ARTICLE pubs.acs.org/crystal
Polytype Selection by Intercalated Anions: Design and Synthesis of the 3R2 Polytype of the Layered Double Hydroxide of Zn and Al Published as part of a virtual special issue on Structural Chemistry in India: Emerging Themes S. Radha, S. V. Prasanna, and P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India
bS Supporting Information ABSTRACT: The layered double hydroxides are obtained by the stacking of metal hydroxide layers one above another. The stacking sequence is determined by the molecular symmetry of the intercalated anion. Anions select for those stacking sequences which provide interlayer sites having a local symmetry compatible with their own molecular symmetry. Oxoanions, XO3n (X = S, I), are unique in that their molecular symmetry, C3v, is compatible with two different polytypes of rhombohedral symmetry. The SO32 ion selects for the more ubiquitous 3R1 polytype, whereas the IO3 ion selects for the much rarer 3R2 polytype. Structure refinement shows that neither anion departs significantly from the structure of the free species on intercalation. The higher charge-to-size ratio of the SO32 ion compared to the IO3 ion and its consequent stronger basicity is responsible for the nucleation of the 3R1 polytype, wherein the trigonal prismatic interlayer sites facilitate the superior hydrogen bonding capacity of the SO32 ion. The poorer basicity of the IO3 ion and its consequent poorer proclivity to form hydrogen bonds select for the 3R2 polytype wherein the IO3 ion is lodged in interlayer sites of octahedral symmetry.
’ INTRODUCTION Layered materials exhibit the phenomenon of polytypism owing to anisotropy in bonding.1 Among different polytypes of a given material, two of the three cell dimensions remain invariant, while the third is variable and depends on the stacking sequence of the layers. Polytype diversity among layered solids is limited by close packing considerations, when the interlayer is unoccupied. On the other hand, if the interlayer is occupied by guest species (cations, anions, or neutral species), a greater diversity of polytypes is theoretically possible and experimentally observed.2 Polytype selectivity among such solids depends on several factors such as the method of preparation, extent of hydration of the interlayer, symmetry and mode of coordination of the intercalated ion and charge and size of the intercalated species.3 Layered double hydroxides (LDHs) are a class of inorganic solids comprising a stacking of positively charged brucite-like layers having the composition [MII1xM0 xIII(OH)2]xþ (M = Mg, Ca, Zn, Co, Ni, Cu; M0 = Al, Cr, Fe). Anions, An, along with the water molecules are incorporated in the interlayer region for charge compensation.4 We abbreviate the molecular formula with the symbol [M-M0 -A]. The most commonly found anion in the naturally occurring LDHs is the carbonate ion. The carbonate-LDHs crystallize in the structure of the 3R1 polytype,5 which has the metal hydroxide stacking sequence AC CB BA AC----. Here the symbol AC or more completely AbC stands for a metal hydroxide layer (Figure 1). Upper case symbols A and C represent hydroxyl r 2011 American Chemical Society
positions and b represents the octahedral interstitial site, in which the metal ion is located.6 The layers CB and BA are obtained by successive translations of the AC layer by the vector (2/3, 1/3, z). This stacking yields a three-layer cell of rhombohedral symmetry, R. A crystal of a different structure can be envisaged, in which the metal hydroxide layer AC, is successively translated by the vector (1/3, 2/3, z). Such a translation would lead to a metal hydroxide stacking sequence AC BA CB AC----, designated by the symbol 3R2 (Figure 1). There is only one difference between the two polytypes: in the 3R1 polytype the OH groups lining the interlayer space are eclipsed with respect to one another while in the 3R2 polytype they are staggered. However, a LDH belonging to the 3R2 polytype has to date not been unequivocally characterized. The first attempt at explicitly synthesizing a 3R2 polytypic modification of the MgAl LDH was done by Newman and Jones7 in a modification of the oxide precursor route to LDHs reported by Rajamathi and co-workers.8 Further development of this theme by the Jones group was published in 2007 and later again in 2008, and their work suggested that the 3R2 structure could be realized by nucleating the LDH at a high temperature and CO32 starved conditions.9,10 Further attempts to synthesize a 3R2 polytype and examine its transformation to the 3R1 was reported by Budhysutanto and co-workers.11 However, the Received: December 23, 2010 Revised: April 9, 2011 Published: April 21, 2011 2287
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Figure 1. Schematic representation of the structures of 3R1 and 3R2 polytypes. Vertical lines demonstrate the eclipsed arrangement of the OH groups in the 3R1 polytype and staggered arrangement in the 3R2 polytype.
Table 1. Precipitation Conditions, Cell Parameters, and Compositions of Different LDH Samples precipitation conditions
cell parameters
LDH system
pH
T (°C)
a (Å)
c (Å)
[ZnAlSO3] LDH S1
10
60
3.079
23.605
LDH S2
9
60
3.068
33.05
LDH S3
9 (HT)a
110
3.069
8.774
LDH S4
8
60
3.07
8.88
LDH I1
8
60
3.075
29.95
LDH I2
8.5
60
3.07
9.9
LDH I3 LDH I4
9 10
60 60
3.069 3.07
9.8 9.9
LDH I5
anion exchange
90
3.072
29.95
[ZnAlIO3]
LDH system
a
Zn/Al
anion content
water content
approximate formula [Zn0.67Al0.33(OH)2][SO3]0.165 3 0.75H2O [Zn0.66Al0.34(OH)2][IO3]0.34 3 0.7H2O
LDH S1
2.1
0.164
0.75
LDH I5
1.92
0.34
0.7
Hydrothermally treated.
preparations in all these cases were not single phase and no structure refinements were attempted. Why is the 3R1 polytype greatly favored, especially among the CO32-LDHs? Taylor12 suggested that the molecular symmetry of the carbonate ion (D3h) matches the local symmetry of the interlayer sites in the 3R1 polytype (also D3h), on account of which, hydrogen bonding between the layer hydroxyl and interlayer carbonates is maximized. As a corollary of this argument, the 3R2 polytype, which comprises interlayer sites having octahedral symmetry, is not favored. It appears therefore that polytype selection is mediated by the molecular symmetry of the intercalated anion.
We then ask the question: what kinds of anions are most likely to nucleate the growth of the 3R2 polytype? In an earlier paper,13 we synthesized SO42 containing LDHs under a wide matrix of precipitation conditions. Implicit in this attempt was the understanding that the coordination symmetry of the intercalated SO42 ion, C3v on the basis of the application of the Halford’s rule,14 is a subset of both trigonal prismatic and octahedral interlayer site symmetries and hence should mediate the formation of polytypes containing either of these interlayer sites with equal ease. The formation of both 3R1 and 1H polytypes was found to be facile, under appropriate conditions. The 1H polytype has a stacking sequence of AC AC AC----, and like the 3R2 polytype, includes 2288
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only octahedral interlayer sites. SO42 containing mineral LDHs are also known to belong to 3R1 and 1H polytypes.15 In continuing our pursuit of the synthesis of new polytypes, we turned our attention to a new set of anions having the molecular symmetry, C3v. These are SO32, IO3, BrO3, and ClO3. In this paper, we report the successful synthesis of the IO3-LDH in the structure of the 3R2 polytype. The SO32-LDH however crystallizes in the structure of the 3R1 polytype. We refine the crystal structures of the two polytypes using the Rietveld technique. Other anions yielded compounds with extensive turbostratic disorder and are not discussed.
’ EXPERIMENTAL SECTION The [ZnAlA] LDHs (A = SO32, IO3) were synthesized by coprecipitation at different constant pH values and by anion exchange reactions using [ZnAlNO3] LDH used as a precursor at different temperatures. In a typical coprecipitation, 50 mL of a mixed metal nitrate ([Zn]/[Al] = 2) salt solution was added dropwise to a reaction vessel containing a solution of sodium salt of the required anion, taken 10 times in excess of the stoichiometric requirement. A constant pH was maintained during the preparation by simultaneous addition of 0.5 M NaOH using a Metrohm model 718 STAT Titrino, operating in the pH STAT mode. The temperature was kept constant at 60 °C and N2 gas was bubbled throughout the experiment. The resulting slurry was then divided into two portions. One part was further aged at 60 °C (16 h) while the other part was subjected to hydrothermal treatment (110 °C, 15 h) in a Teflon lined autoclave (50% filling). The precipitate obtained was then separated out by centrifugation, washed with decarbonated water several times and finally with acetone, and dried in a desiccator. The preparations were carried out at constant pH values of 8, 8.5, 9, and 10 for all the anions. For a preparation by anion exchange, the [ZnAlNO3] LDH precursor (0.5 g) is suspended in a solution containing the sodium salt of the incoming anion, taken 20 times in excess of the stoichiometric requirement and stirred for a period of 40 h. In separate batches, the exchange reactions were carried out at the ambient temperature (2528 °C) and at 90 °C. The precipitate was then washed several times with decarbonated water and finally with acetone and dried in a desiccator. The preparative conditions used for the synthesis of different LDH samples are summarized in Table 1.
’ CHARACTERIZATION All samples were characterized by powder X-ray diffraction using a Bruker D8 Advance powder diffractometer (source Cu KR radiation, λ = 1.5418 Å). The data were collected with a step size = 0.02°2θ and counting time = 1.2 s/step. The cell parameters evaluated as c = l d00l, a = 2 d110 are listed in Table 1. Many of the samples listed in Table 1 are faulted and thereby not suited for structure refinement. Two samples, S1 and I5, were ordered and selected for structure refinement. Sample S1 is the sulfite containing LDH obtained at pH = 10 and sample I5 is the iodate containing LDH obtained by anion exchange at 90 °C. For the structure refinement by the Rietveld method, data were collected at a step size of 0.02°2θ with a counting time of 10 s/step. IR spectra of the samples were recorded using Bruker model Alpha-P IR spectrophotometer (diamond ATR cell, 4 cm1 resolution, 4004000 cm1). The Zn and Al content of S1 and I5 were determined by atomic absorption spectroscopy using Varian model AA240 atomic absorption spectrometer. The anion content of the LDHs was determined by ion chromatography using a Metrohm model
Figure 2. A Rietveld fit of the PXRD pattern of the [ZnAlSO3] LDH (Sample S1).
861 Advanced Compact Ion chromatograph fitted with a Metrosep SUP 5 150 column. For determining the anion content, a preweighed amount of the LDH was dissolved in a minimum amount of HCl (sulfite-LDH) or HNO3 (iodate-LDH) and injected into the column. Standard solutions of the corresponding anion were used for calibrating the chromatograph response. The intercalated water content was determined by thermogravimetry using a Metler Toledo TGA/SDTA 851e, stare 7.01 system (30 to 750 °C, heating rate of 5 °C min1, flowing N2). The complete chemical composition of these two LDHs is also given in Table 1. The IR spectra and the thermogravimetric analysis (TGA) data of these samples are given in the Supporting Information, SI1 and SI2. The observed Zn/Al ratio is close to 2, the nominal composition. The observed anion content matches that expected of the Zn/Al ratio.
’ STRUCTURE REFINEMENT The powder X-ray diffraction (PXRD) patterns were first indexed using the program PROZSKI16 and the figures of merit (FOM) were determined. The structures were then refined by the Rietveld method using the GSAS software.17 The refinement of the structure of the 3R1 polytype was carried out using the structure of the [ZnAlSO4] LDH (CC No. 91859) as the model. For refinement of the structure of the 3R2 polytype, the structure of the O3 modification of LiCoO2 (CC No. 155284) was used as a model. In both cases, the coordinates corresponding to the metal hydroxide/oxide layer were obtained from the corresponding model structures and used in the first phase of the refinement. The Zn/Al ratio was taken in accordance with the values obtained from AAS measurements and refined further. The coordinates of the anions were determined using the difference Fourier technique inbuilt in the GSAS software in further cycles of refinement. ’ RESULTS AND DISCUSSION The PXRD patterns of different polytypes having the same crystal symmetry and the same number of layers per unit cell exhibit reflections at identical Bragg angles. PXRD patterns corresponding to such polytypes differ only in the relative intensities of select reflections. Often these differences are very slight leading to ambiguities in polytype identification.18 Both 2289
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Table 2. Observed and Calculated d-Spacings of [ZnAlA]0.33 (A = SO32, IO3) LDHs LDH S1
LDH S3
LDH I5
a = 3.079(9) Å; c = 23.60(7) Å
a = 3.069(1) Å; c = 8.774(1) Å
a = 3.072(9) Å; c = 29.954(1) Å
FM = 44
FM = 22.2
FM = 22.3
d (obs) (Å)
d (calc) (Å)
hkl
d (obs) (Å)
d (calc) (Å)
hkl
d (obs) (Å)
d (calc) (Å)
hkl
7.85
7.81
003
8.77
8.77
001
9.96
9.98
003
3.89
3.90
006
4.38
4.38
002
4.99
4.99
006
2.64
2.64
101
2.65
2.65
100
3.33
3.32
009
2.59
2.60
012
2.54
2.54
101
2.64
2.65
101
2.42
2.42
104
2.27
2.27
102
2.50
2.50
104
2.31 2.09
2.31 2.08
015 107
1.96 1.70
1.96 1.69
103 104
2.26 1.99
2.25 1.99
107 0015
1.97
1.97
018
1.53
1.53
110
1.53
1.53
111
1.76
1.76
1010
1.51
1.51
111
1.51
1.51
113
1.66
1.66
1011
1.54
1.54
110
1.51
1.51
113
1.49
1.49
1013
1.43
1.43
116
Table 3. Results of the Rietveld Refinement of LDH S1 molecular formula
[Zn2Al(OH)6][SO3]0.5 3 2.25H2O
crystal system
rhombohedral
space group
R3m
cell parameters/ Å
a = 3.0762(7); c = 23.420(5)
volume/ Å3
191.94 (1)
data points
4750
reflections fitted
44
Rwp Rp
0.088 0.065
R(F2)
0.089
Rexp
0.029
reduced χ2
8.82
3R1 and 3R2 polytypes generate reflections at identical positions in their PXRD patterns and can be distinguished only by the differences in relative intensities of the h0l/0kl reflections. While the 3R1 polytype is characterized by the presence of strong 0kl reflections, the 3R2 polytype exhibits intense h0l reflections in the diffraction pattern.6 The basal reflections on the other hand depend only on the size of the intercalated anion and remain invariant for different polytypes containing anions of similar size. The PXRD pattern of the [ZnAlSO3] LDH prepared at pH 10 (Sample S1 in Table 1) is shown in Figure 2. It contains many sharp Bragg reflections indicating that the material is highly crystalline. The pattern could be indexed to a hexagonal cell (a = 3.079 Å; c = 23.60 Å) with an acceptable figure of merit (FM = 44.6). The d-spacings and the corresponding indices are given in Table 2. The reflection conditions 00l (l = 3n) and h þ k þ l = 3n indicate that the crystal symmetry is rhombohedral.19 Further, the observation of Bragg peaks due to the 01l (l = 2, 5, 8) reflections and the extinction of peaks due to the 10l (l = 1, 4, 7) reflections shows that the crystal structure is of 3R1 polytype.6 The structure of the LDH-S1 was then refined by the Rietveld method using the data in Figure 2. As no structure model of the SO32-LDHs is reported, the structure of the [ZnAlSO4]
Table 4. Refined Atomic Position Parameters of the LDH S1 x
y
z
3a
0.000
0.000
0.000
0.3391
3a 6c
0.000 0.3333
0.000 0.6666
0.000 0.3728(5)
0.6609 1.000
S
6c
0.000
0.000
0.1645(5)
0.084
O2
18h
0.2741(2)
0.7259(2)
0.1495(7)
0.208
atom
Wyckoff Position
Zn Al O1
) distances (Å
SOF
angles (deg)
(Zn,Al)O1
2.0028(4)
O1ZnO1
O1O1
2.566(4)
O1ZnO1
100.346 (1) 79.654(1)
SO2
1.5594(4)
O2SO2
110.449(7)
O2O1
2.9641(3)
LDH (CC No. 91859) was used as a model. In the first phase of the refinement procedure, a partial structure excluding the interlayer atoms was used. The goodness of fit parameters obtained at this stage of refinement are given in Supporting Information SI3. The fit could not be improved any further. At this stage of the refinement, the difference Fourier map was computed to get the residual electron density. A maximum in the residual electron density was observed at the 6c position (0.000, 0.000, 0.164). The S atom was included in this position and its occupancy was refined. After further cycles of refinement, difference Fourier analysis showed another maximum in the residual electron density at the 18h site (0.274, 0.725, 0.149). The O atom was included here in subsequent cycles of the refinement. The final refinement after including all the interlayer atoms is shown in Figure 2, and the fit obtained is satisfactory with a featureless difference profile. The various refined structural and nonstructural parameters are listed in Table 3. The position parameters obtained after the refinement along with the bond lengths and bond angles are listed in Table 4. The S and O atoms of the sulfite ion occupy the 6c and 18h sites, respectively, similar to the S, O atoms in the sulfate-LDH structure. The SO bond 2290
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Figure 3. (a) Structure of the [ZnAlSO3] LDH (Sample S1). (b) The structure viewed down the c-axis showing the S atom in the prismatic interlayer site. The upper (U) and lower (L) hydroxyl groups are shown by symbols of different size for clarity.
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Figure 5. A Rietveld fit of the PXRD pattern of the [ZnAlIO3] LDH (Sample I5).
Table 5. Results of the Rietveld Refinement of LDH I5
Figure 4. PXRD patterns of the [ZnAlSO3] LDHs prepared at (a) pH 8 (b) pH 9 (c) (b) hydrothermally treated at 110 °C.
distances and the OSO bond angles deviate only slightly from that of the free SO32 ion. The plot of the structure obtained is shown in Figure 3. The SO32 ion occupies the trigonal prismatic position in the interlayer. The PXRD patterns of the SO32-LDHs prepared under other conditions (samples S24) are shown in Figure 4. The LDH S4 is turbostratically disordered and shows no improvement in ordering upon hydrothermal treatment. The LDH prepared at pH 9 (sample S2) is also disordered, but shows reasonable ordering after the hydrothermal treatment (sample S3). While the as-prepared sample has a basal spacing of 11 Å, the hydrothermally treated sample exhibits reduced basal spacing of 8.7 Å and is indexed to 1H polytype (FM = 22.2). The sulfite ion being pyramidal (molecular symmetry C3v) occupies both trigonal prismatic (3R1 polytype) and the octahedral (1H polytype) interlayer sites very similar to the sulfate ion in LDHs.13 It also exhibits different basal spacings owing to different degrees of hydration of the interlayer and exchanges the interlayer water with the vapor phase. While we could demonstrate that the SO32 could select for polytypes with prismatic and octahedral interlayer sites owing to symmetry
molecular formula
[Zn2Al(OH)6][IO3] 3 2.1H2O
crystal system
rhombohedral
space group
R3m
cell parameters/ Å
a = 3.0724(2); c = 30.112(2)
volume/ Å3
236.64
data points reflections fitted
4750 44
Rwp
0.1253
Rp
0.094
R(F2)
0.134
Rexp
0.050
reduced χ2
6.022
compatibility, our aim to synthesize a new polytype, 3R2, was unsuccessful. The iodate intercalated LDHs were synthesized both by coprecipitation at different constant pH values and by anion exchange at different temperatures. The LDH prepared at pH 8 (Sample-I1) and the LDH obtained by exchange at 90 °C (Sample-I5) show similar PXRD patterns with sharp Bragg reflections (Figure 5). The samples obtained under other conditions were poorly crystalline making the task of polytype identification difficult. Indexing the Bragg peaks of sample I5 shows that it is a three-layered rhombohedral polytype (Table 2, FM = 19). In addition to the indexed peaks, a few minor reflections were seen in the 2532°2θ region due to unassigned impurities. This region was excluded in further analysis (see Supporting Information SI4 for the complete pattern). The presence of strong peaks due to 10l (l = 1, 4, 7) reflections points to the 3R2 polytype.6 The synthesis of a 3R2 polytype is hitherto not reported in the LDH literature except for the few reports where the OH is the anion.7,9 In order to provide unequivocal evidence that the sample I5 belongs to 3R2 polytype, we attempted a structure refinement of the LDH. In contrast to other more commonly found anions such as CO32, Cl and NO3, IO3 is a strong scatterer. Consequently, its position parameters can be refined with lesser uncertainty than that of light atoms. The reflection conditions obtained by indexing the pattern (h þ k þ l = 3n; l = 3n) are consistent with five spacegroups:19 R3m, R3m, R3, R3, and R32. 2291
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Table 6. Refined Atomic Position Parameters of the LDH I5 Wyckoff Position
x
y
Zn
3b
0.000
0.000
0.50
Al
3b
0.000
0.000
0.50
0.341
O1
6c
0.6667
0.3333
0.1456 (1)
1.000
I
3a
0.000
0.000
0.000
0.335
O2
6c
0.6667
0.3333
0.0228(4)
0.33
O3
6c
0.6667
0.3333
0.05621(1)
0.36
atom
) distances (Å
z
SOF 0.659
angles (deg)
(Zn,Al)-O1 O1O1
1.944(1) 2.566(4)
O1ZnO1 O1ZnO1
104.385 (3) 75.606(3)
IO2
1.902(1)
O2IO2
102.418(5)
O2O1
3.001(3)
O3O1
2.692(1)
A Lebail fit was carried out in all the five spacegroups and the goodness of fit parameters were compared. R3 and R32 were ruled out as the goodness of fit parameters obtained by Lebail refinement were high. Since the refinement cannot be done by direct methods due to the poor crystallinity of the sample, we looked for model structures belonging to the other three space groups R3m, R3, and R3m. No structure models of the 3R2 polytype could be found among the LDHs. Hence we decided to look for a model among structurally related materials where the layer has a similar composition as the LDHs. The basic building block of the LDHs is the metal hydroxide layer, which is obtained by the edge sharing of [MX6]n (M = Zn2þ, Al3þ; X = OH) octahedra yielding a layer composition [MX2]. The LDH crystal is generated by the stacking of these layers. The structural motif corresponding to the [MX2] layer is widely observed across a very diverse series of inorganic solids. In an earlier paper,20 we defined the [MX2] layer as a “structural synthon”. A structural synthon represents a specific packing of atoms that repeats itself in many systems. One of the systems that comprises a stacking of [MX2] layers is the LiCoO2. LiCoO2 has a layered structure, wherein the edge sharing CoO6 octahedra form [CoO2] layers with Liþ ions distributed in the interlayer.21 We chose the structure of the O3 polytype of LiCoO2 which belongs to the spacegroup R3m as the model. The O3 nomenclature among LiCoO2 stands for the three-layered periodicity with octahedral interlayer sites.22 The 3R2 polytype of the LDHs is also expected to have a three-layered periodicity with octahedral interlayer sites.18 The partial structure based on the O3 polytype was used for the first few cycles of refinement of the LDH sample I5, and the interlayer atom positions were found by difference Fourier analysis as in the earlier case. The difference Fourier computed after the early cycles of refinement showed a significant residual electron density at the 3a position (0.000, 0.000, 0.000) and the I atom was placed there. Further cycles of refinement showed residual electron densities at 6c positions (0.667, 0.333, 0.0228 and 0.667, 0.333, 0.0562) where the O atom of the IO3 and the intercalated water were placed, respectively. The fit obtained after the final refinement, excluding the impurity peaks is satisfactory (Figure 5). Given the fact that the peaks in the PXRD pattern are broad the goodness of fit parameters obtained are acceptable for a powder pattern (Table 5). The atomic coordinates, bond lengths, and bond angles obtained after the final refinement are listed in Table 6, and the plot of the structure
Figure 6. (a) Structure of the [ZnAlIO3] LDH obtained from the refinement. (b) The structure viewed down the c-axis showing the I atom in the octahedral interlayer site. The upper (U) and lower (L) hydroxyl groups are shown by symbols of different colors for clarity.
obtained is shown in Figure 6. In this structure, the layers are stacked one above the other generating octahedral interlayer sites. The I atom of the IO3 ion is positioned exactly at the center of this octahedral site (Figure 6b) in a manner similar to the way the C atom is positioned at the center of the prismatic site in the 3R1 structure of the CO32-LDH.23 Similarly, the O atoms of the IO3 ion are located just above or below the layer hydroxyls. The IO bond distance and the OIO bond angles are comparable with that of the free IO3 ion. The interatomic distances obtained between the hydroxyl ion of the layer and the interlayer water atoms are in the range of hydrogen bonding distance. Using IO3 as an anion we could thus demonstrate anion mediated polytype selection to realize the synthesis of a new polytype. The poor crystallinity observed in the case of IO3-LDHs as opposed to the highly crystalline SO32-LDHs can be attributed to the charge/size ratio of the anion. The SO32 ion has a higher charge and is comparatively smaller in size, while the IO3 has a lower charge/size ratio. This renders SO32 to be more basic and the IO3 a nonbasic anion making the latter a weak hydrogen bonding species. This is reflected in the bond distances obtained from the refinement of the two LDHs: OSOH = 2.98 Å; OIOH = 3.6 Å (OS: O of the SO32 group, OI: O of the IO3 group and OH: O of the metal hydroxide layer). The weak interaction among the layer and the interlayer hence leads to the poor crystallinity of the IO3-LDHs Further while most of the laboratory-synthesized LDHs tend to crystallize in a structure with prismatic interlayer site, IO3 seems to prefer octahedral interlayer sites generating a 3R2 polytype. For a given “a” parameter the size of the octahedral interlayer site is smaller than the prismatic site. Thus, IO3 being a weak H-bonding species would destabilize the LDH if located in the prismatic site. The octahedral site offers a greater packing efficiency and stabilizes the LDH. The question now arises as to why the IO3 ion does not mediate the growth of the 1H polytype where the interlayer site is octahedral similar to that of 3R2? According to Verma and Krishna,1 the structure with a 2292
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Crystal Growth & Design higher symmetry is more stable compared to the ones with lower symmetry. The 1H polytype belongs to the spacegroup P3 and the 3R2 belongs to R3m space group which is of higher symmetry and hence is more favorable. Recent DFT calculations also point to the greater stability of the 3R structure compared to the 1H.24 On exchanging the IO3 ion with CO32 and Cl, the product LDHs (details of which are not shown here) were found to belong to 3R1 polytype indicating that the IO3 ion is unique in mediating the formation of 3R2 polytype.
’ CONCLUSION In conclusion, anions of C3v symmetry are shown to succeed in nucleating new polytype structures. Two factors play a crucial role in polytype selection: (i) the molecular symmetry of the anion and (ii) the ability of the anion to form hydrogen bonds with metal hydroxide layer. Anions that form strong hydrogen bonds promote the 3R1 polytype with trigonal prismatic interlayer sites, while others promote the 3R2 polytype with octahedral interlayer sites. ’ ASSOCIATED CONTENT
bS
Supporting Information. IR spectra and TG-DTG curves of LDH samples, S1 & I5, goodness of fit parameters obtained at various stages of refinement of LDH S1 and PXRD pattern of the LDH I5. This information is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
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’ ACKNOWLEDGMENT Authors thank the Department of Science and Technology (DST), Government of India (GOI), for financial support. S.R. and S.V.P. are grateful for the award of Senior Research Fellowships of the Centre for Scientific and Industrial Research, GOI. P. V.K. is a recipient of the Ramanna Fellowship of the DST. ’ REFERENCES (1) Verma, A. R.; Krishna, P. Polymorphism and Polytypism in Crystals; John Wiley: New York, 1966. (2) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 558. (3) Khaldi, M.; De. Roy, A.; Chaouch, M.; Besse, J. P. J. Solid State Chem. 1997, 130, 66. (4) Trifiro, F.; Vaccari, A. Comprehensive Supramolecular Chemistry; Atwood, J. L., Macnicol, D. D., Davies, J. E. D., Vogtle, F., Eds.; Pergamon: Oxford, 1996; Vol. 7; p 251. (5) Allmann, R.; Jepsen, H. P. Neues Jahrb. Mineral., Monatsh. 1969, 544. (6) Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551. (7) Newman, S. P.; Jones, W.; O’Connor, P.; Stamires, D. N. J. Mater. Chem. 2002, 12, 153. (8) Rajamathi, M.; Nataraja, G. D.; Ananthamurthy, S.; Kamath, P. V. J. Mater. Chem. 2000, 10, 2754. (9) Mitchell, S.; Biswick, T.; Jones, W.; Williams, G.; O' Hare, D. Green Chem. 2007, 9, 373. (10) Mitchell, S.; Baxendale, I. R.; Jones, W. Green Chem. 2008, 10, 629. (11) Budhysutanto, W. N.; Agterveld, D. V.; Schomaker, E.; Talma, A. G.; Kramer, H. J. M. Appl. Clay Sci. 2009, 48, 208. 2293
dx.doi.org/10.1021/cg101707n |Cryst. Growth Des. 2011, 11, 2287–2293
