is one of the biggest challenges facing human kind today.1 Large quantities of CO2 are dissolved in natural water bodies, wherein it is transformed into carbonate ions as CO2 + H2O → CO32− + 2H+. CO2 dissolution causes the acidification of natural water sources. There is an urgent need to mineralize the dissolved carbonate ions, in the form of insoluble inorganic carbonates.2 If this is not done, even a slight increase in the average ambient temperature has the potential to release massive amounts of dissolved CO2 back into the atmosphere. Layered double hydroxides (LDHs) are among the most important candidate materials for CO2 amelioration.3−6 LDHs are obtained by the partial, isomorphous substitution of M(II) ions by M′(III) ions in the structure of M(II)(OH)2. The resulting metal hydroxide layer, [M(II)1−xM′(III)x(OH)2]x+, has a positive charge, to compensate which anions, An−, are included in the interlayer galleries.7,8 Mineral LDHs as well as laboratory-synthesized samples are generally obtained from the aqueous medium. Natural water bodies, as well as laboratory water, unless specially treated, are rich in dissolved CO2. LDHs therefore crystallize with carbonate ions in the interlayer region.9 LDHs comprising other anions are known to readily exchange their anions for incoming carbonates.10 The carbonate ions once incorporated into the LDHs cannot be exchanged for other anions, unless they are discharged first by the use of a mineral acid.11,12 On the basis of these empirical observations, it is suggested that the LDHs have a high affinity for carbonate ions.12 The origin of this affinity is traced to the crystal structure of LDHs. Numerous crystal structures have been refined in this diverse family of hydroxides with M(II) = Mg, Ca, Co, Ni, Cu, Zn; M′(III) = Al, Cr, Fe, V, Ga, In; An− = Cl−, Br−, NO3−, CO32−, SO42−, among others.13−15 A recent detailed review makes a critical appraisal of these reported structures.13 All the structures reviewed by Richardson14,15 comprise a cation-disordered metal hydroxide layer. The metal hydroxide © 2015 American Chemical Society

layer is obtained from a hexagonally packed array of hydroxyl ions, with the cations, M(II) and M′(III), occupying alternative layers of octahedral sites randomly. A single-metal hydroxide layer can be described as AbC within the Bookin and Drits scheme.16 Here A and C represent the close-packed positions of hydroxyl ions and “b” represents the octahedral interstitial site occupied by the cations statistically. Carbonate-containing LDHs are shown to have the sequence AC CB BA AC·······, wherein successive metal hydroxide layers are translated by (2/3, 1/3) relative to one another. Such a stacking sequence yields a three-layer cell of rhombohedral crystal symmetry. Taylor9 suggested that this mode of stacking is most conducive to the incorporation of carbonate ions in the interlayer gallery. The interlayer gallery lined by identical arrays of hydroxyl ions include prismatic interlayer sites (local site symmetry D3h). The molecular symmetry of the carbonate ions is also D3h. The coincidence of the molecular symmetry of the carbonate ion with the local symmetry of the interlayer site maximizes the hydrogen-bonding interactions, leading to a tight packing of the interlayer space, greater thermodynamic stability, and enhanced affinity for CO32− ions. In opposition to Taylor’s views, there are contrarian studies that show the heat of formation of carbonate LDHs to be nearly zero.17,18 There are other suggestions that CO32− LDHs are kinetically nonlabile (metastable) but thermodynamically unstable,19 to account for the poor leaving nature of intercalated carbonate ions. Evidence is now emerging which militates against the widely accepted cation-disordered structure model: (1) X-ray diffraction studies of single crystals of mineral LDHs reveal weak reflections arising out of a supercell of a cation-ordered metal hydroxide layer.20,21 (2) EXAFS spectra22 of Mg−Fe Received: Revised: Accepted: Published: 11075

August 31, 2015 October 20, 2015 October 22, 2015 November 2, 2015 DOI: 10.1021/acs.iecr.5b03207 Ind. Eng. Chem. Res. 2015, 54, 11075−11079

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Industrial & Engineering Chemistry Research LDHs reveal that there are no near neighbor Fe3+···Fe3+ contacts, which are inevitable in a cation-disordered layer. (3) Solid-state NMR spectra of LDH samples give clear and unmistakable signs of cation ordering within the metal hydroxide layer.23 The cation-ordered metal hydroxide layer comprises edgeshared coordination polyhedra of different sizes. The [M(II) (OH)6] is larger than the [M′(III) (OH)6] polyhedron. Consequently, the array of hydroxyl ions lose the hexagonal symmetry and are defined by three different HO···OH in-plane nonbonded distances.24,25 The stacking of the metal hydroxide layers does not fall any more within the scheme of polytypism described by Bookin and Drits.17 Taylor’s model9 also needs a re-evaluation. There is now a critical need for a complete reappraisal of structure of the carbonate LDH, with a view to understand the factors at work, behind the ready incorporation of dissolved carbonates within the LDH gallery. Such an understanding would be vital in view of the applications of LDH for CO2 amelioration. In this manuscript, we propose a cation-ordered structure model for the [Zn−Al−CO3] LDH, refine the structure by the Rietveld method, and discuss the salient features of the proposed structure.

positions of the water molecules were computed by difference Fourier methods. The local point group symmetry of the metal hydroxide polyhedra as well as that of the interlayer sites was determined using code SYMGROUP.27,28 This generates scores for each of the possible symmetry elements. A score of 0 indicates perfect symmetry. A score departing from 0 indicates the extent of variation from exactness. We have used a score below 0.05 as indicative of the existence of the corresponding symmetry element any score >0.05 as an indication of the absence of the symmetry element. The criterion was reasonable as most scores higher than 0.05 were in excess of 1. Where the point group symmetry is constructed on the basis of approximate symmetry elements, it is referred to as the approximate point group. CCDC-1402449 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

3. RESULTS AND DISCUSSION The a-parameter of the widely accepted cation-disordered structure is close to that of mineral brucite (a = 3.13 Å).14 Zn2+ and Al3+ ions share a single crystallographically defined site (3a) and yield a single M(Zn2+, Al3+)−OH distance of ∼2.01 Å. Such a construct is unphysical as the octahedral ionic radius of Zn2+ (0.88 Å) is much larger than the octahedral ionic radius of Al3+ (0.675 Å).29 Consequently, the Zn(II)−OH distance in the isotypic layered hydroxide Zn5(OH)8(NO3)2·H2O is 2.19 Å30 and the Al(III)−OH distances in mineral gibbsite are in the range 1.83−1.95 Å.31 Given these differences, any structure based on a single (Zn2+, Al3+)−OH distance is chemically untenable. It is therefore necessary to identify a structure model wherein Zn2+ and Al3+ ions occupy crystallographically distinct sites. Furthermore, Pauling’s rule32 prohibits two Al3+ cations from occupying neighboring sites within a metal hydroxide layer. For a LDH with the composition [Zn]/[Al] = 2, the imposition of these constraints yields a unique arrangement of cations,33 with a large a-parameter (a = √3 × a0). Such an arrangement of cations leads to a metal hydroxide layer, comprising an orderly arrangement of [Al(OH)6] polyhedra sharing edges with the larger [Zn(OH)6] polyhedra.25 The metal hydroxide layer retains the threefold symmetry. When such layers are stacked one over another, a trigonal crystal with a single-layered unit cell is obtained (space group P3)̅ .25 However, the observed pattern could not be indexed to the P3̅ space group. The literature offers other instances of cation-ordered metal hydroxides but with monoclinic crystal symmetry. In these, the metal hydroxide layer is defined by a = √3 × a0 and b = 3 × a0.26 The stacking of layers is determined by β ∼ 103°. Thiel and co-workers26 proposed this model for the [LiAl2(OH)7]· nH2O LDH, which was later adopted for the I−III LDH obtained by the imbibition of LiCl by bayerite Al(OH)3.34 Krivovichev and co-workers20 proposed a similar model for mineral quintinite. As a prelude to adapting this structure model for refinement, the observed reflections were first indexed to a cell of monoclinic symmetry (Table S1 in the Supporting Information). The cell parameters were further refined within the space group C2/m by a le Bail fit of the whole profile using GSAS.35 The fit was satisfactory and a featureless difference profile was obtained. For structure refinement, the metal hydroxide layer was constructed using the structure model proposed by Thiel and co-workers.26 The C2/m space group provides two potential

2. EXPERIMENTAL SECTION 2.1. Preparation. All the chemicals used for synthesis were procured from Merck, India, and used without further purification. Stock solutions of Zn(NO3)2 and Al(NO3)3 were prepared and standardized prior to use. In a typical precipitation reaction, 50 mL of the mixed-metal nitrate solution (total concentration 1 M, [Zn]/[Al] = 2.0) was slowly added (0.4 mL min−1 dropping rate) to a Na2CO3 solution (100 mL) containing 5 times the stoichiometric requirement of CO32− ions, with constant stirring. A pH 10 is maintained by dosing simultaneously 0.25 M NaOH solution using a Metrohm Model 718 STAT Titrino operating in the pH stat mode. Temperature was maintained constant at 60 °C. After complete addition, the resulting slurry was aged for 18 h in mother liquor at the same temperature. The precipitate was then separated by centrifugation and washed with deionized water (Type II, ELIX 3 Millipore Ion Exchange system) and dried in an oven at 60 °C. 2.2. Characterization. Phase formation was confirmed by powder X-ray diffraction (PXRD) (Bruker D8 Advance powder diffractometer, source Cu Kα radiation, Ni filter, λ = 1.5418 Å) operating in reflection geometry. For structure refinement data were collected at a scan rate 0.12° 2θ min−1 (step size 0.02°, step time 10 s/step, 5−70° 2θ). Thermogravimetric analysis (TGA) was carried out with a Metler Toledo TGA/SDTA 851e system driven by STARe 7.01 software. The sample was initially heated from 30 to 100 °C and equilibrated at 100 °C for 1 h. Then the temperature was ramped from 100 to 800 °C (heating rate 5 °C min−1, N2 atmosphere). 2.3. Indexing and Structure Refinement. The PXRD pattern of the [Zn−Al−CO3]0.33 LDH was indexed using code PROSZKI (POWDER) to a cell of monoclinic symmetry (Table S1) with a Dewolff’s Mn value of 16.17 (FM value 10.12). Structure refinement was carried using the cation-ordered structure model provided by Thiel and co-workers26 for the I−III LDH of the formula [LiAl2(OH)7]·nH2O. The metal hydroxide layer was obtained by replacing Li+ by Al3+ and Al3+ by Zn2+ in the layer of the I−III LDH. Positions of the hydroxyl oxygen atoms, O1 and O2, were refined initially in the General Structure Analysis System (GSAS) to transform the I−III LDH layer into the II−III LDH layer. Code FOX (Free Objects for Crystallography) was used to refine the position of the carbonate anions in the interlayer. The structure model obtained from FOX was exported back into code GSAS for further refinement and determination of positions of the intercalated water molecules. The 11076

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Industrial & Engineering Chemistry Research sites for cations, 4g (0, 1/3, 0) and 2a (0, 0, 0), having the correct multiplicity ratio compared to that of [Zn]/[Al] = 2. Zn2+ was placed in the 4g site and Al3+ in the 2a site. There are two different positions for the hydroxyl oxygen atoms (O1, 8j; O2, 4j). The metal hydroxide layer was input into GSAS as a partial structure model. The coordinates of O1 and O2 were refined first along with the other nonstructural parameters to obtain the correct description of the metal hydroxide layer. To determine the position and orientation of the carbonate anion in the interlayer, this partial structure model was taken into the code FOX (Free Objects for Crystallography).36,37 The CO32−anion was introduced into the interlayer region, as a free molecule. This is done by holding the C−O bond length and O−C−O bond angle flexible within restraints around the value of a free CO32− ion with the default sigma value of 0.01 in appropriate units for the bond lengths and bond angles. The position of the C atom is allowed to vary randomly. At each position, the CO32− ion is also rotated and the calculated pattern is compared with the observed pattern. The structure refinement is based on a Monte Carlo procedure, with the various R parameters used as the cost function. For a stable refinement, the position of the C atom was restricted to a plane midway in the interlayer of the gallery (z = 0.5). The carbonate ion in the process of optimization explored all possible orientations before converging to an orientation with its molecular plane parallel to the metal hydroxide layer (see graphic in the TOC for asymmetric unit of the cell obtained at the end of the FOX procedure). At this stage the structure was exported into code GSAS to complete the refinement by inclusion of water molecules. The water positions Ow1 (2d) and Ow2 (4h) were determined by successive difference Fourier cycles. Two water molecules had to be added to meaningfully obliterate the residual electron density in the difference Fourier maps. Once the water positions were obtained the refinement converged rapidly to yield a fit (Figure 1) with a featureless difference profile and acceptable R values (Tables 1 and 2). The refined structure (Figure 2) has a number of notable features not observed before: (1) There are three different Zn− OH bond lengths at 2.07, 2.118, and 2.117 Å, respectively (Table S2). A SYMGROUP27,28 analysis of the symmetry of

Table 1. Results of Rietveld Refinement of the Structure of the [Zn−Al−CO3]0.33 LDH Using a Cation- Ordered Structure Model crystal system space group cell parameters/Å

monoclinic C2/m a = 5.3188(10) b = 9.2145(17) c = 7.7504(6) β = 102.869(15)° 370.31(4) 38 0.1611 0.1186 0.1472 2.743

volume /Å3 parameters refined Rwp Rp R(F2) χ2

Table 2. Refined Atomic Position Parameters of [Zn−Al− CO3]0.33 LDH atom type

Wyckoff position

x

y

z

SOF

Zn Al O1 O2 C O11 O12 O13 Ow1 Ow2

4g 2a 8j 4i 8j 8j 8j 8j 2d 4h

0 0 0. 88919 0.32918 0.8397 0.9184 0.5962 0.9841 0.5 0

0.3321 0 0. 13864 0.0 0.6365 0.5127 0.6478 0.7539 0 0.12462

0 0 0.11076 0.12124 0.5116 0.4623 0.5163 0.5096 0.5 0.5

1 1 1 1 0.125 0.125 0.125 0.125 0.2184 0.4144

Figure 2. Asymmetric unit of the [Zn−Al−CO3]0.33 LDH showing the carbonate ions in the interlayer (left panel) and the complete unit cell (right panel).

the [Zn(OH)6] polyhedron yields an approximate point group of D3 (Table 3). This is in comparison to D3d expected for the R3̅m cation-disordered structure, based on a single M−OH distance. It is noteworthy that despite the reduction in the crystal symmetry to monoclinic, the [Zn(OH)6] polyhedron has an approximate threefold symmetry arising out of the nearly equal values for two of three Zn−OH bond lengths. (2) There are two different Al−OH bond lengths at 1.72 and 1.79 Å, respectively, yielding an approximate point group symmetry of D3d for the [Al(OH)6] polyhedron. (3) The carbon atom of the carbonate anion is in a slightly acentric position in the

Figure 1. Rietveld fit of the PXRD pattern of the [Zn−Al−CO3]0.33 LDH in cation-ordered model. 11077

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Industrial & Engineering Chemistry Research Table 3. Local Site Symmetries Computed by Code SYMGROUP for Different Sites in the Monoclinic (C2/m) Structuresa C2/m [Zn(OH)6] polyhedron

[Al(OH)6] polyhedron

interlayer site below Al3+ interlayer site below Zn2+ anion

C2

C3

0.00000

0.05806

0.04354 0.04354 0.00000

0.05553

0.04164 0.04164 0.00000

i

r(m)

∼D3

0.00000

0.00000

∼D3d

0.00000

0.04164 0.04164 0.00000

C2h C2

0.00000 0.21040

symmetry

0.02575

0.00528 0.00789 0.02545

∼C3v

Figure 3. [Zn−Al−CO3] LDH viewed along the c-axis. (a) Carbonate ions are in interlayer sites proximal to the Zn2+ cation. Sites proximal to Al3+ are vacant. (b) The arrangement of carbonate ions in the interlayer. O13 shown by the arrow is not hydrogen-bonded to either O1 or O2.

a

The entry under each symmetry element gives the score. The score measures the extent of deviation from exactness.

interlayer gallery (z = 0.5116) and there is a slight loss of planarity. There are three different C−O bond lengths at 1.302, 1.308, and 1.329 Å and the O−C−O bond angles vary from 117° to 121°. But these variations are more likely numerical artifacts of the refinement rather than being chemically significant. However, the anion symmetry is reduced to C3v from D3h in the free anion. (4) It is particularly noteworthy that while all the atoms of the carbonate anion occupy general positions (8j), the water molecules are predicted to occupy special positions 2d (0.5, 0, 0.5) and 4h (0, y, 0.5) (Table 3), thereby enhancing our confidence in the refined structure. The positions of Ow1 and Ow2 are distinct from the positions of the O atoms of the carbonate ion. This is in contrast to the cation-disordered structure model, wherein the O atoms of the intercalated carbonates and water molecules are indistinguishable. Furthermore, the O atoms of the intercalated water are at hydrogen-bonding distances (∼2.9 Å) with the OH group and thereby contribute to the bonding between the layer and the interlayer. (5) The interlayer sites defined by the six nearest OH groups, three from each of the two layers lining the gallery, yield a local symmetry of C2h and C2. It is noteworthy that these local site symmetries are, as expected, subsets of the laue symmetry 2/m of the crystal. This is in contrast to the D3h local symmetry generated in the cation-disordered cell in the R3̅m space group. (6) CO32− ions are located in the interlayer sites proximal to the Zn2+ ions, which have C2h symmetry (Figure 3). Of the three oxygen atoms of the carbonate anion, two (O11 and O12) are within hydrogen-bonding distances with the hydroxyl oxygens (O1 and O2) at 2.88 and 2.58 Å, respectively (Table S2). The third oxygen (O13) does not hydrogen bond with the metal hydroxide layer. This is in contrast to the cationdisordered model, wherein all the three carbonate oxygen atoms are at hydrogen-bonding distances with the metal hydroxide layer consistent with Taylor’s criterion.9

parameters obtained in this work are very close to those reported by Krivovichev and co-workers20 from their study on single crystals of mineral quintinite. However, the model proposed in this work differs slightly from that of Krivovichev. In the latter, two positions are proposed for the CO32− ions. The intercalated water is located in a single position. In this work, the CO32− is located in a single site and positional disorder is observed for the intercalated water molecules which occupy two sites. From a purely structural point of view, it is likely that the two models are very close to one another. However, these competing models have profound implications for the energetics and thereby the thermodynamics of CO2 mineralization. CO2 mineralization studies have hitherto been carried out using the oxide products obtained by the thermal decomposition of LDHs. CO2 uptake from flue gases takes place along with that of water vapor. The end product of the combined uptake of CO2 and H2O is the LDH either in bulk38 or at the surface.39 It is therefore important to have a good understanding of the structure of the LDH. This study is an effort toward this objective.

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ASSOCIATED CONTENT

S Supporting Information *

Table of 2θ values and indexing of the Bragg reflections of [Zn−Al−CO3]0.33 LDH; refined bond distances and bond angles. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.iecr.5b03207. (PDF)

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4. CONCLUSIONS In conclusion, the carbonate-intercalated layered double hydroxides crystallize in monoclinic symmetry (space group C2/m) and comprise cation-ordered metal hydroxide layers. These layers are stacked by a vector which is inclined to the plane of the metal hydroxide layer at β = 103°. The cell

AUTHOR INFORMATION

Corresponding Authors

*Fax: +91-802296 1354. E-mail: [email protected] (P.V.K.). *E-mail: [email protected] (S.M.). Notes

The authors declare no competing financial interest. 11078

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Evidence of a Monoclinic Polytype in M2+-M3+ Layered Double Hydroxides. Mineral. Mag. 2010, 74, 833. (21) Cooper, M. A.; Hawthorne, F. C. The Crystal Structure of Shigaite, [AlMn2+2 (OH)6]3(SO4)2 Na(H2O)6{H2O}6, HydrotalciteGroup Mineral. Can. Miner. 1996, 34, 91. (22) Vucelic, M.; Jones, W.; Moggridge, G. D. Cation Ordering in Synthetic Layered Double Hydroxides. Clays Clay Miner. 1997, 45, 803. (23) Sideris, P. J.; Nielsen, G. U.; Gan, Z.; Grey, C. P. Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy. Science 2008, 321, 113. (24) Jayanthi, K.; Kamath, P. V. Observation of Cation Ordering and Anion-Mediated Structure Selection among the Layered Double Hydroxides of Cu(II) and Cr(III). Dalton Trans. 2013, 42, 13220. (25) Radha, S.; Jayanthi, K.; Breu, J.; Kamath, P. V. Relative Humidity-Induced Reversible Hydration Of Sulfate- Intercalated Layered Double Hydroxides. Clays Clay Miner. 2014, 62, 53. (26) Thiel, J.; Chiang, C.; Poeppelmeier, K. Structure of Lithium Aluminum Hydroxide Dihydrate (LiAl2(OH)7.2H2O. Chem. Mater. 1993, 5, 297. (27) Casanova, D.; Alemany, P.; Alvarez, S. SYMOP, Program for the Calculation of Continuous Symmetry Operation Measures; Universidad de Barcelona, Barcelona, Spain; 2007. (28) Pinsky, M.; Casanova, D.; Alemany, P.; Alvarez, S.; Avnir, D.; Dryzun, C.; Kizner, Z.; Sterkin, A. Symmetry Operation Measures. J. Comput. Chem. 2008, 29, 190. (29) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751. (30) Stählin, W.; Oswald, H. R. The Crystal Structure of Zinc Hydroxide Nitrate, Zn5(OH)8(NO3)2.2H2O. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1970, 26, 860. (31) Saalfeld, B. H.; Wedde, M. Refinement of the Crystal Structure of Gibbsite, AI (OH) 3. Z. Kristallogr. 1974, 139, 129. (32) Pauling, L. The Principles Determining The Structure Of Complex Ionic Crystals. J. Am. Chem. Soc. 1929, 51, 1010. (33) Richardson, I. G. Clarification of Possible Ordered Distributions of Trivalent Cations in Layered Double Hydroxides and an Explanation for the Observed Variation in the Lower Solid-Solution Limit. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2013, 69, 629. (34) Britto, S.; Kamath, P. V. Structure of Bayerite-Based LithiumAluminum Layered Double Hydroxides (LDHs): Observation of Monoclinic Symmetry. Inorg. Chem. 2009, 48, 11646. (35) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Report LAUR 86-748, Los Alamos National Laboratory, Los Alamos, USA; 2004 (36) Favre-Nicolin, V.; Č erný, R. Free Objects for Crystallography (FOX). J. Appl. Crystallogr. 2002, 35, 734. (37) Č erný, R.; Favre-Nicolin, V. FOX: A Friendly Tool to Solve Nonmolecular Structures from Powder Diffraction. Powder Diffr. 2005, 20, 359. (38) Constantino, V. R. L.; Pinnavaia, T. J. Basic Properties of Mg2+1‑xAl3+x Layered Double Hydroxides Intercalated by Carbonate, Hydroxide, Chloride, and Sulfate Anions. Inorg. Chem. 1995, 34, 883. (39) Rebours, B.; d'Espinose de la Caillerie, J. B.; Clause, O. Decoration of Nickel and Magnesium-Oxide Crystallites with SpinelType Phases. J. Am. Chem. Soc. 1994, 116, 1707.

ACKNOWLEDGMENTS Authors thank the Department of Science and Technology (DST), Government of India, for financial support. P.V.K is a Ramanna Fellow of the DST. Authors thank one of the reviewers for her/his useful comments.

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