Synthon Approach to Structure Models for the Bayerite-Derived

Apr 13, 2017 - The Br– ion intercalated layered double hydroxide (LDH) of Li and Al obtained from the bayerite-Al(OH)3 precursor crystallizes in a s...
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Synthon Approach to Structure Models for the Bayerite-Derived Layered Double Hydroxides of Li and Al Supreeth Nagendran and P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India S Supporting Information *

ABSTRACT: The Br− ion intercalated layered double hydroxide (LDH) of Li and Al obtained from the bayerite-Al(OH)3 precursor crystallizes in a structure different from that of the gibbsite-Al(OH)3 derived counterpart. Additionally, it undergoes temperature- and humidity-induced reversible interpolytype transformations. The dehydrated LDH (T ≈ 120 °C) adopts a structure of hexagonal symmetry (space group P3̅1m) and comprises a stacking of the metal hydroxide layers arranged one above another. On cooling and rehydration, the LDH adopts a structure of monoclinic symmetry with a stepwise increase in the stacking angle, β. Using the structural synthon approach, based on the systematic elimination of the principal symmetry elements of the hexagonal crystal, structure models were generated for each of the two hydration steps (relative humidity ∼50%, >70%) and the structures refined (space group C2/m). The refined structures show that the interpolytype transitions are a result of rigid translations of successive metal hydroxide layers relative to one another by translation vectors (1/6, 0, 1) and (1/3, 0, 1), respectively.

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INTRODUCTION The layered double hydroxide (LDH) of Li and Al, having the general formula [LiAl2(OH)6](An−)1/n·yH2O (A = Cl−, Br−, CO32−, NO3−, SO42−) is generally prepared by the imbibition of the corresponding lithium salt LinA into Al(OH)3.1 Al(OH)3 crystallizes in a layered structure, wherein the layer composition is [Al2□(OH)6] (□ = octahedral cation vacancy).2 When Al(OH)3 is soaked in a solution of a lithium salt, LiBr in this work, the Li+ ion enters the vacancy through the triangular face of the [□(OH)6] octahedron, by a mechanism known as “diadochy”.3 The resulting metal hydroxide layer acquires the composition [Al2Li(OH)6]+. Simultaneous with this, the Br− ion is intercalated into the interlayer gallery to restore charge neutrality. The totality of the reaction is topotactic in nature, and no bond breaking or bond making is envisaged in this imbibition process. Al(OH)3 crystallizes in four different polymorphic modifications, of which two, gibbsite4 and bayerite,5 are common. These differ from one another in the interlayer relationship. If a single metal hydroxide layer is represented by the symbol P and its mirror image by the symbol P̅, the stacking sequence in a gibbsite crystal is PP̅ P··· and the stacking sequence in a bayerite crystal is PPP···. Gibbsite is therefore a two-layer polytype (a = 8.676 Å, b = 5.070 Å, c = 9.721 Å, β = 94.57°, space group = P121/n1), whereas bayerite is a one-layer polytype (a = 5.062 Å, b = 8.672 Å, c = 4.713 Å, β = 90.27°, space group P121/a1). Of these, gibbsite is the thermodynamically most stable form6 and is used extensively for the synthesis of Li-Al LDHs.7,8 When a gibbsite precursor is used in the imbibition reaction, the resulting LDH preserves the PP̅P··· stacking sequence and © 2017 American Chemical Society

crystallizes in the structure of a two-layer hexagonal polytype (a = 5.10 Å, c = 14.30 Å, space group P63/mcm).7−9 The question naturally arises: would the use of a bayerite precursor yield a LDH of a different structure? Given the use of [Li-Al] LDHs in (i) the removal of toxic anions such as arsenate10 and fluoride11 (ii) the shape-selective intercalation of anions12 (iii) the storage and triggered release of drugs, agrochemicals, and vitamins13 this question has added significance in view of the wide range of applications. An early attempt to synthesize LDHs by the imbibition of LiCl into bayerite was made by Fogg and co-workers.14 They assigned the LDH to a three-layered polytype of rhombohedral symmetry. However, the reflection conditions did not obey the requirement of rhombohedral symmetry, and later work reassigned the structure to a one layer polytype of monoclinic symmetry (C121/m1).15 A monoclinic structure model had already been proposed by Poeppelmeier,16 although the LDH in that instance was prepared by a gas−solid interaction. It is now well established that the products obtained from the bayerite precursor crystallize in structures different from those obtained from the gibbsite precursor.15 However, structure refinement has been reported only for the b-[Li-Al-Cl] LDH (b = bayerite derived).15 In this work, we report the synthesis, structure refinement, and temperature-/humidity-induced Received: January 14, 2017 Published: April 13, 2017 5026

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Inorganic Chemistry Table 1. Results of Compositional Analysis of the as-Prepared [Li-Al-Br] LDH (Phase I)a

a

Li+ (wt %)

Al3+ (wt %)

Br− (wt %)

mass loss from TGA (%)

approx composition of the sample

2.5 (2.34)

22.18 (21.16)

31.102 (29.45)

42

[Li0.43Al(OH)3][Br]0.47·0.5H2O

Values given in parentheses are calculated based on the approximate composition given in the last column. LDH phases, structure models were constructed in this work by a twostep procedure. Step I: the stacking motif of the metal hydroxide layers was first established using the DIFFaX code.18 Within the DIFFaX formalism, a single metal layer is completely described including all the symmetryrelated atoms. The crystal is then constructed by stacking metal hydroxide layers one above another using a stacking vector. The diffraction intensity is computed for an infinitely extended stacking. The observed Bragg reflections are broadened using a Lorentzian function of fwhm = 0.2° 2θ. The resulting profile is compared with the observed pattern. In each case, the DIFFaX code also computes the Laue symmetry. The stacking vector is systematically varied until the simulated pattern matches the observed pattern. Once a good visual match is established, the partial structure is evident and the corresponding stacking vector defines the stacking motif and the polytype. Step II: the partial structure model was used as input in the FOX code (free objects for crystallography).19 The Br− ion and O atom of the H2O molecule were introduced into the interlayer. FOX code enabled the Br− ion and the O atom to diffuse in the interlayer gallery by random successive translations. After each translation, the computed pattern was compared with the observed pattern. A Monte Carlo procedure was employed to determine if the translation is acceptable using the various R values as the cost function. At each step, the structure was visualized in real time. The Br− ion and O atom were allowed to sample the entire interlayer space without the imposition of any constraints. At the end of this optimization procedure carried out in direct space, a structure model was obtained. Finally the structure model, now comprising the metal hydroxide layer and the interlayer species, was used as input data in the FULLPROF20 suite to complete the Rietveld refinement in the conventional way in the reciprocal lattice.

structural transformations observed in the bayerite-derived [LiAl-Br] LDH. Br− ion is interesting for many reasons. (1) Unlike the Cl− ion, the Br− ion does not participate in hydrogen bonding with either the metal hydroxide layer or with the intercalated water molecules. It therefore occupies positions in the interlayer gallery not accessed by other anions, yielding interesting interlayer structures.17 (2) Given its low hydration enthalpy, in comparison to the Cl− ion, the Br− ion changes its position in the interlayer gallery by facile and reversible diffusion as a function of ambient humidity. For instance, the g-[Li-Al-Br] LDH (g = gibbsite derived) shows a structural change without any variation in the basal spacing.9 (3) Br− being a heavy scatterer, it leaves its unmistakable stamp on the powder patterns, making the structure refinement relatively facile.

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EXPERIMENTAL SECTION

Bayerite was prepared by ammonia precipitation following a literature procedure.1 The [Li-Al-Br] LDH was prepared by soaking 0.5 g of bayerite in 10 mL of ∼10 M LiBr solution followed by hydrothermal treatment in a Teflon-lined autoclave at 125 °C for 24 h. The sample was centrifuged, washed with type II water (specific resistance 15 MΩ cm, Millipore Academic water purification system), and dried at 65 °C. A wet chemical analysis of the as-prepared product LDH was carried out. The Al content was estimated by gravimetry. A preweighed quantity (∼0.05 g) of the sample was dissolved in concentrated HCl. Al(OH)3 was precipitated by adding 25% NH3 as described in Vogel’s Textbook of Quantitative Inorganic Analysis. The precipitate was filtered, ignited, and weighed as Al2O3. The Li content was estimated by flame photometry and Br− content by ion chromatography (Metrohm Model 861 Advanced Compact Ion Chromatograph fitted to a Metrosep SUP5 150 column). The idealized composition of the I−III LDH system corresponds to [Li]/[Al] = 0.5 (observed 0.43) and [Li]/ [Br] = 1 (observed 0.92). Thermogravimetric analysis (Mettler Toledo TG/SDTA Model 851e system, 30−900 °C, heating rate 5 °C min−1, flowing air) (Figure S1 in the Supporting Information) shows that the as-prepared LDH loses ∼7% of its mass below 200 °C, and this mass loss is ascribed to the intercalated water content. This corresponds to 0.54 mol of water per empirical formula unit (Table 1). The composition of the LDH reported here is closer to the ideal value in comparison to that reported earlier ([Li]/[Al] = 0.33)15 for the [Li-AlCl] LDH. Other authors have reported [Li]/[Al] = 0.577 and did not comment on the layer charge, although this value is conceptually incompatible with the structure model. Given the uncertainties in the measurements, and the proximity of the measured to the ideal composition, we approximate the composition observed here to the ideal value, resulting in the molecular formula LiAl2(OH)6Br·H2O. The as-prepared sample was characterized by powder X-ray diffraction (PXRD) using a Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5418 Å) operated in reflection geometry. In situ variable-temperature and -humidity measurements were carried out using a Anton Paar CHC plus+ humidity chamber attachment. For Rietveld refinement, the data were recorded over a 5−90° 2θ range (step size of 0.02° 2θ, counting time 10 s step−1). The success of a refinement is critically dependent upon the choice of a suitable structure model. On the basis of the invariance of the ionocovalently bonded metal hydroxide layer [LiAl2(OH)6]+ in all the

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RESULTS AND DISCUSSION In both bayerite and gibbsite, Al3+ occupies an acentric position in the [Al(OH)6] polyhedron and there are six different Al−O distances.21 Consequently, the layer group of a single [Al2□(OH)6] layer is p121/a1.21 On the incorporation of Li+ ions in the [Al2□(OH)6] layer, certain significant changes take place within the metal hydroxide layer: (i) the Al3+ cations move to a centric position and (ii) the layer group symmetry evolves to p3̅12/m (Figure 1). The overall symmetry of the metal hydroxide layer is higher. Different polymorphic and polytypic modifications within the [Li-Al] LDH system arise due to the different stacking motifs of this metal hydroxide layer. Each stacking motif has its own characteristic PXRD pattern. The PXRD pattern of the as-prepared LDH (Figure S2a in the Supporting Information) yields a basal reflection at 7.7 Å (11.5° 2θ) followed by a higher order reflection appearing at 3.8 Å (23.2° 2θ). However, the higher order reflection is riding on a broad prominent hump spanning the region 20−30° 2θ. Samples prepared under a wide matrix of imbibition conditions exhibited this feature, whenever the bayerite precursor was used. A similar feature was also reported by other authors,22 who refrained from commenting on its origin. This feature is not observed in g-[Li-Al-Br],9 and therefore it appears characteristic of the bayerite-derived phase. We refer to this as phase I. In the first instance, phase I was judged to be unsuitable for any detailed structure refinement. In light of an 5027

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Figure 2. Relevant portion of the graph of the translationengleiche subgroups with the summit P63/mcm. The space group numbers are given in parentheses.

Figure 1. (a) Structure of a single layer of Al(OH)3 viewed down the c axis. (b) Layer group representation of Al(OH)3. (c) Structure of a single layer of [Li-Al] LDH viewed down the c axis. (d) Layer group representation of the [Li-Al] layer. UO and LO are respectively the upper and lower hydroxyl oxygen atoms of the metal hydroxide layer.

Table 2. Observed 2θ Values (deg) of [Li-Al-Br] LDH with Corresponding Indices Obtained by Using PROSZKI phase I, monoclinica

phase II, hexagonalb

phase III, monoclinicc

2θ (deg)

hkl

2θ (deg)

hkl

2θ (deg)

hkl

11.6 23.3 35.3 36.2 38.0 41.6 49.2 59.0 62.5 62.6 63.4 64.7 68.6 71.3 74.8

001 002 003 −201 131 −132 −133 −242 −115 134 233 044 153 −333 −260

11.9 20.1 23.4 23.8 31.4 35.2 36.1 37.3 40.9 41.6 47.8 53.2 55.6 56.4 60.7 61.4 63.1 64.5 65.9 67.5 68.4 79.2

001 100 101 002 102 110 003 111 200 103 202 104 203 211 212 114 300 301 105 213 302 311

11.3 20.5 22.6 34.1 36.1 38.5 40.3 44.6 47.2 51.8 52.9 56.1 63.3 64.5 68.1 74.2

001 −110 002 003 −131 131 −132 132 042 222 133 203 044 061 062 135

Figure 3. Rietveld fit of the PXRD pattern of [Li-Al-Br] LDH (phase II).

Table 3. Results of Rietveld Refinement of the Structures of the Dehydrated and Hydrated [Li-Al-Br] LDHs cryst syst space group cell params a (Å) b (Å) c (Å) β (deg) V (Å3) no. of data points no. of rflns fitted no. of params refined Rwpa Rpa Rexpa RF χ2 RBragg preferred orientation G1 G2

a a = 5.076 Å, b = 8.815 Å, c = 7.674 Å, β = 96.61°, FM = 9.14, De Wolff’s Mn value 17.16. ba = 5.104 Å, c = 7.477 Å, FM = 23.80, De Wolff’s Mn value 38.75. ca = 5.064 Å, b = 8.818 Å, c = 8.052 Å, β = 101.98°, FM = 4.03, De Wolff’s Mn value 8.34.

phase I

phase II

phase III

monoclinic C121/m1

hexagonal P3̅1m

monoclinic C121/m1

5.09354 8.79898 7.67697 96.50019 341.854 4251 203 50 11.9 (12.8) 9.2 (12.9) 7.5 10.3 2.66 11.5

5.12366

5.08039 8.80269 8.03770 102.14426 351.411 4251 249 46 7.4 (7.1) 5.7 (6.3) 5.6 4.55 1.72 6.60

0.909990 0.228420

7.51315 170.810 4251 79 32 8.8 (8.8) 6.9 (7.8) 5.8 3.89 2.41 6.63

0.786070 0.042540

a

R values are the current CIF standard. R values obtained from FOX code are given in parentheses.

7

earlier report that this class of LDHs undergoes ordering on temperature-induced dehydration, the sample was equilibrated at 120 °C (phase II). At this temperature the loss of intercalated water is complete (Figure S1). The in situ PXRD data recorded at 120 °C (Figure S2b) shows that the broad hump is completely eliminated and the PXRD pattern

comprises sharp lines. The basal spacing contracts by 0.2 Å, showing that dehydration takes place without any significant change in the interlayer distance. The PXRD pattern of phase II could be indexed to a hexagonal cell (Table 2) (ah = 5.104 Å, ch 5028

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Inorganic Chemistry Table 4. Refined Atomic Position Parameters of Dehydrated and Hydrated [Li-Al-Br] LDHs atom type

Wyckoff position

O1 O2 Al Li Br1 Br2 Ow1 Ow2

8j 4i 4g 2a 8j 4i 8j 4j

O1 Al Li Br

6g 2d 1a 1b

O1 O2 Al Li Br1 Br2 Ow1

8j 4i 4g 2a 8j 4i 8j

x Phase I 0.84478 0.38078 0.00000 0.00000 0.07331 0.09758 0.00902 0.05932 Phase II 0.00000 0.33333 0.00000 0.00000 Phase III 0.86213 0.40364 0.00000 0.00000 0.72355 0.28675 0.38619

y

z

occ

0.17365 0.00000 0.32539 0.00000 0.35582 0.00000 0.34395 0.23409

0.12646 0.12605 0.00000 0.00000 0.50000 0.50000 0.50000 0.50000

1.000 1.000 1.000 1.000 0.113 0.229 0.120 0.213

0.63615 0.66667 0.00000 0.00000

0.12732 0.00000 0.00000 0.50000

1.000 1.000 1.000 1.000

0.17283 0.00000 0.33130 0.00000 0.81230 0.00000 0.33159

0.12462 0.12470 0.00000 0.00000 0.50000 0.50000 0.50000

1.000 1.000 1.000 1.000 0.125 0.249 0.287

Figure 6. Structure of a layer of [Li-Al] LDH showing the relationship between the unit mesh of hexagonal and monoclinic symmetry. UO and LO are respectively the upper and lower hydroxyl oxygen atoms of the metal hydroxide layer.

Figure 7. PXRD pattern of [Li-Al-Br] LDH: (a) phase III (hydrated at RH ≈ 70%) compared with (b) the pattern simulated using the stacking vector (a/3, 0, z). (c) PXRD pattern of as-prepared phase I compared with (d) the pattern simulated using the stacking vector (a/ 6, 0, z). Reflections marked with # are due to the 003 plane. The reflections contributing to the hump are marked with asterisks.

Figure 4. Structures of bayerite derived [Li-Al-Br] LDHs (a) phase I, (b) phase II, and (c) phase III viewed along the y axis showing the change in the stacking vector due to reversible hydration−dehydration behavior. UO and LO are respectively the upper and lower hydroxyl oxygen atoms of the metal hydroxide layer.

Figure 5. Structure of bayerite derived [Li-Al-Br] LDHs (a) phase I, (b) phase II, and (c) phase III viewed along the z axis (parallel to the stacking direction) showing the structural transformation due to reversible hydration−dehydration behavior. UO and LO are respectively the upper and lower hydroxyl oxygen atoms of the metal hydroxide layer.

= 7.477 Å; h = hexagonal). From an examination of the c parameter, it is evident that phase II is a one-layer polytype (symbol 1H). Such a polytype is obtained by stacking the [LiAl2(OH)6]+ layers shown in Figure 1 one above another using the stacking vector (0, 0, 1). This corresponds to the stacking sequence PPP···. To proceed further for structure

Figure 8. Rietveld fit of the PXRD pattern of [Li-Al-Br] LDH (phase III).

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five groups P6322, P63cm, P6̅c2, P63/m, and P3c̅ 1 are governed by the reflection condition 00l (l = 2n) and are rejected for being incompatible with phase II. Of the two remaining subgroups, P6̅2m is rejected, as it generates a prismatic coordination for the metal ions. This leaves P3̅1m as the only possible space group for phase II (Figure 2). The g-[Li-Al-A] (A = Cl−, NO3−) LDHs7 are reported to crystallize in the P63/m space group. Thereby, the subgroups of P63/m were also examined. There are three subgroups of P63/ m. Of these, P63 is rejected, as it is governed by the reflection condition 00l (l = 2n). P6̅ is also rejected, as it generates a prismatic coordination for the metal ions. This leaves P3̅ as the only other possible choice of space group. The space groups P3̅1m and P3̅ both provide appropriate special positions 2d and 1a, whose site multiplicities satisfy the 2:1 ratio of Al:Li. Accordingly Li was placed in the 1a (0, 0, 0) site and Al in 2d (2/3, 1/3, 0) site. The position of the oxygen of the hydroxyl ion (O1) was taken from that of the published structure of g-[Li-Al-Br] (CCDC 929942) and placed in the equivalent site 6k (x, 0, z) (space group P3̅1m) or 6g (x, y, z) (space group P3̅). In the dehydrated structures, the halide ion is known to occupy a position in the interlayer closest to the Li+ ion which is the seat of the positive charge in the metal hydroxide layer. This corresponds to the 1b site (0, 0, 1/2) in both of the chosen space groups. Rietveld fits were secured using both the structure models. The fit obtained by the use of the space group P3̅1m was quantitatively better (Figure 3 and Tables 3 and 4). The bond length and bond angle values (Table S1 in

Figure 9. Rietveld fit of the PXRD pattern of [Li-Al-Br] LDH (phase I).

refinement, a suitable structure model is required. However, all of the reported structure models in this class of LDHs are based on the structure of the 2H polytype having the stacking sequence PP̅ P··· derived from the gibbsite precursor. One such typical dehydrated g-[Li-Al-Br] LDH7 is reported to crystallize in the P63/mcm space group. The transformation of the PP̅P··· to the PPP··· requires the elimination of the mirror plane perpendicular to the stacking direction. The translationengleiche graph23 with P63/mcm at the apex has seven subgroups of hexagonal symmetry. Of these, the

Figure 10. SEM images of (left top and bottom) gibbsite-derived and (right top and bottom) bayerite-derived [Li-Al-Br] LDH. 5030

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subgroup of P31̅ m of monoclinic symmetry is C2/m (Figure 2). A structure model with similar cell parameters (space group C12/m1) has been proposed earlier by Thiel and Poeppelmeier16 and further adapted by Britto and co-workers.15 The layer was extracted from the reported structure of b-[Li-Al-Cl] and used as input into FOX code as a partial structure. The Br− ion and O atom of the intercalated water were inserted into the interlayer region and allowed to diffuse. A satisfactory fit of the computed pattern with the observed pattern could be secured only after the introduction of two Br atoms. The Br− ions occupy a 8j general position and 4i (x, 0, 0.5) position. The positional disorder observed in the location of the Br− ion is characteristic of a non-hydrogen-bonding species. The structure model obtained from FOX was exported to code FULLPROF, and the refinement was completed (Figure 8 and Tables 3 and 4). The final refined structure (Figures 4 and 5) does not vary significantly from the model obtained from FOX code and shows the rigid relative translation of successive layers. The bond lengths and bond angles (Table S1 in the Supporting Information) are in the range expected for Li−O and Al−O bonds. The successful refinement of the structure of phase II and phase III led us to a close examination of the PXRD pattern of phase I for three reasons. (1) When phase III was equilibrated in situ at a relative humidity of ∼50% (T = 30 °C), phase I was recovered and the broad hump at the 20−30° 2θ reappeared, showing that it is characteristic of phase I. (2) The Bragg reflections of phase I were generated in the PXRD pattern simulated using a (a/6, 0, 1) stacking vector (Figure 7). (3) The simulated pattern has numerous Bragg reflections in the 20−30° 2θ region whose combined profile resembles the observed broad hump, suggesting that the observed hump is structural in origin. A translation of (1/6, 0, 1) along ah of the metal hydroxide layer yields the stacking angle β ≈ 96°. Therefore, the pattern in Figure S2a in the Supporting Information corresponding to phase I was indexed to a monoclinic cell of β ≈ 96°, with an acceptable figure of merit (FM in Table 2). A structure model was constructed by applying (1/6, 0, 1) translation to successive layers, and the Rietveld refinement of the structure was carried out (Figure 9 and Tables 3 and 4). The resultant structure (Figures 4 and 5) is obtained by a translation intermediate between those of phase II (0, 0, 1) and phase III (1/3, 0, 1). The reversible hydration behavior of phase I differs from that of its gibbsite-derived counterpart.9 In the latter, the relative orientation of the layers, the crystal symmetry, and the space group (P63/m) remain invariant throughout the hydration cycle. The structural changes are limited to the migration of the Br− ion in the interlayer with a variation in the degree of hydration. The morphology of phase I also differs substantially from that of the gibbsite-derived counterpart (Figure 10). The latter comprises large hexagonal faceted crystallites (20−50 μm). The crystallites in phase I have a grainy morphology. Consequently, the March function parameter G1 used in the structure refinement of phase I has a value of 0.91 (Table 3) (G1 = 1 corresponds to an unoriented sample). In contrast, G1 = 0.4 (Table 3 in ref 9) for the gibbsite-derived counterpart, showing

the Supporting Information) are reasonable. Plots of the structure are given in Figures 4 and 5. When phase II was cooled in situ to 30 °C and rehydrated at a relative humidity value of ≥70%, an entirely new phase was obtained (phase III). The PXRD pattern of this phase (Figure S2c in the Supporting Information) had reflections different from those in phase I. The PXRD pattern at Figure S2c could not be indexed to hexagonal symmetry. On the other hand, it was indexed to a cell of monoclinic symmetry (Table 2). A clear topotactic relation could be established wherein am = ah and bm = √3 × ah, thus showing that the metal hydroxide layer remains invariant in phases II and III and the monoclinic cell is derived from the hexagonal cell by (i) a different choice of the unit mesh in the a−b plane (Figure 6) (ii) the choice of a nonorthogonal stacking angle β for the monoclinic cell. A nonorthogonal stacking angle is realized by the rigid translation of successive layers relative to one another. The problem now reduces to one of determining the correct stacking vector. Structural Synthon Approach.21,24,25 A symmetry-based approach to arrive at the stacking vector which provides the correct translation between successive layers involves the choice of a suitable structural synthon. In the present instance, the metal hydroxide layer (layer group p3̅12/m) (Figure 1) itself is the structural synthon. In this layer, there is a 3̅-axis perpendicular to the layer passing through each Li atom and a 3-fold axis passing through each of the two Al atoms within the unit mesh. There are in-plane 2-axes perpendicular to the a (and b) axis. When these layers are stacked one above another in such a way that the 3-axes (3 or 3̅) of successive layers coincide, then the principal 3-axis of each layer is conserved in the whole stack, leading to a crystal of hexagonal/trigonal symmetry. Thus, stacking vectors (0, 0, 1), (1/3, 2/3, 1), and (2/3, 1/3, 1) lead to crystals of hexagonal/trigonal symmetry. The stacking vector (0, 0, 1) yields the hexagonal structure of phase II. It is readily seen that the stacking vectors (1/3, 2/3, 1) and (2/3, 1/3, 1) yield a three-layer crystal of rhombohedral symmetry. To obtain a crystal of lower symmetry, the principal symmetry elements of the structural synthon (3 or 3̅) have to be systematically eliminated by the appropriate choice of stacking vectors. In the present instance, phase III has monoclinic symmetry. To obtain a crystal of monoclinic symmetry, the metal hydroxide layer has to be stacked in such a way that there should be no coincidence of the 3 (or 3̅) axes of successive layers, while at the same time the in-plane 2-axes are conserved. This can be realized by translating successive layers rigidly along the a axis. An n-layer polytype can be obtained by using the stacking vector (a/n, 0, 1). The resulting n-layer orthogonal cell can be replaced by a 1-layer nonorthogonal cell (β ≠ 90°) of lower volume having monoclinic symmetry. A number of PXRD patterns were simulated for model structures corresponding to n = 2−6 (Figure S3 in the Supporting Information). These n values cover a range of stacking angles β = 110−96°. A comparison of the simulated patterns with the observed patterns (Figure 7) shows that the PXRD pattern of phase III corresponds to that simulated for n = 3. The translation vector (1/3, 0, 1) corresponds to β = 102°, which agrees with the angle β used to index the pattern (Table 2). The symmetry of this one-layer nonorthogonal cell should be a subgroup of the one-layer orthogonal cell. The only 5031

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Article

Inorganic Chemistry

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a much higher degree of orientation (G1 = 0 corresponds to a fully oriented sample).

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CONCLUSION A rigorous symmetry-based structural synthon approach was proposed earlier24,25 to arrive at the complete universe of polytypes in any layered system. This work is a model application of the structural synthon approach to arrive ab initio at structure models. The dehydrated, partially hydrated, and fully hydrated LDHs of [Li-Al-Br] are shown to be obtained by the rigid translations of successive layers relative to one another along the hexagonal a axis of the metal hydroxide layer. These rigid translations do not affect the strong ionocovalent intralayer bonding but alter only the interlayer connectivities. The reversible diffusion of water molecules in the interlayer region during temperature and humidity cycling lubricates the rigid relative translations of the metal hydroxide layers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00119. CCDC 1525200 (phase I), 1525198 (phase II), and 1525199 (phase III) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures-beta/. TGA and DTG curve of phase I LDH, simulated PXRD pattern for model structures of b-[Li-Al-Br] with stacking vectors (a/n, 0, z) (n = 2−6), and bond distances and bond angles of phase I, phase II, and phase III of b-[LiAl-Br] LDH obtained from Rietveld refinement (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.V.K.: [email protected]. ORCID

Supreeth Nagendran: 0000-0002-9843-0214 P. Vishnu Kamath: 0000-0002-3549-7024 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Department of Science and Technology (DST), Government of India. S.N. thanks the University Grants Commission for the award of a Senior Research Fellowship (NET). P.V.K. is a recipient of the Ramanna Fellowship of the DST. The authors thank the Solid State and Structural Chemistry Unit, Indian Institute of Science, for providing SEM images.

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REFERENCES

(1) Poeppelmeier, K. R.; Hwu, S.-J. Synthesis of Lithium Dialuminate by Salt Imbibition. Inorg. Chem. 1987, 26, 3297−3302. (2) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; English Language Book Society and Oxford University Press: London, 1975. (3) Komarneni, S.; Kozai, N.; Roy, R. Novel Function for Anionic Clays: Selective Transition Metal Cation Uptake by Diadochy. J. Mater. Chem. 1998, 8, 1329−1331. (4) Megaw, H. D. The Crystal Structure of Hydrargillite, Al(OH)3. Z. Kristallogr. - Cryst. Mater. 1934, 87, 185−204. 5032

DOI: 10.1021/acs.inorgchem.7b00119 Inorg. Chem. 2017, 56, 5026−5033

Article

Inorganic Chemistry (24) Britto, S.; Kamath, P. V. Polytypism in the Lithium-Aluminum Layered Double Hydroxides: The [LiAl2(OH)6]+ Layer as a Structural Synthon. Inorg. Chem. 2011, 50, 5619−5627. (25) Britto, S.; Kamath, P. V. Structural Synthon Approach to the Study of Stacking Faults in the Layered Double Hydroxides of Lithium and Aluminum. Z. Anorg. Allg. Chem. 2012, 638, 362−365.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on April 13, 2017, with a minor text error in the first paragraph of the Results and Discussion section. The corrected version was reposted on April 18, 2017.

5033

DOI: 10.1021/acs.inorgchem.7b00119 Inorg. Chem. 2017, 56, 5026−5033