Preparation of the layered double hydroxide (LDH) LiAl2(OH)7·2H2O, by gel to crystallite conversion and a hydrothermal method, and its conversion to lithium aluminates M. Nayak,a T. R. N. Kutty,*a V. Jayaramanb and G. Periaswamy,b aMaterials Research Centre, Indian Institute of Science, Bangalore 560 012, India bMaterials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India A layered double hydroxide (LDH) with chemical composition LiAl2(OH)7·2H2O was prepared via a wet chemical route of gel to crystallite (G–C) conversion at 80 °C involving the reaction of hydrated alumina gel, Al2O3·yH2O (80<y<120) with LiOH (Li2O/Al2O30.5) in presence of hydrophilic solvents such as ethanol under refluxing conditions.The hydrothermal synthesis was carried out using the same reactants by heating to140 °C in a Teflon-lined autoclave under autogenerated pressure (20 MPa).Transmission electron microscopy showed needle-shaped aggregates of size 0.04–0.1 mm for the gel to crystallite conversion product, whereas the hydrothermal products consisted of individual lamellar crystallites of size 0.2–0.5 mm with hexagonal morphology. The LDH prepared through the gel to crystallite conversion could be converted into LiAl(OH)4·H2O or LiAl(OH)3NO3·H2O by imbibition of LiOH or LiNO3, respectively, under hydrothermal conditions.Thermal decomposition of LDH above 1400 °C gave rise to LiAl5O8 accompanied by the evaporation of Li2O. LiAl(OH)4·H2O and LiAl(OH)3NO3·H2O decomposed in the temperature range 400–1000 °C to a- or b-LiAlO2.The compositional dependence of the product, the intermediate phases formed during the heat treatment and the possible reactions involved are described in detail. The compounds LiAl5O8 and LiAlO2 are luminescent hosts The gel can be converted directly to crystallites in the presence of an organic solvent owing to instability of the gel caused by when doped with Fe3+, emitting in the red spectral region.1,2 These red-emitting phosphors are useful for artificial illumi- influx of aliovalent ions.The merits of this process over the conventional ceramic processing are the increased homogeneity nation in plant growth applications.2–4 One of the polymorphic forms of LiAlO2, viz. c-LiAlO2, has received much attention of the products and the reduction in the processing temperature.G–C conversion can take place even with coarser gels so because of the possibility of its use as a tritium breeding material in fusion reactors5,6 and as an electrolyte matrix for that the raw materials need not include expensive organometallics or alkoxides. The general reaction involved in this molten carbonate fuel cells.7,8 High surface area a-LiAlO2 is used as a catalyst support9 and for the preparation of the technique is the breakdown of the gel network owing to the ionic pressure caused by the influx of aliovalent ions. delithiated transitional alumina compounds.10 The present work aims to decipher routes to obtain phase-pure LiAlO2 Hydrothermal synthesis is based on supersaturated solvents under elevated pressure–temperature (P–T) conditions; accord- (diVerent polymorphic forms) and LiAl5O8 wherein LDH [LiAl2(OH)7·2H2O] is used as a precursor and also has many ingly the end product may diVer.applications of its own. Layered lithium dialuminium hydroxide, LiAl2(OH)7·2H2O, which is analogous to the mineral Experimental hydrotalcite, [Mg6Al2(OH)16]CO3·4H2O, has received a lot of attention because of its potential application in the field of The principle involved and the experimental details of G–C sensors, and also as antacid, by way of selective sorption of conversion technique have been presented in our previous weak acids (H2S, CO2, etc.,).It also finds uses in ion exchange publications.18–20 Hydrated alumina gel was prepared through for poisonous anions such as [Fe(CN)6]4-, solid-state anion precipitating Al3+ (aq) with 30 mass% ammonia solution, conductors and in catalysis.11–17 Moreover, the compound is washed free of anion contaminants using hot water (tested for a precursor for the preparation of LiAl5O8 and LiAlO2.the absence of sulfate by adding Ba(OH2) solution and for A number of publications exist in the literature on the chloride ions by adding AgNO3 solution to the filtrate) and synthesis and physicochemical properties of these com- suspended in a conical flask containing ethanolic lithium pounds.1–17 Poppelmeir et al.17 prepared LiAl2(OH)7·2H2O by hydroxide.The presence of anionic contaminants such as the insertion of LiOH into Al(OH)3 and studied the phase SO42- and Cl- impedes the reaction.The reaction vessel was relations and stability of the compounds at diVerent tempera- fitted with a water-cooled condenser and an alkali guard tube ture regimes. Their papers do not deal with the phases stabilised to prevent the ingress of CO2 and refluxed for 5–6 h at 80 °C at temperatures >1200 °C and the possible conversion of while continuously stirring using a magnetic stirrer.The solid LiAlO2 to LiAl5O8. The diYculty in the imbibition technique product obtained was washed free of unreacted LiOH and airto maintain the Li2O/Al2O3 molar ratio, often leads to the dried in a desiccator. The lithium content in the washings was presence of unreacted reactants and phase purity is diYcult to monitored to measure the extent of reaction and to compare attain. Most of the literature is on the ion exchange, intercal- the composition of the solid product obtained via the wetation products and the possible applications thereof.We chemical route (AAS). prepared LiAl2(OH)7·2H2O through the novel route of gel to Hydrothermal preparation was carried out in a Teflon crystallite (G–C) conversion18–20 as well as via a hydrothermal (PTFE)-lined Morey-type autoclave SS318.The autoclave was method,21–23 which acted as a precursor for the preparation of charged with the reactants, viz. hydrated alumina gel mixed LiAlO2 and LiAl5O8. The advantages of the G–C conversion with LiOH in the desired molar ratio; deionised water was include procedural simplicity and economy of the method as added to the required percentage so as to autogenerate pressure in the range 5–40 MPa and heated in the temperature range the starting materials are cheap water-soluble inorganic salts.J. Mater. Chem., 1997, 7(10), 2131–2137 2131100–240 °C for 12 h. The temperature was varied to check the phase stability region. Hydrothermal imbibition was performed by charging the autoclave with previously prepared LDH and LiOH or LiNO3 in the desired molar ratio.Imbibition was carried out at 140 °C for 12 h. The chemical compositions of the products were determined by wet chemical analyses using atomic absorption spectroscopy (AAS). Thermal analyses were performed on a simultaneous thermogravimetry–diVerential thermal analysis (TG–DTA) instrument from Polymer Laboratory STA-1500 at a heating rate of 5 °C min-1.Phase identification of the powders was carried out by X-ray powder diVraction using a Scintag/USA diVractometer. IR absorption spectra were recorded on a BIORAD FTIR spectrometer in the range 4000–400 cm-1. Solid-state 27Al MAS NMR spectra were obtained at 78.2 MHz using a high-resolution NMR spectrometer (BRUKER 300 MHz) at room temperature fitted with a magic angle spinning probe (MAS) for rotating the sample at a frequency of 7 kHz. 27Al MAS NMR chemical shifts (d) were referenced to 1 M Al(H2O)6Cl3 (d=0). The morphology and the particle size were determined by the intercept method on the micrographs obtained from a JEOL 200 CX, 200 kV transmission Fig. 1 Variation of Li2O/Al2O3 molar ratio in the product as a electron microscope (TEM) having 2 A ° resolution.function of initial Li2O/Al2O3 molar ratio in the reaction mixture Results and Discussion Gel to crystallite conversion EVect of initial composition on phase formation. The composition of the solid phases prepared through G–C conversion were dependent on the initial Li2O/Al2O3 mol ratio in the reaction mixture (Fig. 1). It is evident that as the Li2O/Al2O3 ratio is increased in the reaction mixture, the lithium retained in the product is also increased and attained a limiting value of ca. 0.5. Above this concentration, the excess lithium added was washed out. Table 1 gives the chemical composition of the as-prepared samples for various starting ratios and the resultant products on subsequent heat treatment at 900 or above Fig. 2 TG–DTA traces of LiAl2(OH)7·2H2O prepared through G–C 1400 °C. At Li2O/Al2O30.05, the phase formed is pseudo- conversion boehmite which decomposed on heat treatment to give a-Al2O3 and LiAl5O8.For Li2O/Al2O3 ratios between 0.05 and 0.5, a mixture of nordstrandite [Al(OH)3] and A mass loss (16%) between 100 and 200 °C accompanied by a very strong endothermic peak in the DTA is due to the removal LiAl2(OH)7·2H2O was formed.Above a Li2O/Al2O3 ratio of 0.5, a voluminous mass having the chemical composition of structural water. Between 200 and 500 °C, the mass loss is ca. 32% which can be attributed to dehydroxylation. DTA LiAl2(OH)7·2H2O was obtained. The formation of LDH is confirmed by X-ray diVraction (XRD) (Fig. 4, later). Preparing shows a corresponding broad and shallow endotherm centred around 270 °C.A subsequent endotherm at 640 °C a phase of composition LiAl(OH)4·H2O was not possible via this route owing to leaching of LiOH. accompanied by a mass loss of 6% is due to the dehydroxylation of the residual hydroxy groups. Continuous mass loss between 1000 and 1300 °C, and the endotherm which shows a Thermal analyses. Fig. 2 shows thermal analyses traces of LiAl2(OH)7·2H2O prepared by G–C conversion.The sample peak at 1143 °C, is due to the evaporation of Li2O. Samples prepared at higher initial Li2O/Al2O3 ratios (>0.5) show the showed a total mass loss of 53% up to 540 °C in agreement with the literature,17 however the sequence of mass losses same trends as above during thermal analyses. Isothermal mass loss measurements were also carried out at diVer.The TG curve shows that the major mass loss occurred below 540 °C in three steps. The initial mass loss (5%) below various temperatures until a constant mass was obtained and the results are in good agreement with the dynamic mass loss 100 °C is probably due to the loss of physically absorbed water. Table 1 Chemical compositions of the as-prepared LDH and the phases obtained upon calcination above 1400 °C Li2O/Al2O3 molar ratio after calcination at reaction mixture product as-prepared 900 °C 1400 °C 0.05 0.04 pseudoboehmite a-Al2O3+LiAl5O8 a-Al2O3+LiAl5O8 0.19 0.18 Al(OH)3+LiAl2(OH)7·2H2O c-LiAlO2+LiAl5O8 c-LiAlO2+LiAl5O8 0.22 0.2 Al(OH)3+LiAl2(OH)7·2H2O LiAl5O8 LiAl5O8 0.29 0.29 Al(OH)3+LiAl2(OH)7·2H2O (major) c-LiAlO2+LiAl5O8 LiAl5O8 0.5 0.49 LiAl2(OH)7·2H2O c-LiAlO2+LiAl5O8 LiAl5O8 0.72 0.49 LiAl2(OH)7·2H2O c-LiAlO2+LiAl5O8 LiAl5O8 1 0.5 LiAl2(OH)7·2H2O c-LiAlO2+LiAl5O8 LiAl5O8 2132 J.Mater. Chem., 1997, 7(10), 2131–2137measurements. Static mass loss measurements shows a total were observed between 1000 and 400 cm-1. These sharp peaks are attributed to AlO6 groups.24 Dm of ca. 58% between room temperature and 1300 °C.Based on the thermal analyses results, the following reaction scheme can be proposed for the formation of LiAl5O8 from X-Ray diVraction. XRD patterns as a function of calcination temperature are shown in Fig. 4. An oven dried (105 °C) sample LiAl2(OH)7·2H2O: does not show any diVerence in its XRD pattern compared LiAl2(OH)7·2H2O CA 150 °C LiAl2(OH)7+2H2O Dm=17%(1) with as-prepared specimens.The sample annealed at 150 °C shows a similar pattern but with broadened diVraction peaks and diminished intensity. The reason for this is the removal of LiAl2(OH)7CA 500 °C LiAl2O3(OH)+3 H2O Dm=30%(2) interlayer water, a fact confirmed by TG–DTA studies and IR absorption spectra. Further heating of the sample to 250 °C, resulted in a complete change of the diVraction pattern, which 4 LiAl2O3(OH) CA 600 °C 2 ‘Li2Al4O7’+2H2O Dm=7% consists of very broad and weak diVraction peaks corresponding to LiAlO2 with small crystallite size.The change in the (�3 LiAlO2+LiAl5O8) (3) XRD pattern is due to the complete destruction of brucite type layers. As the temperature of calcination is increased to 5 LiAlO2 CA 1400 °C 24 h LiAl5O8+2Li2OF Dm=18% (4) 450 °C, b-LiAlO2 started nucleating, and become a major phase at 600 °C.At 1000 °C the phases stablised are c-LiAlO2 (major) and LiAl5O8 (minor). Upon further increase in tempera- IR absorption spectra. Samples isolated from the isothermal ture, c-LiAlO2 decomposed and the formation of LiAl5O8 was mass loss studies were analysed by IR absorption spectra for favoured.This change is associated with the evaporation of the presence of hydroxy groups in the intermediates. Fig. 3 lithia, Li2O, as detected from the eZuent gas when purged shows the IR absorption spectra of the sample as a function with dry argon while the sample is heated in a tubular furnace. of temperature. The sample heated to 105 °C shows a broad Prolonged heat treatment above 1400 °C yielded monophasic absorption band centred around 3450 cm-1 owing to the LiAl5O8.Further phase separation occurred on raising the OMH stretching frequencies from hydrogen bonded as well as temperature above 1600 °C. The resultant phases are abridged hydroxy groups. Sharp peaks at 1025, 825 and Al2O3(minor) and LiAl5O8. 550 cm-1 and shoulders at 675 and 650 cm-1 are characteristics of AlO6 octahedra.On heating the sample to 250 °C, the Hydrothermal preparation of LiAl2(OH)7·2H2O peak at 3450 cm-1 is considerably reduced in intensity, indicat- The products from the hydrothermal preparative runs are ing that the hydroxy groups are removed from the sample. shown in Table 2. The temperature of the hydrothermal treat- Also, the peak at 1375 cm-1 vanishes, and a doublet appears ment was varied in order to study the conditions of preparation at ca. 1550 cm-1. A new peak was observed at 1200 cm-1. at which the phase is stable. Essentially, our results indicate Also, the number of peaks between 1000 and 500 cm-1 reduces that, along with the composition, P–T conditions have great to one (centred around 550 cm-1). When the sample was influence on the phase stability.LDH is stabilised only if heated at 800 °C, the absorption band at 3450 cm-1 was completely lost indicating the disappearance of all hydroxy groups. On further heating at 1400 °C, sharp multiple bands Fig. 4 XRD traces of the LiAl2(OH)7·2H2O as a function of Fig. 3 IR spectra of LiAl2(OH)7·2H2O as a function of temperature: temperature: (a) 105 °C, (b) 150 °C, (c) 250 °C, (d) 450 °C, (e) 1000 °C and ( f ) 1400 °C (6, c-LiAlO2; *, LiAl5O8) (a) 105 °C, (b) 250 °C, (c) 1000 °C and (d) 1400 °C J.Mater. Chem., 1997, 7(10), 2131–2137 2133Table 2 Results of the hydrothermal preparative runs at three diVerent temperatures as-prepared Li2O/Al2O3 in the reaction mixture 140 °C 180 °C 240 °C 1 LiAl2(OH)7·2H2O b-LiAlO2+c-AlOOH b-LiAlO2 0.5 LiAl2(OH)7·2H2O b-LiAlO2+c-AlOOH b-LiAlO2+c-AlOOH 0.33 LiAl2(OH)7·2H2O+c-AlOOH c-AlOOH c-AlOOH 0.25 c-AlOOH+LiAl2(OH)7·2H2O c-AlOOH c-AlOOH 0.2 c-AlOOH c-AlOOH c-AlOOH Li2O/Al2O30.5, at temperatures below 140 °C.Larger crystallite size (0.5 mm) and better crystallinity were obtained using higher concentrations of LiOH. As the temperature of the hydrothermal preparation was increased above 150 °C, irrespective of the composition, the phases stabilised were b- LiAlO2+c-AlOOH (boehmite) at Li2O/Al2O30.5 and only boehmite at Li2O/Al2O3<0.5.Pure phase b-LiAlO2 was obtained for Li2O/Al2O3=1 at a temperature 240 °C. Fig. 5 shows the X-ray diVraction patterns of LDH prepared through the hydrothermal route at 140 °C. All the diVraction peaks due to the basal planes are split, as a relt of anion insertion in the intermediate layer.DiVraction peaks arising Fig. 6 TG–DTA traces of LiAl2(OH)7·2H2O prepared via the hydrofrom the (330) and (600) reflections are totally absent. Fig. 5 thermal route (inset) shows enlarged portions of (002) and (004) reflections. On increasing the lithium concentration, the extent of splitting planes and the peaks are broadened accompanied by a shift was not very much increased. to higher angles, yet retaining the LDH pattern.Shallow, DTA traces of the hydrothermal products (Fig. 6) show broad endotherms at 250 and 275 °C arise from the removal multiple thermal events in the temperature range 30–400 °C of hydroxy groups attached to the Al and Li in the brucite unlike the G–C prepared sample.Sharp endotherms at 46 and type layers. X-Ray analysis of the heat-treated sample showed 77 °C are due to the desorption of water and endotherms at a completely amorphous pattern. A very broad and shallow 132 and 192 °C arise from the removal of interlayer water. endotherm centred around 950 °C is due to the loss of residual XRD analysis shows diminished intensity of all the basal hydroxy groups; the phases formed thereupon are mixtures of c-LiAlO2 and LiAl5O8. The intermediate phases, on heat treatment, are the same as that of the sample prepared through G–C conversion; above 1400 °C the phase stabilised is LiAl5O8.Table 3 shows TG data of the intermediate products formed. TG curves indicate that the major mass loss occurred below 500 °C.Between 500 and 1050 °C, the mass loss is only 3% owing to removal of residual hydroxy groups. The continuous mass loss above 1050 °C without any thermal events is due to the evaporation of Li2O which is responsible for the formation of LiAl5O8 above 1400 °C, according to reaction (4). Hydrothermal imbibition to prepare LiAl(OH)4·H2O To achieve higher Li2O/Al2O3 molar ratios in the LDH prepared through G–C conversion or hydrothermally, samples were further hydrothermally imbibed with LiOH and LiNO3 at 140 °C.Hydrothermal imbibition of LiOH into the LDH prepared through the hydrothermal route resulted in the formation of a-LiAlO2 and boehmite (c-AlOOH) [Fig. 5(c)]. Further insertion of LiOH created instability in the compound resulting in phase separation into a-LiAlO2 and boehmite.In Table 3 TG analyses data of the LDH prepared through hydrothermal route at 140 °C and the intermediate phases stabilised temperature/°C mass loss (%) phase formed RT–100 °C 6 LiAl2(OH)7·2H2O 100–200 °C 17 LiAl2(OH)7·2H2O (intensity of all the basal diVraction peaks diminished) 200–500 °C 30 very broad peaks corresponding to a-LiAlO2 , amorphous Fig. 5 XRD patterns of LiAl2(OH)7·2H2O prepared via the hydrother- background indication of mal route at 140 °C with initial Li2O/Al2O3 ratios of (a) 1, (b) 0.5 and poorly crystallised second phase (c) sample on further imbibition with LiOH. Inset shows the enlarged 500–1050 °C 3 b-LiAlO2+LiAl5O8 portions of the basal reflections of hydrothermally prepared samples 1050–1300 °C 7 LiAl5O8+c-LiAlO2 (a) (002) and (b) (004).(#, Boehmite; 6, a-LiAlO2). 2134 J. Mater. Chem., 1997, 7(10), 2131–2137contrast, G–C prepared samples allow imbibition. The imbibed IR absorption spectra (Fig. 8) of the imbibed samples show clear diVerences from that of the starting composition in terms sample has a Li2O/Al2O3 ratio of ca. 1, corresponding to the composition LiAl(OH)4·H2O.Fig. 7 shows the XRD patterns of the intensity and sharpness of the absorption bands for the LiOH imbibed sample and splitting of the bands and broaden- of the imbibed samples in relation to the starting composition (G–C prepared). The major diVerence is that some of the basal ing are observed for the LiNO3 imbibed sample. However, the essential pattern is the same, indicating that the basic structure reflections, (004) and (006), are split, whereas splitting is not observed for (002) reflection.The splitting of the XRD peaks remains undisturbed. Both samples show absorption due to hydrogen-bonded, bridged hydroxy groups similar to that of is due to the insertion of LiNO3, LiOH or [Li(OH)2]- ions into the intermediate layer. Splitting is not very pronounced the starting composition.The LiOH imbibed sample shows a very strong and sharp peak at 1380 cm-1 band and the for LiNO3 imbibed samples, whereas LiOH inserted samples show clear splittings. On further increase of LiOH, the phases shoulders at 1618 and 1491 cm-1 almost the same as in the starting material. A gain in intensity of the 1380 cm-1 band is stabilised are LiAl2(OH)7·2H2O and b-LiAlO2.Table 4 summarises the decomposition products of these compositions and due to the intercalation of Li(OH)2- in the intermediate layer. An increase in sharpness as well as the absorption intensity the polymorphic forms of LiAlO2 stabilised at diVerent temperatures. The decomposition product is c-LiAlO2 which is a beyond 1000 cm-1 (characteristics of the AlO6 group) indicate cation ordering in the octahedral sites.In contrast, insertion luminescent host for Fe3+ ions having an emission maximum at ca. 708 nm. of LiNO3 leads to intensification of bands at 1618, 1491 (clearly split) and 1380 cm-1 along with a new band at 1436 cm-1 corresponding to the asymmetric stretching of the NO3- group (n3) while the absorption band due to symmetric stretching (n1) is observed at 1112 cm-1; absorption bands beyond 1000 cm-1 are much broadened. 27Al MAS NMR spectra of diVerent polymorphs of LiAlO2 are shown in Fig. 9.The polymorphs show clear diVerences in chemical shift. The low temperature form (<400 °C) of a- LiAlO2 shows a chemical shift of d 9.5 corresponding to 27Al Fig. 7 XRD tracings of the hydrothermally imbibed samples: (a) asprepared LiAl2(OH)7·2H2O prepared through G–C conversion; (b) imbibed with LiNO3; (c) imbibed with LiOH, LDH/LiOH Fig. 8 IR spectra of LiAl2(OH)7·2H2O: (a) before imbibition, (b) imbibi- =1; (d) imbibed with LiOH, LDH/LiOH=0.5 [LDH= LiAl2(OH)7·2H2O]; #, b-LiAlO2; 6, LiAl2(OH)7·2H2O tion with LiOH and (c) imbibition with LiNO3 Table 4 Phases prepared through hydrothermal imbibition of LiAl2(OH)7 prepared through G–C conversion and the polymorphic forms of LiAlO2 composition of the charge as-prepared 400 °C 500 °C 1000 °C 1300 °C LDH51LiNO3 LiAl(OH)3NO3·H2O a-LiAlO2 b-LiALO2 c-LiAlO2 c-LiAlO2 LDH52LiNO3 LDHa+b-LiAlO2 b-LiAlO2+amorphous b-LiAlO2+LiAl5O8 c-LiAlO2+LiAl5O8 c-LiAlO2+LiAl5O8 background LDH51LiOH LiAl(OH)4·H2O a-LiAlO2 b-LiAlO2 c-LiAlO2 c-LiAlO2 LDH52LiOH LDH+b-LiAlO2 b-LiAlO2+amorphous b-LiAlO2+LiAl5O8 c-LiAlO2+LiAl5O8 c-LiAlO2+LiAl5O8 background aLDH=LiAl2(OH)7·2H2O.J. Mater. Chem., 1997, 7(10), 2131–2137 2135SAED photographs [Fig. 10(c)] show spotty rings indicating the polycrystalline nature of the sample. Hydrothermally prepared samples show lamellar crystals [Fig. 10(b)] with sizes ranging from 0.2 to 0.5 mm and SAED [Fig. 10(d)] shows a spotty single crystalline pattern. The results indicate that G–C produces aggregates of fine particles, whereas hydrothermal treatment yields lamellar individual single crystallites. LiAl2(OH)7·2H2O prepared either through G–C conversion or hydrothermally acts as a precursor for the preparation of LiAl5O8. Conversion is made possible owing to Li2O evaporation at higher temperatures, a fact not previously clarified in the literature.LiAl5O8 was also prepared from a reaction mixture having Li2O/Al2O3=0.22. LiAl5O8 exists in two forms viz. an ordered low-temperature form and a disordered hightemperature form. The ordered form is primitive cubic (P4332) while the disordered form has a true spinel structure (F41/d32m).The order–disorder transformation occurs at 1295±5 °C according to the literature.26 However, samples prepared through the present route did not undergo this type of transformation even at 1600 °C and the phase remained ordered. This indicates that the order–disorder transformation is dependent on the preparative route through which the phase is formed. When fast cooled (600 °C min-1), the samples were more disordered as shown by the decreased intensity of (100) and (110) reflections in comparison with (111) and (220) reflections.Monophasic LiAlO2 was prepared hydrothermally and also by the imbibition of LiAl2(OH)7·2H2O prepared through G–C conversion by LiNO3 or LiOH. The structure of the LDH consists of the positively charged brucite-like layers [LiAl2(OH)6]+ bridged by interlayer anions as well as water molecules.Depending upon the radius of the anion in the intermediate layer, the width of the interlayer varies. The existence of this intermediate layer is responsible for the unique properties of the material viz. anion exchange and intercalation, Fig. 9 Solid-state 27Al MAS NMR of (a) a-LiAlO2, (b) b-LiAlO2 and (c) c-LiAlO2 which makes preparation of LiAlO2 possible.LiAlO2 exists in diVerent polymorphs a-, b-, and c-LiAlO2, which diVer in the structure and cation coordination as established by solid-state NMR. The low-temperature form, a-LiAlO2, is stable below in an octahedral site. By contrast, b-LiAlO2 shows a chemical shift of d 78.6 and the c-form shows a minor splitting at d 70 400 °C and is prepared via hydrothermal imbibition.The bform is stable in the temperature range 500–650 °C prepared and 66. 27Al chemical shifts in the range d 50–85 are characteristic of 27Al in tetrahedral sites.25 The diVerence between the by heating the a-form at 500 °C or by hydrothermal synthesis at 240 °C. The compound c-LiAlO2 is prepared by heating b and c polymorphs is that the latter shows a clear splitting indicating site occupancy of 27Al in two diVerent types of the b-form at 1000 °C and is very stable in contrast to the material prepared via the sol–gel method11 which decomposes tetrahedra, one of which may diVer from the other in the degree of distortion.around 600 °C to LiAl5O8 and c-LiAlO2. The above results indicate that the two preparative techniques, G–C conversion and hydrothermal treatment, are Conclusions potential routes to synthesise LiAl5O8 and LiAlO2.The basic reactions in both the methods are the same; however the Gel to crystallite conversion and the hydrothermal method preparation conditions diVer. Continuous influx of aliovalent can be easily adapted for the preparation of important mateions into the gel network in the presence of hydrophilic solvents rials such as LiAl2(OH)7·2H2O, LiAl(OH)4·H2O, LiAl5O8 and such as ethanol will bring about rapid disintegration of the LiAlO2.The products formed are superior, in terms of homogel network. Hydrophilic solvents prevent H2O molecules re- geneity and phase content, to those prepared by conventional entering into the gel network and, thereby impede the reaction.solid-state methods. The samples prepared through G–C conversion always have very low crystallite size (0.04–1 mm), whereas hydrothermally prepared samples yield higher crystallite sizes (0.2–0.5 mm). References LDH [LiAl2(OH)7·2H2O] prepared through the two routes 1 N. T. Melamed, F. de S. Barros, P. J. Viccaro and J. O. Artman, diVer in their characteristics although the decomposition prod- Phys. Rev.B, 1972, 5, 3377. uct is the same both cases. The discrepancy may be due to the 2 J. G. Rabatin, J. Electrochem. Soc., 1978, 25, 920. particle size and cation distribution. The diVerences in the 3 J. Van Broekhoven, Illuminating Engineers Society Meeting, New particle morphology and crystallite size are further established York, July 1, 1973. 4 M. W. Parker and H. A. Borthwick, Plant Phys., 1949, 24, 345. by TEM studies. 5 B. Rasneuer, Fusion T echnol., 1985, 8, 1909. Fig. 10 shows TEM micrographs and corresponding selected 6 J. Jimnez-Beceril, P. Bosch and S. Bulbulian, J. Nucl.Mater., 1991, area diVraction patterns (SAED) of LiAl2(OH)7·2H2O pre- 185, 304. pared via G–C and hydrothermal routes. The morphology 7 A. J.Appleby and F. R. Foulkes in Fuel Cell Handbook, Van and crystallite size are diVerent in each case. The sample Nostrand Reinhold, New York, 1989, p. 297. prepared by G–C conversion shows [Fig. 10(a)] fibrous needle- 8 K. Kinoshita, J. W. Sim and J. P. Ackerman, Mater. Res. Bull., 1978, 13, 445. shaped aggregates, with particle sizes in the range 0.04–0.1 mm. 2136 J. Mater. Chem., 1997, 7(10), 2131–2137Fig. 10 TEM micrographs of LiAl2(OH)7·2H2O prepared through (a) G–C conversion, (b) hydrothermal route, the white patches are due to the defect in the carbon grid used to hold the sample; (c) and (d) are SAED patterns of the samples prepared through G–C conversion and the hydrothermal method, respectively 9 K. R. Poeppelmeier, C. K. Chiang and D. O. Kipp, Inorg. Chem., 20 T. R. N. Kutty, V. Jayaraman and G. Periaswami, Mater. 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Kutty and P. Padmini, Mater. Chem. Phys., 1995, 39, 200. 19 T. R. N. Kutty and M. Nayak,Mater. Res. Bull., 1995, 30, 325. Paper 7/02065A; Received 25thMarch, 1997 J. Mater. Chem., 1997, 7(10), 2131–2137 21