Order, Disorder and Crystallinity in Layered. Hydroxides. P. Vishnu Kamath. Central College, Bangalore University. Department of Chemistry

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1 Order, Disorder and Crystallinity in Layered Hydroxides P. Vishnu Kamath Department of Chemistry Central College, Bangalore University Bangalore

2 Layered Hydroxides Mg(OH) 2 [Mg (1-x) Al x (OH) 2 ] x+ [(CO 3 )x/2 mh 2 O] Brucite Mg(OH) 2 and its isoelectronic derivatives M(II)(OH) 2 (M=Ca, Mn, Fe, Co, Ni, Cd) LiAl(OH) 4 ; K 2 Sn(OH) 6 Rajamathi, M; Thomas, G. S.; Kamath, P. V. Proc. Ind. Acad. Sci. 2001, 113, 871. LDH [Mg 1-x Al x (OH) 2 ](CO 3 )x/2 mh 2 O

3 Study of Structural Disorder in Ni(OH) 2 ph>13 ph 9 Positive electrode material of alkaline secondary batteries (Ni-Cd, Ni-Fe, Ni-Zn and Ni-MH) Polymorphism: α(hydrated) & β modifications Aldrich NH 3 pptn Different oxidation states: II, III, IV Electrochromism: Ni(II) Green, Ni(III) Black Topotactic decomposition to NiO [001] Ni(OH)2 // [111] NiO

$Broadening of Reflections in the Powder X-ray Diffraction Patterns of Layered Materials Instrumental Factors Particle Size Effects: Nanoparticulate? Estimated particle sizes are too unrealistic!$

4 Broadening of Reflections in the Powder X-ray Diffraction Patterns of Layered Materials Instrumental Factors Particle Size Effects: Nanoparticulate? Estimated particle sizes are too unrealistic! Ordered Stacking faults Turbostratic Interstratified Disorder in Layered Solids Cation vacancies Stacking faults ABABABCBCBCBBABABAB---- ABCABCABCBCABCABC---- Turbostraticity Random orientation of layers Interstratification Intergrowths of motifs with different interlayer spacings Disorder in the intercalated layer

6 DIFFaX Simulations of the PXRD Patterns of Layered Hydroxides Principle Treat a crystal as a stacking of layers of atoms Compute the diffraction intensity per layer Integrate the intensity over a given number/infinite number of layers Layers are interconnected by stacking vectors Methodology Instrumental broadening (Lorentzian/Gaussian) Define a layer (using ICSD data) Define a stacking vector (0,0,1) Crystalline phase; (0,0,r) (r 1) Interstratification (1/3,2/3,1) and (2/3,1/3,1) Stacking faults (x,y,1) (x,y Random) Turbostraticity; (x,y,r) Turbostraticity with interstratification Engineering Disorder Combination of two or more stacking vectors with varying probabilities Probability = % incidence of the corresponding disorder

7 Layer Thickness (N) N= N=37 N=Infinite Intensity (arb.units) Turbostaticity % 15% 0% θ (degrees) Disc diameter Å Å Infinite Interstratification % % 5% 0% θ (degrees) Intensity (arb.units)

8 Conclusions of DIFFaX Simulations Intensity (arb. units) Particle size Thickness broadens 00l reflections Disc diameter broadens hk0 reflection Cation vacancies marginally decrease the intensity of hk0 reflections 001 Cation vacancies (Ni) % % Stacking faults broaden the h0l reflections θ (degrees) Turbostraticity causes the asymmetric broadening of hk0 reflections; 00l unaffected Interstratification causes the pronounced broadening of the non- hk0 reflections, that is, 00l and hkl; hk0 unaffected Ramesh, T. N.; Jayashree, R. S.: Kamath, Clays and Clay Min., 2001, 51, 570-6

9 Methodology Used for Analysis θ (degrees) Expt. Simu. Diff. 111 Intensity (arb. units)

10 Observed Patterns Simulated Patterns ph>13 ph>13 ph 9 ph 9 Aldrich Aldrich NH 3 pptn NH 3 pptn

11 Results of DIFFaX Simulations of PXRD Patterns of β-nickel Hydroxide Preparative Method Sample Crystallite Size (Å) Cation vacancy (%) Thick -ness Disc diameter Turbostr aticity (%) Interstr atificati on (%) Ammonia pptn A , Aldrich Chemical Co. (U. S) B , Strong alkali pptn (ph = 9) C Strong alkali pptn (ph>13) D

Structural Disorder in Mg(OH) 2 Electro synthesis ph=10 & HT (120 C) ph=10

only from particle size effects but also from incidence of a variety of

Disorders can be engineered into the crystal by varying the precipitation

12 Structural Disorder in Mg(OH) 2 Electro synthesis ph=10 & HT (120 C) ph=10 The broadening of reflections in the PXRD patterns of Mg(OH) 2 arises not only from particle size effects but also from incidence of a variety of disorders Disorders are incorporated during the crystallization process Disorders can be engineered into the crystal by varying the precipitation conditions Radha, A. V.; Kamath, P. V.; Subbanna, G. N. Mater. Res. Bull. 2003, 38, 731.

13 Classification of Stacking Faults Hydroxide layer - AbC or AC Where hydroxyl ion positions: A, B, C; Cation positions: a, b, c Stacking sequence in an ordered Ni(OH) 2 crystal: AC AC AC AC Stacking Faults: Insertion of an AB or a BC layer: AC AC AC AB AC AC or AC AC AC BC AC AC close packed Insert BA CB motif: AC AC BA CB AC AC close packed Insertion of a BA layer: AC AC BA AC AC AC not close packed Insertion of a CA layer: AC AC CA AC AC not close packed Insertion of CB BA motif: AC AC CB BA AC AC not close packed

= Prismatic interlayer site; Octahedral interlayer site 2H 1 polytype 3R 1 polytype Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551.

14 Polytypism in Layered Hydroxides One-dimensional polymorphism! One layer AC-AC-AC 1H Two Layer AC=CA=AC 2H 1 AC AB AC 2H 2 AC BA=AC 2H 3 Three Layer AC=CB=BA=AC 3R 1 AC BA CB AC 3R 2 Seven other hexagonal polytypes - 3H 1-7 = Prismatic interlayer site; Octahedral interlayer site 2H 1 polytype 3R 1 polytype Bookin, A. S.; Drits, V. A. Clays Clay Miner. 1993, 41, 551. Compare the stacking fault motifs with the stacking sequences of pure polytypes Insertion of AB or BC layers generates 2H 2 type of stacking faults Insertion of CB or BA layers generates 2H 3 type of stacking faults Insertion of CA layer generates 2H 1 type of stacking faults

15 DIFFaX Simulated PXRD Patterns of Different Polytypes β-ni(oh) R H Intensity (arb.units) 102 1H θ (degrees)

001 Stacking faults with 2H 2 motifs 20% 15% 10% 101 100 110 102 10 20 30 40

16 001 Stacking faults with 2H 2 motifs 20% 15% 10% Local structure of 2H 2 stacking fault Local structure of 3R 2 stacking fault Intensity (arb. units) 001 Stacking faults with 3R 2 motifs 30% % 10% θ (degrees) 111 Intensity (arb.units)

Stacking faults with 2H 1 motifs 001 101 100 102 30% 20% 10% 110 10 20 30 40

17 Stacking faults with 2H 1 motifs % 20% 10% Stacking faults with 3R 1 motifs 111 Local structure of 2H 1 stacking fault Intensity (arb.units) % % 10% θ (degrees) 111 Local structure of 3R 1 stacking fault Intensity (arb.units)

18 Each type of stacking fault leaves its characteristic stamp on the PXRD pattern Ramesh, T. N.; Kamath, P.V.; Shivakumara, C. Acta Cryst. B. 2006, B62, 530. Ordered crystal Faulted crystal Polytype Point group Stacking fault motif Point group Reflection most affected 1H -3m - -3m - 2H 1 6/mmm 2H 1 6/mmm 102 2H 2 6/mmm 2H 2 6/mmm 101,102 2H 3-3m 2H 3-3m 102 3R 1-3m 3R 1-3m 101 3R 2-3m 3R 2-3m 102

19 001 ACC β-nickel hydroxide Expt. Simu H with 11% 3R θ (degrees) Intensity (arb. units)

20 001 HT β-nickel hydroxide Expt. Simu H with 20% 2H θ (degrees) Intensity (arb.units)

21 Layered Double Hydroxides (LDHs) General formula [M(II) 1-x M'(III) x (OH) 2 ] (A n- ) x/n mh 2 O M(II) =Ca, Co, Cu, Mg, Ni, Zn ; M'(III) =Al, Cr, Fe, Ga, In; A n- = Inorganic/organic anions Minerals in Nature Hydrotalcite (3R 1 ) and Manasseite (2H 1 ) Same Composition Mg 6 Al 2 (OH) 16 CO 3 4H 2 O! Mg(OH) 2 [Mg (1-x) Al x (OH) 2 ] x+ [(CO 3 )x/2 mh 2 O] Brucite LDH

22 DIFFaX Simulated PXRD Patterns of Different Polytypes H R θ (degrees) Intensity (arb. units) R 2 Regions in the PXRD patterns 1. Low angle region (5-25º2θ) basal 00l reflections - Characteristic of interlayer anion size 2. The high angle region (50-65º2θ) (hk0) & (hkl) reflections -Characteristic of metal hydroxide layer 3. The mid.-2θ region (30-50º2θ) (h0l)/(0kl) reflections -Characteristic of polytypes & disorders Thomas, G. S.; Rajamathi, M.; Kamath, P. V. Clays Clay Miner. 2004, 52, 693.

23 Effect Incorporation of 3R 1 and 2H 1 motifs in 3R 1 Polytype 3R R 1 3R 1 with 15% 2H 1 3R 1 with 35% 2H 1 Intensity (arb. units) 3R 1 with 15% 3R 2 3R 1 with 35% 3R 2 Intensity (arb. units) θ (degrees) θ (degrees)

24 Co-Fe-CO 3 LDH ph=10 Expt. Simu. Diff. 3R 1 with 30% 3R 2 3R 1 with 30% 2H 1 0kl FWHM in 2θ in 2θ Intensity (arb. units) θ (degrees) Relative intensity of 00l reflections high mid-2θ peaks can be indexed to the 01l (l = 2, 5, 8) 3R 1 polytype Non-uniform broadening of 0kl reflections reflections Structural disorder

25 Effect of ph on Disorder ph 10 Co-Al-CO 3 LDH Expt. Simu. Diff. 3R 1 with 55% 2H 1 ph 8 3R 1 with 20% 2H θ (degrees) Intensity (arb. units)

26 Results of DIFFaX Simulations of Different LDHs 3R 1 (%) 2H 1 (%) 3R 2 (%) Point group LDH Co-Fe-CO 3 ph /mmm Co-Fe-CO 3 ph /mmm Co-Al-CO 3 ph /mmm Co-Al-CO 3 ph /mmm Ni-Fe-CO /mmm Ni-Fe-CO C /mmm Ni-Al-CO /mmm Ni-Al-CO C /mmm

27 Ordering of the Poorly Crystalline β-nickel Hydroxide-Single Step? Strategy: Hydrothermal treatment (80-200ºC) Solid formed immediately on precipitation is most disordered and least stable (Ostwald, 1897) Disorders that reduce enthalpy (Verma & Krishna, 1966) Interstratification: [Ni(OH) 6-x (H 2 O) x ] or [Ni(OH) 6-x (NO 3 ) x ] Cation vacancy: [Ni 1-x x (OH) 2x (H 2 O) 2x ] Disorders that conserve enthalpy and increase entropy Stacking faults: [Ni(OH) 6 ] Interstratification and cation vacancies are eliminated at 140ºC, stacking faults persist up to 170 ºC

28 Simu. with 23% interstratifcation, 15% stacking faults & 17% cation vacancies Expt. NH65 Simu. Crystalline Intensity (arb.units) θ (degrees)

29 Observed Patterns Simulated Patterns NH NH % 2H 2 & 10% Ni vacancies NH140 NH140 11% interstratification, 15% 3R 2 & 17% vacancies NH110 NH θ (degrees) θ (degrees) Intensity (arb. units)

30 Ni(OH)2 Hydrothermally treated at 170 C

31 Mechanism of Elimination of Stacking Faults in Ni(OH) 2 As precipitated nickel hydroxide has 3R 2 type of stacking faults (17-20%) [Randomly selected layers translate by (1/3,2/3,1)] Vary temperature C and duration of hydrothermal treatment 5-48 h T 125 C and t=5-48 h: 17-20% 3R 2 stacking faults T 140 C and t>18 h: 20% 2H 2 stacking faults T=170 C: 1.5-4% 2H 2 stacking faults persist T=200 C Stacking faults are completely eliminated Evolution of structural order is step wise: ACBACBAC ACABACAC ACACACAC 3R 2 2H 2 Ordered Stacking faults do not affect the enthalpy as they have the same packing fraction Ramesh, T. N.; Kamath, P.V.; Shivakumara, C. Acta Cryst. B. 2006, B62, 530.

32 Cycle Life Data of Electrodes Ramesh, T. N.; Kamath, P.V.; Shivakumara, C. J. Electrochem. Soc. 2005, 152, A806. NH65 NH110 NH140 NH cycle number NEE

33 Effect of Hydrothermal Treatment Ni-Al-CO 3 LDH Expt. Simu. HT 180 o C 3R 1 with 50% 2H 1 ph>12 3R 1 with 50% 2H θ (degrees) Intensity (arb. units)

34 Thermodynamic and Structural Aspects of Polytypism in LDHs Interactions contributing to the enthalpy in LDH system (1) Bonding within the brucite layer (2) Bonding within the interlayer (3) Ionic bonding between the +ly charged brucite layer & -ly charged interlayer (4) Hydrogen bonding between the brucite layer and the interlayer atoms For a given LDH system, Factors (1) to (3) are the same for all polytypes Different polytypes generate different types of sites in the interlayer! The requirement of hydrogen bonding dictates the nature of interlayer sites and thereby selects certain specific stacking sequences among the range of possibilities Factor (4) dictates the preferential formation of different polytypes For carbonate anion in planar orientation Prismatic interlayer site are essential to maximize H-bonding Both 3R 1 and 2H 1 polytypes provide prismatic interlayer sites 3R 1 and 2H 1 polytypes have comparable enthalpies Entropy factor comes into play to produce a random mix of 3R 1 and 2H 1 polytypes

35 Entropy Consideration Ordered structures are not favored Favours disorders that do not affect the enthalpy Stacking faults belong to this category (Metal ion coordination sphere is unaffected) Restricts the stacking faults to 3R 1 and 2H 1 motifs (H-bonding is unaffected) Faulted crystals with stacking disorders have a greater thermodynamic stability! Stacking faults once incorporated cannot be eliminated even by HT treatment Look at point group symmetry of the faulted crystals Crystal with a higher symmetry being thermodynamically more stable (Verma and Krishna) Laue symmetry for a mix of 3R 1 with 3R 2 motifs is 3m for a mix of 2H 1 with 3R 1 motif is 6/mmm Radha, A. V.; Shivakumara, C.; Kamath, P. V. Clays Clay Miner. 2005, 53, 521.

36 Mechanism of Elimination of Stacking Faults in Ni(OH) 2 As precipitated nickel hydroxide has 3R 2 type of stacking faults (17-20%) [Randomly selected layers translate by (1/3,2/3,1)] Vary temperature C and duration of hydrothermal treatment 5-48 h T 125 C and t=5-48 h: 17-20% 3R 2 stacking faults T 140 C and t>18 h: 20% 2H 2 stacking faults T=170 C: 1.5-4% 2H 2 stacking faults persist T=200 C Stacking faults are completely eliminated Evolution of structural order is step wise: ACBACBAC ACABACAC ACACACAC 3R 2 2H 2 Ordered Stacking faults do not affect the enthalpy as they have the same packing fraction Ramesh, T. N.; Kamath, P.V.; Shivakumara, C. Acta Cryst. B. 2006, B62, 530.

37 001 t =18h 100 NH Expt. Simu. 111 t =18h t =5 h θ (degrees) Ramesh, T. N.; Kamath, P.V.; Shivakumara, C. Acta cryst. B. 2006, Intensity (arb.units)

38 Rietveld Refinement of Ni(OH) 2 HT at 200º C for 48 h

40 Ordering of Carbonate LDHs by Homogeneous Precipitation Crystallinity and phase formation depends on precipitation conditions Factors governing the Solid formation are Nucleation and growth steps Precipitation kinetics Kinetics of precipitation can be altered by controlled release of alkali by hydrolysis of urea Urea hydrolysis generates NH 3 and CO 3 ions Generation NH 3 alkaline ph; CO 3 ions ordered stacking of hydroxide layers Urea hydrolysis at different temperatures varies the kinetics of precipitation

41 Ni-Al-CO 3 LDH by Urea Hydrolysis at Different Temperatures HT 180 o C Ni-Al-CO 3 LDH Expt. Simu. 3R 1 with 7% 2H 1 General features Gives pattern with sharp peaks more ordered samples non-uniformly broadened peaks structural disorder HT 140 o C 3R 1 with 10% 2H 1 in FWHM with in temp. progressive elimination of disorders with in temp. Stacking faults persists till 180º C! 100 o C 3R 1 with 25% 2H θ (degrees) Intensity (arb. units)

42 Rietveld refinement of Ni-Al-CO 3 LDH HT at 200º C R-3m a =3.03 Å; c =22.85 Å Goodness of fit R wp R Bragg R F R p χ Refinement is satisfactory but shows systematic residual intensity under the 018 reflection Other profile function (Pearson VII) does not improve the fitting Refinement by other authors also shows a residual intensity in this region ( θ) incidence of stacking disorders Bellotto et al., 1996, J. Phys. Chem. 100, 8527 ; Costantino et al., 1998, Eur. J. Inorg. Chem

43 Combined Rietveld and DIFFaX Approach to Determine Nature of Disorder Output parameters of the Rietveld refinement were input into the DIFFaX program Same difference profile two fortran based packages are internally consistent Ni-Al-CO 3 HT at 200 o C Expt. Simu. Diff. 3R 1 with 9% 2H 1 Intensity (arb. units) 3R 1 (Rietveld data) Incorporation of 9% 2H 1 polytype Corrects residual intensity in the difference profile Improves R wp from 35.8% to 24.9% θ (degrees) Combined Rietveld-DIFFaX approach ordered LDHs includes residual stacking disorders

44 Mg-Al-CO 3 LDH HT at 140º C Co-Al-CO 3 LDH HT at 140º C R-3m a =3.04 Å; c =22.66 Å R-3m a =3.06 Å; c =22.59 Å Homogeneous precipitation at different temperatures ordering of LDHs Hydrolysis offers a delicate kinetic control over both nucleation and growth steps Temperature of hydrolysis determines the rate of generation of base affecting the rate of nucleation and growth is dependent on the concentrations of various species in solution Combined Rietveld-DIFFaX approach insights into nature of structural disorder that neither method can give on its own.

45 Effect of Anions on Order/Disorder and Polytype Selectivity in LDHs of Zn with Al Stacking sequences in LDHs are determined by Crystal chemical considerations Symmetry match between of the guest moieties and the interlayer sites Thermodynamic considerations Strength of bonding between the atoms of the layer and the interlayer No atoms in interlayer Favours 1H, 2H 2 and 3R 2 polytypes or their intergrowths (Ramesh et al., Acta cryst. B, 2006) CO 3 ions in interlayer Favours 3R 1 and 2H 1 polytypes or their intergrowths (Radha et al., Clays Clay Min., 2005) Objective of this study Polytype selectivity of in Zn 2 Al(OH) 6 (A n- ) 1/n mh 2 O LDH with simple inorganic anions monoatomic anions of different sizes (F -, Cl -, Br -, I - ) C 2v (NO 2 -, S2 O 3 ); C3v (BrO 3 - ); D3h (NO 3 -, CO3 ); Td (SO 4, ClO4 - )

46 Sites Available in Interlayer of LDHs for Anions Space group R -3m (3R 1 polytype) Interlayer sites available are (18h), (18g), (3b) and (6c) (3b) site - offers the best prospects of H-bonding; (18h)/(18g) sites - next best set of sites for H-bonding (6c) - least suited for H-bonding (largest distance from the hydroxyl ions)

35 Å CO 3 ion (D 3h ) plane perpendicular to the c- axis O atoms of CO 3 & H 2 O in

47 LDHs with Single Atom thick Interlayer R-3m a =3.07 Å; c =22.74 Å R-3m a = 3.08 Å; c = Å CO 3 ion (D 3h ) plane perpendicular to the c- axis O atoms of CO 3 & H 2 O in (18h) sites NO - 2 ion (C 2v ) plane perpendicular to the c- axis O atoms of NO - 2 & H 2 O in (18h) sites C atoms of CO 3 in (6c) sites N atoms of NO - 2 in (6c) sites

48 Cl - in (18h) sites d = 7.8 Å LDHs with Halides in the interlayer Br - in (6c) sites d = 7.96 Å

49 Zn-Al-I - LDH Expt. Simu. Ex. 3R 1 with 35% turbo Co-pptn * turbostratic θ (degrees) Choice of interlayer sites by halides Relative H- bonding ability Size of the interlayer sites Intensity (arb. units)

50 LDHs with non-planar Anions in the Interlayer NO 3 - ions intercalates with N-O bonds collinear to c- axis 3 atoms thick interlayer Ex from Cl - LDH d = 8.5 Å Zn-Al-NO 3 - LDH Interstratified - - (NO 3 /Cl ions) Expt. Simu. Diff. Symmetry mismatch Disorder Interlayer site Symmetry - D 3h NO - 3 ion coordination symmetry - C 2v Co-pptn d = 8.8Å 3R 1 with 30% 2H 1 Co-pptn Stack faults Exchange reactions from ordered Cl - LDH Interstratified disordered ClO - 4 LDH Stack faults θ (degrees) Intensity (arb. units)

51 LDH with Anions with Pyramidal Structure (C 3v Symmetry) Different synthetic method different interlayer distance Ex from Zn-Al-Cl - LDH d = 9.5 Å Zn-Al-BrO 3 - LDH Expt. Simu. 1H with 50% turbo Incorporation of BrO 3 - ions Changes polytype to 1H Turbostratic disorder Co-pptn d = 10.8 Å 1H with 35% turbo θ (degrees) Intensity (arb. units)

52 Tetrahedral Anion Ex from Zn-Al-Cl - LDH d = 8.8 Å Zn-Al-SO 4 LDH Expt. Simu. Diff. 3R 1 with 35% 1H & 30% turbo Co-pptn d = 11.2 Å 1H with 35% turbo * θ (degrees) Role of synthetic method Changes symmetry of intercalated SO 4 ions Different interlayer distance Changes Polytype selectivity Intensity (arb. units)

53 IR spectra of Zn-Al-SO 4 LDHs IR vibrations T d SO 4 ion SO 4 in T d Antisym stretch (ν 3 ) cm -1 Antisym deformation (ν 4 ) cm -1 IR vibrations C 3v SO 4 ion ν 3 (F 2 ) splits to A 1 +E modes SO 4 in C 3v (1155 and 1114 cm -1 ) Characteristic ν 1 mode cm -1 Halford s rules - molecule in a solid, the crystallographic axis is defined as the principal axis Exchange sample - c-crystallographic axis ill-defined due to stacking disorders SO 4 exhibit T d

54 Conservation of Order/Disorder & Crystallinity During Anion Exchange Reactions Zn-Al-CO 3 LDH d = 7.6 Å - Zn-Al-ClO 4 LDH d = 9.0 Å Expt. Simu. Diff. 3R 1 with 25% 2H 1 3R 1 with 25% 1H & 30% turbo - Ex from NO 3 precursor d = 7.6 Å Ex from Cl - precursor d = 7.6 Å Zn-Al-CO 3 LDH Expt. Simu. Diff. 3R 1 with 10% 2H 1 Intensity (arb. units) 3R 1 with 10% 2H θ (degrees) θ (degrees) Exchanging ClO - 4 ions with CO 3 ions removes turbostratic disorder Mutual translations of layers during exchange prismatic interlayer to maximize H-bonding, which includes both 3R 1 and 2H 1 stacking motifs

55 Zn-Al-Cl - LDH Ex from NO 3 - LDH Zn-Al-NO 3 - LDH Ex from Cl - LDH Expt. Simu. Diff. 3R 1 with 30% 2H 1 Interstratified (Cl - - /NO 3 ions) Intensity (arb. units) θ (degrees) During anion exchange reactions CO 3 & Cl - ions having matching symmetries with interlayer site ordered structures ClO - 4, BrO3 -, SO4 & NO - 3 ions with different intercalate symmetry stacking disorders

56 Results of DIFFaX Simulations of the Different Zn-Al LDHs LDH method 1H (%) 2H 1 (%) 3R 1 (%) Turbo a (%) Loren b (º 2θ) Disc dia (Å) No. of layers Zn-Al-ClO 4 - Cp Zn-Al-CO 3 Ex(ClO 4 - ) Zn-Al-CO 3 Ex(Cl - ) Zn-Al-CO 3 Ex(NO - 3 ) Zn-Al-Cl - Ex(NO - 3 ) Zn-Al-NO - 3 Cp Zn-Al-NO - 3 Ex(Cl - ) c Zn-Al-NO - 3 Ex(ClO - 4 ) Zn-Al- SO 4 Cp Zn-Al-SO 4 Ex(Cl - ) Zn-Al-BrO 3 - Cp Zn-Al-BrO 3 - Ex(Cl - ) a Turbostratic disorder; b Lorentzian line shape width; c Interstratified with 50% 3R1 polytype of Zn-Al-Cl - LDH

57 Interlayer interactions dictate the stacking order in LDHs Ionic interaction and H-bonding between brucite-like layer and interlayer H- bonding being directional determines the stacking sequence Interlayer water & anions participate in H-bonding with hydroxide layer H-bond formation in LDHs depends on 1 Orientation of hydroxyl ions and interlayer species 2 Interlayer sites - stacking sequence of the brucite-like sheets 3 Configuration of anion - symmetry, size, charge and polarizability Three factors act cooperatively to maximize the bonding interactions Factors (1) & (3) are predetermined by the anion structure and its coordination Monoatomic/ high symmetry anions - Orientation degrees of freedom for H-bonding non-planar/low symmetry anions- Reorganization of stacking sequence for H-bonding Long range ordering energy of metal hydroxide slabs >Hydrogen bond formation Turbostratic disorder due to incompatibility of anion symmetry with interlayer site In co-pptd Zn-Al-SO 4 LDH, SO 4 ions goes to C 3v from T d symmetry to mediate the long range ordering to generate octahedral interlayer sites 1H polytype Exchange Zn-Al-SO 4 LDH is turbostratic as long range stacking is predetermined by the structure of the precursor LDH.

58 Acknowledgements Dr. T. N. Ramesh Dr. A.V. Radha Grace S. Thomas Dr. Michael Rajamathi Prof. R. Seshadri (Currently at UCSB) Dr. C. Shivakumara (SSCU, IISc.) Solid State and Structural Chemistry Unit, IISc. (PXRD facilities) Department of Science and Technology, Govt. of India UGC/CSIR for student fellowships (NET)

Thermally Induced Polytype Transformations among the Layered Double Hydroxides (LDHs) of Mg and Zn with Al

J. Phys. Chem. B 2006, 110, 12365-12371 12365 Thermally Induced Polytype Transformations among the Layered Double Hydroxides (LDHs) of Mg and Zn with Al Grace S. Thomas, A. V. Radha, P. Vishnu Kamath,*,