Polymorphism and Structural Disorder in the Carbonate Containing

The stacking faults cause the nonuniform broadening of peaks due to the hkl ..... However, the doublet appearing in the 35−37° 2θ range shows a po...
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J. Phys. Chem. C 2008, 112, 9510–9515

Polymorphism and Structural Disorder in the Carbonate Containing Layered Double Hydroxide of Li with Al Sylvia Britto,† Grace S. Thomas,† P. Vishnu Kamath,†,* and S. Kannan‡,* Department of Chemistry, Central College, Bangalore UniVersity, Bangalore 560 001, India, and Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, G.B. Marg, BhaVnagar 364 002, India ReceiVed: January 14, 2008; ReVised Manuscript ReceiVed: April 10, 2008

The carbonate containing layered double hydroxide (LDH) of Li with Al having the formula [LiAl2(OH)6](CO3)0.5 · 1.5H2O crystallizes in the monoclinic system. The single layered cell belongs to the 1M polytype. However, the crystallites are extensively faulted. The structural disorder is on account of the incorporation of stacking faults. The stacking faults cause the nonuniform broadening of peaks due to the hkl reflections in the powder X-ray diffraction pattern. Using DIFFaX simulations, we compare the local structure of the stacking faults with the structure of the different theoretically possible polytypes within the monoclinic crystal system. Such a comparison shows that the stacking faults arise due to the random incorporation of motifs having orthorhombic symmetry (2O polytype) within the matrix of monoclinic symmetry. On heating, the progressive dehydration of the interlayer region introduces turbostratic disorder. This is in contrast with the case of halide containing LDHs, which exhibit an ordering of the interlayer upon dehydration. Introduction Brucite [Mg(OH)2]-based layered double hydroxides (LDHs) crystallize with either the rhombohedral or hexagonal symmetry1 and exhibit extensive polytypism. Bookin and Drits2–4 outlined all the theoretically possible polytypes in this system and predicted the powder diffraction pattern characteristic of each polytype. Among the brucite-based LDHs, the symmetries of the possible polytypes are restricted by the requirement of maintaining a close packing of atoms. Polytypism among the cubic and hexagonal crystals such as SiC and CdI2, respectively, has been extensively studied.5 Al(OH)3-based LDHs comprise another class of layered materials with potential applications in catalysis,6 preferential ion exchange, and intercalation reactions.7 Al(OH)3 crystallizes in four naturally occurring polymorphic modifications known as gibbsite,8 bayerite,9 nordstrandite,10 and doyleite.11 The most well-studied of these are bayerite and gibbsite. Bayerite is structurally related to brucite, with three Mg2+ ions substituted by two Al3+ ions. This results in a layer of composition [Al20(OH)6]. The cation vacancies are ordered, resulting in a larger unit cell (a ) 5.047 Å, c ) 4.73 Å) when compared to Mg(OH)2 (a ) 3.15 Å, c ) 4.77 Å). The metal-hydroxide layer stacking sequence in these hydroxides is AC AC...12 In contrast, gibbsite has the stacking sequence AC CA AC... and is a two-layer polytype of Al(OH)3 (ICSD No. 27 698; space group: P121/n1, a ) 8.676 Å, b ) 5.070 Å, c ) 9.721 Å, β ) 94.58°). “Imbibition”13 of a lithium salt, LiX (X ) Cl-, Br-, OH-), into Al(OH)3 results in the formation of a layered double hydroxide (LDH) of Li with Al, having a layer of composition [LiAl2(OH)6]+ and an interlayer comprising X- · H2O. The Li-Al-X LDH was synthesized starting from both bayerite and gibbsite; the former yielded a rhombohedral LDH,14 and the latter yielded * To whom correspondence should be addressed. E-mail: vishnukamath8@ hotmail.com (P.V.K.); [email protected] (S.K.). † Bangalore University. ‡ Central Salt and Marine Chemicals Research Institute.

a hexagonal LDH,15 pointing to a possible topochemical pathway for the LDH formation. Bayerite has also been reported to crystallize in the monoclinic symmetry [ICSD No. 26 830; space group: P121/a1, a ) 5.062 Å, b ) 8.671 Å, c ) 4.713 Å, β ) 90.27°]. It is therefore expected that the Li-Al LDHs could also crystallize in the monoclinic crystal system. Although the Li-Al-CO32- LDH has been indexed to a hexagonal cell in earlier papers,16 certain reflections are split, a feature indicative of a possible lowering of the crystal symmetry. In later work, the Li-Al-CO32- LDH was indexed to a monoclinic cell.17 Most interesting is the case of the Li-Al-OH- LDH.18 While this LDH could be indexed in both the hexagonal and monoclinic crystal systems, the structure could not be satisfactorily refined in either, in part due to the nonuniform broadening of lines in the powder X-ray diffraction (PXRD) pattern. Such nonuniform broadening of lines is due to stacking disorders. We have, of late, been interested in the classification and quantification of stacking disorders in layered hydroxides.19–21 In particular, we have devised a scheme whereby the local structure of the stacking fault is compared with the stacking sequence of the different theoretically possible polytypes as a means of classification of the stacking faults and evolving a nomenclature for them.19 In this manner, the stacking disorders are describable within a crystal chemical terminology. This approach has been successful for the brucite-based LDHs, given the extensive understanding of polytypism in close-packed systems. In this paper, we extend this approach to the Li-Al-CO32LDH, which crystallizes in the monoclinic crystal system. We first describe the various possible polytypes in this crystal symmetry, predict their PXRD patterns using DIFFaX simulations, and then classify and quantify the stacking disorders in the as-prepared samples. We also employ variable temperature PXRD (VTPXRD) studies and monitor the evolution of structural disorder with temperature leading up to the dissociation of the LDH and formation of the oxide residue.

10.1021/jp800341n CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

Structure of the Li-Al-CO32- LDH

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Experimental Section Solid urea was added to a mixed metal ([Al(III)] + [Li(I)] ) 0.5 M) chloride solution while maintaining the molar ratio of urea/([Al(III)] + [Li(I)]) at 3.3. The suspension was aged at 90 °C for 48 h. The solid was then recovered by filtration (approximate yield 80%), rinsed with acetone, and dried in an air oven at 80 °C. In situ VTPXRD was carried out on a Philips X’pert MPD system connected to an Anton-Paar high temperature XRK assembly using Cu KR radiation. The sample was mounted in a high temperature cell and heated at 5 °C min-1 in steps of 25 °C and stabilized for 10 min before measurements. The operating voltage and current were 40 kV and 40 mA, respectively. The step size was 0.05° 2θ with a step time of 1 s. DIFFaX Simulations. The Fortran-based computer code DIFFaX22 was used for the simulation of the PXRD patterns of the ordered as well as faulted crystals. Within the DIFFaX formalism,23 a crystalline solid is described as a stacking of layers of atoms, interconnected by a suitable stacking vector, also called a “transition”. The stacking unit in LDH comprises a metal-hydroxide layer and an interlayer of intercalated anions and water molecules. The DIFFaX code requires the stacking direction to be along the c crystallographic axis and the cell parameters to be defined as a, b, c, and γ. The assumption is that R ) β ) 90°. Such a convention is suited for hexagonal and rhombohedral crystal systems. The Li-Al-CO32- LDH crystallizes in the monoclinic crystal system. To make this structure compatible with the DIFFaX code, the cell was “orthogonalized” as ao ) am, bo ) bm, co ) cm sin β (o, orthogonalized cell parameter; m, monoclinic cell parameter), and γo ) 90°. The stacking vector (cm/am cos β, 0, 1) generates the monoclinic symmetry. A single layer (stacking unit) is defined using the position coordinates taken from the model structure (space group: C12/m1, a ) 5.086 Å, b ) 8.8088 Å, c ) 7.758 Å, β ) 102.62°). All the symmetry related atoms are explicitly defined, and the point group is declared as “unknown”. Such an option enables the DIFFaX code to evaluate the Laue symmetry. The computed symmetry is 2/m, compatible with the symmetry of the model structure. We define this as the 1M polytype. An additional translation of adjacent layers by ((am/3, 0, 1), leading to a stacking vector (cm/am cos β ( am/3, 0, 1), generates a cell with monoclinic symmetry (computed Laue symmetry: 2/m) and a two-layer periodicity. We define this polytype as 2M. The stacking of two different layers, one of which is the mirror image of the other (by reflection about the bc-plane) alternately by (cm/am cos β, 0, 1) and (-cm/am cos β, 0, 1), generates a two-layer polytype with orthorhombic symmetry, 2O (computed Laue symmetry: mmm). The layer structure and the stacking vectors used in the simulation of the PXRD patterns of ordered polytypes are given in the Supporting Information SI.1. Faulted structures are generated by the random use of the relevant stacking vectors with different probabilities (see the Supporting Information SI.2 for stacking vectors and probabilities used in illustrative simulations of the PXRD patterns of faulted crystals). For model simulations, the calculated Bragg reflections are broadened by using a Lorentzian profile function with a full width at half-maximum (fwhm) ) 0.2o 2θ. For simulation of the experimental patterns, a Lorentzian profile with the fwhm of the 00l (l ) 1) reflection is used.

Figure 1. PXRD pattern of the Li-Al-CO32- LDH. Feature marked by the asterisk is due to residual bayerite.

TABLE 1: Observed d-Spacings of the PXRD Pattern of the Li-Al-CO32- LDH monoclinic cella d (obs) (Å) d (calc) (Å) 7.531 4.334 3.763 2.519 2.483 2.214 1.893 1.693 1.600 1.469 1.442

7.571 4.324 3.785 2.527 2.482 2.219 1.894 1.694 1.603 1.468 1.441

hkl 001 110 002 003 200/-131 201/-132 202/-133 133/-204 -134/204 -331/060 -332/061

hexagonal cellb d (calc) (Å) hkl fwhm (°2θ) 7.557 4.324 3.779 2.527 2.481 2.218 1.893 1.694 1.601 1.468 1.441

003 101 006 111 112 115 118 1110 1111 300 303

0.3 0.4 0.4 0.6 0.7 0.8 1.4 0.5 0.7

a a ) 5.086 Å, b ) 8.8088 Å, c ) 7.758 Å, β ) 102.62°. b a ) 5.086 Å, c ) 22.675 Å.

TABLE 2: Special Positions within the Space Group C12/m1 Used to Model the Structurea of the Li-Al-CO32LDH atom Li Al O1 O2 O3 O4 O5 b

Wyckoff position 2a 4g 8j 4i 2d 4h(a) 4h(b)

x

y

z

SOFb

0.0 0.0 0.866 0.397 0.0 0.0 0.0

0.0 0.335 0.181 0.0 0.5 0.333 0.155

0.0 0.0 0.148 0.142 0.5 0.5 0.5

1.0 1.0 1.0 1.0 0.75 0.625 0.75

a a ) 5.0858 Å, b ) 8.8088 Å, c ) 7.758 Å, β ) 102.62°. SOF: site occupancy factor.

Results and Discussion The PXRD pattern of the Li-Al-CO32- LDH is shown in Figure 1. This pattern can be indexed on the basis of a hexagonal as well as a monoclinic cell. The observed d-spacings and the assignment according to both of the crystal systems are given in Table 1. The following observations can be made: (1) Two prominent basal reflections 00l and 002l appear in the low angle region (5-25° 2θ). Also observed in this region is the peak due to cation ordering (20.5° 2θ), which is assigned to the 110 reflection of the monoclinic cell. (2) At high angles (60-70° 2θ) are observed peaks due to the -33l reflections of the monoclinic cell (63.3 and 64° 2θ). (3) In the mid-2θ region (30-60° 2θ) are seen a series of peaks that are indexed to the 11l (l ) 1, 2, 5, 8, 10, 11) family

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Figure 2. DIFFaX simulated PXRD patterns expected of the (a) 1M, (b) 2M and (c) 2O polytypes of the Li-Al-CO32- LDH.

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Figure 4. DIFFaX simulated PXRD patterns of the 1M polytype incorporating different proportions of planar faults corresponding to the stacking motif of the 2M polytype.

Figure 3. Schematic of the structures of the 1M, 2M, and 2O polytypes of the Li-Al-CO32- LDH.

of reflections of the hexagonal cell. Each of the peaks transforms into a doublet of the type 20l/-13l ( 1 of the monoclinic cell. The monoclinic distortion is very slight, and the calculated peak positions corresponding to each reflection in the doublet coincide. The monoclinic distortion also cannot be discerned in the observed pattern, especially as the peaks are nonuniformly broadened. The fwhm values of the observed peaks are also listed in Table 1. It is evident that the peak due to the 202/ -133 doublet (118 in hexagonal) has a higher fwhm (0.7° 2θ) compared to the 200/-131 doublet (112 in hexagonal) (fwhm ) 0.4° 2θ). Earlier work has shown that, in both the hexagonal24–26 as well as monoclinic crystals,27 the nonuniform broadening of reflections arises out of structural disorder. The most common form of structural disorder manifesting in layered materials are the stacking faults. Stacking faults arise in two ways: (1) due to the random orientation of successive layers about the stacking direction, a kind of disorder known as turbostraticity,28 and (2) growth and deformation faults arising due to the insertion of layers in a sequence that departs from the sequence expected of the ordered solid.24,25 Generally, the former does not conserve the interatomic contact distances and is observed only in layered solids where (a) weak interactions between the layers permit a greater variation in nonbonded interatomic contact distances and (b) the intervention of atoms in the interlayer coupled with a variation in the interlayer spacing restores interatomic contact distances to chemically acceptable values. The growth and deformation faults are more prevalent in close-packed and 3-D solids. Earlier work on hexagonal crystals has shown that the

Figure 5. DIFFaX simulated PXRD patterns of the 1M polytype incorporating different proportions of planar faults corresponding to the stacking motif of the 2O polytype.

two kinds of stacking faults produce characteristic line broadening in the PXRD pattern of the solid.26 The combination of monoclinic distortion and the incidence of stacking faults effectively render structure refinement of the Li-Al-CO32- LDH by the Rietveld method difficult. Thiel and co-workers18 had published a PXRD pattern of Li-Al-OHLDH, which is very similar to our pattern. It was established that the structure crystallizes in the monoclinic system (space group: C2/m), and attempts were made to fit the observed pattern based on a monoclinic model. However, Rietveld refinement of the observed pattern yielded a poor fit due to a mismatch of the relative intensities in the mid-2θ region. This problem was resolved by proposing a novel structure model comprising a 54-layer unit cell. Within such a cell, the layers were stacked using random stacking vectors. This corresponds to a cell with turbostratic disorder. The random vectors were chosen in such a way as to limit the variation in interatomic distances within chemically acceptable limits. More elegant than the use of random stacking vectors is the incorporation of stacking faults whose local stacking sequence matches with that of the possible polytypes in the system. In this scheme, the local symmetry about the fault is the same as that of the corresponding polytype. In this approach to the brucite-based LDHs, we were greatly aided by the work of Bookin and Drits,2,3 who have extensively described all the

Structure of the Li-Al-CO32- LDH

Figure 6. Comparison of the observed pattern of the Li-Al-CO32LDH with (a) the DIFFaX simulated pattern expected of the 1M polytype and (b) the DIFFaX simulated pattern of the 1M polytype containing stacking faults of the orthorhombic symmetry. The difference profile has also been plotted. The low angle region is omitted for clarity.

Figure 7. In situ variable temperature PXRD data of the Li-Al-CO32LDH.

theoretically possible polytypes among the LDHs, which crystallize with hexagonal/rhombohedral symmetry. We also successfully described the stacking disorders in Li-Al-X- (X ) Cl, Br) LDHs in terms of the hexagonal polytypes.21 Although the PXRD pattern of the Li-Al-CO32- LDH can be indexed to a hexagonal cell, all attempts to simulate the observed pattern based on the existing models of hexagonal Li-Al LDHs failed. Therefore, we reject the hexagonal structure model. Our aim is to see if the nonuniform broadening in the PXRD pattern reported in Figure 1 is generated by stacking faults derived from the possible polytypes within the monoclinic crystal system. Within the monoclinic symmetry, however, there is only a limited description of polytypism.29 We therefore describe below the possible polytypes of the Li-Al-CO32LDH. Polytypism in the Li-Al-CO32- LDH. More important than the exact structure of the LDH is the overall view of the stacking pattern of the successive layers. Each layer comprises

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Figure 8. DIFFaX simulation of the PXRD patterns of the Li-Al-CO32- LDH obtained at (a) 125 °C and (b) 175 °C. The low angle region is omitted for clarity.

a metal-hydroxide slab [LiAl2(OH)6] and the intercalated atoms [CO32-, H2O]. Since the exact structure of the Li-Al-CO32LDH is not reported, we use the model of Thiel et al.18 as the starting point for the definition of the layer. Since this structure describes the Li-Al-OH- LDH, more atoms (C, O) need to be accommodated in the interlayer. Two possible interlayer positions were considered in the C2/m space group. The 2c site corresponds to positions wherein the atoms would lie directly in between the Li+ ions and the 4h (0, 0.333, 0.5) site, which corresponds to atoms lying directly in line with the Al3+ ions. The effect on the reflections in the mid-2θ region is the same for both the 2c and 4h interlayer positions. However, placement of O atoms in the 2c position leads to the emergence of additional reflections at 21.6o 2θ and 25.5° 2θ corresponding to -111 and 111, respectively, which are not observed in the experimental pattern (see the Supporting Information SI.3). Therefore, the structural model based on occupancy of 2c positions is discounted. The pattern generated by placing the O atoms in the 4h position correctly simulates the observed pattern in terms of peak positions if not in terms of relative intensities (see the Supporting Information SI.3). The cell parameters and atomic coordinates of the model structure used for the subsequent simulations are given in Table 2. In order to obtain a pattern, which is as close to the experimental pattern as possible, the site occupancies were adjusted as given in Table 2 corresponding to a total interlayer occupancy of 1.166 (carbon included). This is in accordance with the thermogravimetric analysis (TGA) data (Supporting Information SI.4), which indicate a total mass loss of 47.6% for this compound. Assuming a Li/Al ratio of 2, the nominal formula for this compound is found to be [Li0.333Al0.667(OH)2][CO3]0.1667 · 0.5H2O. This layer is the repeating motif. The DIFFaX simulated PXRD patterns of the 1M, 2M, and 2O polytypes are shown in Figure 2. The calculated patterns exhibit the following characteristic features expected of polytypes. (1) The 00l reflections of 1M transform into 002l in the 2M polytype. (2) All the hkl reflections observed in 1M transform into hk2l in the 2M polytype and appear at the same 2θ values.

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TABLE 3: Results of the DIFFaX Simulations of the PXRD Patterns of the Li-Al-CO32- LDH Obtained at Different Temperatures T (°C)

disorder

interlayer oxygen and carbon content (CO3 + H2O)

layer thickness (Å)

25 125 175

12% 2O 5% turbostratic + with 15% 2O 10% turbostratic +15% 2O

1.16 0.87 0.67

∞ ∞ ∞

(3) New hkl reflections are generated in the 2M polytype. (4) The basal reflections and the high angle (>60° 2θ) reflections remain invariant in all the polytypes. The peaks in the mid-2θ region (30-60° 2θ) show changes in their positions and relative intensities in the different polytypes. A schematic of the three polytypes is shown in Figure 3. The observed pattern (Figure 1) does not match in the relative intensities of the reflections in the mid-2θ region with any of the polytypes. Therefore, we proceed to correct for this mismatch by the incorporation of stacking faults. Faulted structures can be generated by randomly introducing stacking sequences of one polytype into the matrix of another. Within the DIFFaX formalism, faulted structures are generated by the simultaneous use of more than one stacking vector with different probabilities. In Figures 4 and 5 are shown the results of DIFFaX simulations of the PXRD patterns of faulted crystals obtained by the random incorporation of different proportions of 2M and 2O polytypes, respectively, in the matrix of the 1M polytype. The following observations are made: (1) The basal reflections are unaffected by the incorporation of stacking disorders, as the periodicity of the electron density along the stacking direction is unaffected by any variation in the stacking sequence of the layers. (2) The relative intensities of the high angle reflections vary marginally, and the variation is similar on the incorporation of both kinds of stacking faults. (3) The peaks in the mid-2θ region are affected the most by the incorporation of stacking disorders. The incorporation of 2M motifs causes the 201/-132 and 202/-133 peaks to widen at the base, with significant wings flaring on either side of a narrow peak maximum. The relative intensities of I003/I200/-131 inverses in ratio at g20% incidence of 2M stacking motifs in the 1M matrix (Figure 4). (4) The incorporation of 2O motifs causes the symmetric and selective broadening of the 201/-132 and 202/-133 reflections (Figure 5). In keeping with the observations of Verma and Krishna,5 stacking disorders conserve the symmetry elements along the stacking direction. Incorporation of the 2M motifs retains the Laue symmetry at 2/m, the same as that of the ordered crystal. Incorporation of the orthorhombic motifs generates the mmm Laue symmetry. Other disorders such as turbostraticity or interstratification destroy the crystal symmetry, and the DIFFaX computed symmetry is -1 (see the Supporting Information SI.5). We compare in Figure 6a the observed pattern in the 30-60° 2θ region with the DIFFaX simulated pattern corresponding to the 1M polytype. While the 2θ values of the calculated and observed patterns match satisfactorily, there is a large mismatch in the relative intensities, especially of the 202/-133 reflection. This mismatch could be corrected for by the incorporation of 12% orthorhombic motifs in the matrix of the 1M polytype (see Figure 6b). However, the doublet appearing in the 35-37° 2θ range shows a poor match on the incorporation of stacking disorders. This is because the low angle feature in the doublet does not belong to the hkl family of reflections.

Wyckoff position/interlayer O 2d, 4h(a), 4h(b) 2d, 4h(a), 4h(b) 4h(a), 4h(b)

Thermal Behavior of the Li-Al-CO32- LDH. The LiAl-CO32- LDH decomposes completely in two steps (280 and 325 °C) (see the Supporting Information SI.4). The first low temperature mass loss corresponds to the loss of interlayer water. The high temperature loss corresponds to the loss of interlayer CO32- and the simultaneous dehydroxylation of the layers. We had observed that, in the Li-Al-X- (X ) Br, Cl) LDHs, the loss of interlayer water opens up new sites for occupation by X- ions. The Cl- ions move to a new position of a lower site degeneracy.21 On the other hand, in the case of the Mg-Al-CO32- LDH, the layers undergo a translation, which converts the trigonal prismatic sites, hitherto occupied by H2O, into octahedral sites.30 This transformation provides for a close packing of the layers once the interlayer atom density falls on account of dehydration. To see the nature of changes taking place upon heating, a variable temperature PXRD study of the Li-Al-CO32- LDH was carried out, and the results are given in Figure 7. On heating the sample to 125 °C, the intensity of the 201/-132 reflection increases relative to the 202/-133 reflection. This pattern was fit by the incorporation of 15% 2O faults and 5% turbostraticity in the 1M structure (Figure 8a). This fit was obtained after accounting for a certain degree of dehydration as shown by a slight shift of the basal reflections to higher 2θ values. The interlayer composition was adjusted as given in Table 3 to account for this dehydration. These changes are further accentuated at 175 °C, and the corresponding pattern was simulated by the incorporation of 15% 2O faults and 10% turbostraticity (Figure 8b). This fit was obtained after removal of all atoms from the 2d positions and with the 4h(a) and 4h(b) positions having a site occupancy of 0.5 each. There is also the progressive growth of the 110 reflection relative to the 002 reflection. At 200 °C (Figure 7), the high angle reflections merge into a single peak with a “sawtooth” line shape, indicating a fully turbostratically disordered phase. The 001 reflection is broadened, and the 002 reflection is extinguished. The 110 reflection remains sharp and grows in intensity. No attempt was made to simulate this pattern. However, the various features are representative of the following changes: (1) The onset of decomposition causes the layers to collapse, and the crystallite size measured along the c-crystallographic direction decreases, leading to the broadening of the 001 reflection. (2) The loss of interlayer atoms effectively brings down the intensity of the 002 reflection relative to the 001 reflection as explained elsewhere.19,30 (3) The sharp 110 reflection shows that translational order is conserved in the ab-plane. This feature is expected of topotactic transformations, where major structural elements along select directions are conserved. Above 200 °C, there is a complete extinction of the basal reflections, showing the collapse of the layered structure and emergence of the oxide residue. The oxide residue is poorly ordered, and reflections due to LiAl5O8 and β-LiAlO2 are seen at 600 °C.

Structure of the Li-Al-CO32- LDH Conclusions The carbonate containing LDH of Li with Al crystallizes in the monoclinic crystal system with a layer stacking sequence corresponding to the 1M polytypes. However, there is a considerable degree of stacking disorder corresponding to the intergrowth of motifs having the orthorhombic symmetry. On heating, the loss of interlayer matter does not substantially alter the nature of disorder up to 175 °C except for the introduction of a small amount of turbostratic disorder. At 200 °C and above, turbostratic disorder dominates as the layers randomly translate relative to one another due to the forces of friction and drag caused by the aggressive dehydration of the interlayer. Acknowledgment. The authors thank the Department of Science and Technology (DST), Government of India for financial support. P.V.K. is a recipient of the Ramanna Fellowship of the DST. S.B. and G.S.T. thank the University Grants Commission for the award of a Senior Research Fellowship (NET) and a Teacher Fellowship, respectively. Supporting Information Available: Typical DIFFaX input files, DIFFaX simulations of model structures, and TG data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Taylor, H. F. W. Miner. Mag. 1973, 39, 377. (2) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551. (3) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 558. (4) Drits, V. A.; Bookin, A. S. Crystal Stucture and X-ray Identification of Layered Double Hydroxides. In Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science: New York, 2001; pp 39-92. (5) Verma, A. R.; Krishna, P. Polymorphism and Polytypism in Crystals; John Wiley: New York, 1966.

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9515 (6) Lei, L.; Zhang, W.; Hu, M.; Zheng, H. J. Solid State Chem. 2006, 179, 3562. (7) Fogg, A. M.; Dunn, J. S.; Shyu, S.-G.; Cary, D. R.; O’Hare, D. Chem. Mater. 1999, 11, 1466. (8) Megaw, H. D. Z. Kristallogr. 1934, 87, 185. (9) Rothbauer, R.; Zigan, F.; O’Daniel, H. Z. Kristallogr. 1967, 125, 317. (10) Bosmans, H. J. Acta Crystallogr. 1970, B26, 649. (11) Hanschild, V. Z. Anorg. Allg. Chem. 1963, 324, 15. (12) Schoen, R.; Roberson, C. E. Am. Mineral. 1970, 55, 43. (13) Poeppelmeier, K. R.; Hwu, S.-J. Inorg. Chem. 1987, 26, 3297. (14) Fogg, A. M.; Freij, A. J.; Parkinson, G. M. Chem. Mater. 2002, 14, 232. (15) Besserguenev, A. V.; Fogg, A. M.; Francis, R. J.; Price, S. J.; O’Hare, D.; Isupov, V. P.; Tolochko, B. P. Chem. Mater. 1997, 9, 241. (16) Serna, C. J.; Rendon, J. L.; Iglesias, J. E. Clays Clay Miner. 1982, 30, 180. (17) Sissoko, I.; Iyagba, R.; Sahai, R.; Biloen, P. J. Solid State Chem. 1985, 60, 283. (18) Thiel, J. P.; Chiang, C. K.; Poeppelmeier, K. R. Chem. Mater. 1993, 5, 297. (19) Radha, A. V.; Shivakumara, C.; Kamath, P. V. Clays Clay Miner. 2005, 53, 521. (20) Ramesh, T. N.; Kamath, P. V.; Shivakumara, C. Acta Crystallogr. 2006, B62, 530. (21) Thomas, G. S.; Kamath, P. V.; Kannan, S. J. Phys. Chem. C 2007, 111, 18980. (22) Treacy, M. M. J.; Deem, M. W.; Newsam, J. M. Computer Code DIFFaX, version 1.807; NEC Research Institute, Inc.: Princeton, NJ, 2000. (23) Treacy, M. M. J.; Newsam, J. M.; Deem, M. W. Proc. R. Soc. London 1991, A433, 499. (24) Delmas, C.; Tessier, C. J. Electrochem. Soc. 1997, 7, 1439. (25) Tessier, C.; Haumesser, P. H.; Bernard, P.; Delmas, C. J. Electrochem. Soc. 1999, 146, 2059. (26) Ramesh, T. N.; Jayashree, R. S.; Kamath, P. V. Clays Clay Miner. 2003, 51, 570. (27) Viani, A.; Gualtieri, A. F.; Artioli, G. Am. Mineral. 2002, 87, 966. (28) Warren, B. E.; Bodenstein, P. Acta Crystallogr. 1966, 20, 602. (29) Bailey, S. W. Clays Clay Miner. 1988, 36, 193. (30) Thomas, G. S.; Radha, A. V.; Kamath, P. V.; Kannan, S. J. Phys. Chem. B 2006, 110, 12365.

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