J. MATER. CHEM., 1994, 4(12), 1875-1881 Wet Chemical Syntheses of Ultrafine Multicomponent Ceramic Powders through Gel to Crystallite Conversion P. Padmini and T. R. Narayanan Kutty Materials Research Centre, Indian Institute of Science, Bangalore -560 012, India Coarse B"+O,,,-xH,O (10 <x <120) gels, free of anionic contaminants react with A(OH), solutions under refluxing conditions at 70-100°C giving rise to nanoparticles of multicomponent oxides (A=Ba, Sr, Ca, Mg or Pb; B=Zr, Ti, Sn, Fe, Al or Cr). These include ABO, perovskites and their solid solutions, polytitanates, hexaferrites and related phases, aluminates with spinel or tridymite structure and chromates. The nanosized crystallites are often in metastable phases, such as cubic BaTiO, at room temperature or superparamagnetic hexaferrites.Through the same route, luminescent phosphors of aluminates doped with rare-earth metals could be prepared. The present results indicate the general features of the gel-crystallite (G-C) conversion involving the instability of the metal hydroxide gel brought about by the disruption of the ionic pressure in the gel as a result of the faster diffusion of A2+ ions through the solvent cavities within the gel frame work. This is accompanied by the splitting of the bridging groups like B-(OH)-B or B-0-6, leading to the breakdown of the gel into crystallites. G-C conversion has advantages as a method of synthesis of ceramics in terms of operational cost and procedural simplicity. Wet chemical synthesis (Table l)i-3of ultrafine ceramic pow- ders continues to be a subject of intense research activity as the products exhibit advantages over powder derived from conventional ceramic routes.The main advantages of these processes are the increased homogeneity and high surface area of the resulting powders which lead to relatively high reactivity and hence low sintering temperatures. Sol-gel pro- cessing is the most widely employed route and involves a colloidal sol that is converted into a gel through ageing. The gel is subsequently calcined, giving rise to crystalline prod- UC~.~-~The powder characteristics, such as particle size, par- ticle shape, crystallinity, phase content, surface area and purity, are mostly dependent on the conditions of ~alcination.~ As an alternative method, the gel can be converted directly to crystallites, even in the presence of solvent, because of the instability of the gels caused by the influx of alien ion^.^,^ The perovskite titanates and their solid solutions have been pre- pared by this method.$ This technique differs from the sol-gel process in that high temperature calcination is not necessary for the formation of multicomponent powders.G-C conver-Table 1 Different chemical processes for the preparation of multicom-ponent oxide systems I. synthesis from complex precursors (thermal decomposition): (a) oxalate route (h)citrate route (c) catecholate route (d) acetate route (t')complex cyanide route 11. (a) co-precipitation (b)freeze-drying 111.evaporative decomposition: (a) spray pyrolysis (b) liquid mix process IV. sol-gel processing: (a) mixed alkoxide route (b) carboxy-alkoxide route (c) hydroxide-alkoxide route V. hydrothermal synthesis VI. gas-phase reactions (plasma or laser technique) VII. self-propagating combustion sion can take place even with coarser gels so that the raw materials need not include expensive organometallics or alkox- ides. G-C conversion is not restricted to titania gel but can be extended to other metal hydroxide gels and ceramic powders of multicomponent oxides, e.g. ferrites, aluminates, zirconates, polytitanates. The general reaction involved in this technique is the breakdown of the gel network because of the change in ionic pressure brought about by the chemical influx of aliovalent ions.It is generally thought that the reactive gels need be prepared from alkoxides or organometallics, but the present results show that coarser gels prepared from any soluble compound can be equally reactive provided there are no interfering anionic contaminants and that the gel is not aged. In comparison to all the known wet chemical techniques, this method has advantages in operational cost and procedural simplicity. Experimental Gels of the hydrated metal hydroxides B"+(OH),*xII,O or B"+O,,,.xH,O (where B=Ti4+, Zr4', Sn4+, A13+. Fe3+, Cr3+ etc.) were prepared by the addition of ammonium hydroxide at 30-40 "C to the corresponding chloride, nitrate or sulfate solutions until the pH was 6-8.The gels were washed free of the anions and ammonium ions. The presence of anionic contaminants such as SO,,-, NO3-or C1-impedes the reaction. No special care was taken to control the particle size of the gel. The extent of water content in the gel was determined quantitatively through weight loss measurements by thermo- gravimetric analysis. The gel was suspended in A(OH), solu- tion, where A =Ba2+, Sr2+, Pb2+, Ca2+ etc., in a flask fitted with a water-cooled reflux condenser. Air in the vessel was displaced by nitrogen and entry of carbon dioxide prevented by an alkali guard tube. The reaction was carried out at 70-100°C for 0.5-4 h. The solid phase remaining in the reaction vessel was filtered off, washed free of A(OH)2and air dried.Unlike the gels, the solid phase formed was crystalline, as identified by electron or X-ray diffraction. The solid phase is not crystalline to X-rays when the concentration of A(OH), is <0.02 mol I-'. The kinetics and mechanism involved in the reaction will be dealt with in a later paper. Phase identification of the powders was carried out by X-ray powder diffraction using a Philips PW 1050/70 J. MATER. CHEM., 1994, VOL. 4 diffractometer with a step scan facility. Particle size and shape were evaluated by the intercept method of micrographs from the transmission elecp-on microscope (TEM) (JEOL 200 CX, 200 kV) having a 2 A resolution. The chemical compositions of the products and any contaminants were determined by wet chemical analysis using atomic absorption spectroscopy (AAS). Mossbauer spectra were recorded at constant acceler- ation in conjunction with a Nuclear Data Instruments ND60 multichannel analyser using a 57C0 source in a rhodium matrix.The experimentally observed Mossbauer spectra were curve-fitted by a least-squares method by computer, assuming Lorentzian lineshapes. The photoluminesence (emission and excitation) spectra were recorded at room temperature using a Hitachi 650-10s fluorescence spectrophotometer equipped with a 150 W xenon lamp and a Hamamatsu R928F photo-multiplier. Thermal analyses were performed on a simul-taneous TG-DTA instrument from Polymer Laboratory, STA 1.500.Results During the initial stages of the reaction a considerable decrease in gel volume was brought about by the disintergration of the coarse gel by the influx of A'+ ions. This decrease in volume was observed only when there was chemical interaction between the A2+ and B"+O,!,*xH,O gel. Otherwise the gel remained as a sticky mass with little volume change. As the reaction proceded, the gel lost its appearance and was con- verted into a flowing powdery mass. The amount of solvent entrapped in the gel was ca. 95% by weight, as it is for most metal hydroxide gels. The relatively open network of gels filled with water may form enhanced diffusion paths for A2+ ions to penetrate the reaction sites. The gel stability could be altered by the use of an organic solvent such as ethanol or acetone. Powders prepared in ethanol exhibited a more uniform particle size distribution. The multicomponent oxide phases prepared during the current investigations are listed in Table 2.These systems were chosen because of their wide-ranging applications. Titanates, Zirconates and Stannates The preparation of the perovskite phases has been reported.8 The perovskite phases prepared by G-C conversion are frequently metastable; e.g. BaTiO, has cubic symmetry at Table 2 Phases formed at 100 "C through G-C crystallite conversion titanates BaTiO,, SrTiO,, PbTiO,, (Ba,Sr)TiO,, (Ba,Pb)TiO,, (Ba,Ca)TiO,, Ba(Zr,Ti)O,, Ba(Sn,Ti)O, (1.05 :1.0 in all cases), BaNd,Ti,O,, (1.05:2.0 :5.0), Nd,Ti,O,, (4:9) zirconates BaZrO,, SrZrO,, (Ba,Sr)ZrO,, Ba(Ti,Zr)O,, Sr(Ti,Zr)O, (1.05 :1.0 in all cases) Ba polytitanates BaTi,O,, +TiO, (1 :6) BaTi,O,, +BaTi,O, (1 :4.5) BaTi,O, (1 :3) Ba,Ti,,O,, (1:2) BaTi,O, (1 :1.5) BaTiO, +BaTi,O, (1:4) [in presence of Mg2+] ferrites BaFe,,O,,, SrFe,,O,, (1.05:6.0 in both cases), BaCo,Fe,,O,, (1.05:2.0: KO), Ba,Ni,Fe,80,, (2.05 :2.0: 14), Ba,MnzFe3,06, (4.05 :2.0 :18) aluminates SrAl,O,, BaA1,0,, CaAl,O, (1.05 : 1.0 in all cases), BaAll2Ol9, SrA1,,0,, (1.05:6.0 in both cases), BaMgAl,,O1, (1.05: 1.0:5.0), BaMg,Al,,O,, (1.05 :2.0 :8.0), Sr,,,Mg,Al,,O,, (5.5:6.0:22.5) chromates BaCr,O,, SrCr,O, (1.05 :1.0 in both cases) The molar ratios in the reaction mixtures are in brackets.room temperature, [Fig.l(a)]. This is true also for titanate solid solutions, even when strongly structure-distorting ions such as Pb2+ or Ca2+ are present. Chemical analyses were carried out to confirm the purity of the resultant phases. The chemical composition of the annealed products have 58 wt.% Ba and 20 wt.% Ti for BaTiO, and 49 wt.% Ba and 32 wt.% Zr for BaZrO, powders, roughly corresponding to 1:1 mole ratios in both cases. Chemical tests of the solutions were performed at different stages of the reaction after ultracentri- fugation in order to detect the B component. These tests were negative. As the temperature of the reaction was 6 100cC, the possibility of dissolution of the B"+On,2*xH20 gel in A(OH)2can be excluded. Moreover, if the dissolution-crystal- lisation process prevailed, definite morphological features of the crystallites would be expected, which is not observed for these products formed at such low temperatures and 1 atm pressure.Hence a G-C transformation may be envisaged where the structural rearrangement caused by A2+ ions within the gel is followed by dehydration. As dehydration is slower at lower temperatures, there is a partial retention of OH- and H20 in the structure. The presence of such network- modifying impurities enhances structural disorder so that the cubic phase remains metastable." The particle size decreases with increasing Ba(OH), concentration or A :B ratio. The extent of aggregation depends upon the initial Ba(OH)2concentration. Crystallites formed from TiO, xH20 and 0.4 mol 1-' Ba(OH), are nearly spherical and are 5-15 nm in diameter.The degree of aggregation is greater for BaTiO, formed from higher A :B ratios and Ba(OH), concentrations.* The particle size ranges from 0.5 to 5 nm. Only H,O was evolved during heating. Heating the sample to nearly 1200"C for 2 h in air, and subsequent cooling to room temperature, yields tetragonal BaTiO,. TG of this sample shows ca. 4-5% weight loss at 200-300 "C. The weight remains nearly constant up to 1200°C. The same trend is observed in the case of zirconates, ferrites and aluminates. Semiconducting BaTi0, ceramics prepared by this route were found to give excellent PTCR (positive temperature coefficient of resistance) characteristics, possibly because of the absence of alkali, which is commonly retained in aged gel products.* This clearly shows that the products obtained by G-C conversion are phase-pure. For A :B <0.67, Ba polytitanates were formed, as shown in Table 2.The actual phase stabilized depends upon the A: B molar ratio and not the effective concentration of A(OH),. It can be seen from Table 2 that the A:B ratio originally maintained in the reaction mixture does not coincide with that of the corresponding solid phases recovered because minor amounts of A(OH), remain unreacted and are finally washed out. Polytitanates such as BaTi,O, and Ba,Ti,O,, are of considerable interest for microwave dielectric appli- cations, particularly as microwave resonators." Fig.1 (b) shows the X-ray diffraction pattern of BaTi,O, formed by this route. This polytitanate has a relative permittivity of ca. 40 at 10 GHz.12Ternary oxides, such as BaNd2Ti5014, have also been synthesized by the present method, with a reported relative permittivity of ca. 80 at 10 GHz.', Impurities play an important role in the stabilisation of the phases. For example, when Mg2+ is present as an impurity, a mixture of BaTiO, and BaTi,O, is obtained with Ba :Ti z 1:1 in the starting composition. When Ba:Ti=1:4 and 5-10 mol% of Mg2+ is present, the only phase formed is BaTi,O,. However, incorporation of Mg2+ into the titania framework was found to be impossible so that ilmenite, MgTiO,, which has 6 :6 coordination, is not formed by this route.Ferrites Reactions were carried out with iron(r1r) hydroxide gels and A(OH), for various compositions indicated by the phase J. MATER. CHEM., 1994, VOL. 4 28/degrees Fig. 1 X-Ray diffraction pattern of (a) BaTiO, exhibiting cubic symmetry (metastable) at room temperature and (b)BaTi,O, diagram of the BaO-MeO-Fe,O, systems14 (Fig. 2) where Me=Mn, Fe, Zn, Co, Ni or Mg. The compositions of the various ferrites are given by the points M, W, X, Y, Z and U in the phase diagram. These hexagonal ferrites with end- member compositions of BaFe12019 or SrFel,Olg are well kno~n.'~~'~The Fe3+ ions occupy five non-equivalent lattice sites in interstices between the oxygen ions. The compositions of the other ferrites are listed in Table 2.The as-prepared powders are found to be X-ray amorphous but crystalline by electron diffraction. The particle size calculated by the linear intercept method from the micrograph [Fig. 3(a)] is of the order of 3-10 nm and the corresponding electron diffractog- 50 50 BaO Me0 Fig. 2 Phase diagram of BaO-MeO-Fe,O, system where Me stands for divalent metals such as Mn, Fe, Zn, Co, Ni or Mg and the points M, W, X, Y, Z and U corresponds to the compositions; M, BaFelzO19; W, BaMe,Fe,,O,,; X, Ba,Me,Fe,,O,,; Y, Ba,Me,Fe,,O,,; Z, Ba,Me,Fe,,O,,; and U, Ba,Me,Fe,,O,, ram [Fig. 3(b)] shows a ring pattern that is characteristic of a mostly polycrystalline material with ultrafine particle size. Ultrafine particles of ferromagnetic and ferrimagnetic mate- rials often exhibit superparamagnetic behaviour, as shown by the Mossbauer spectra of Ba ferrite, Fig. 4(a).The superpara- magnetic state refers to single-domain particles exhibiting a uniform spontaneous magnetisation.'6-'8 The isomer shift, which gives an idea of the chemical bonding, was calculated to be 0.32. Superparamagnetic behaviour was found to persist for the powders even after they had been heat treated at 400-500 "C for 96 h. X-Ray crystallinity was achieved only when the powders were heat treated at temperatures >950 "C: their Mossbauer spectra exhibit multiplets characteristic of Ba ferrite [Fig. 4(b)]. For each hyperfine component, five extra lines are seen, which correspond to five different crystal sites occupied by Fe3+ ions.Thus the microscopic and Mossbauer studies clearly point to the fact that the G-C conversion route gives rise to nanosized ceramic pobders in a metastable state. Aluminates Various aluminate phases formed through the G-C conver-sion route are listed in Table 2. Generally they fall into three structural types: (1)hexa-aluminates," AAl12019; (2) AA1,04, where A =Ba2+, Sr2+ etc., which are related to the distorted 'stuffed' tridymite structure;20$21 (3)hexa-aluminate derivatives with differences in the stacking sequence of the spinel blocks. Phases with distinguishable X-ray patterns can be prepared by varying the A:Al ratio. Unlike the titanate or zirconate systems, the Mg2+ ions can be incorporated in the aluminium- oxygen network because of the similar co-ordination of MgO, and AIO, tetrahedra.AAl,O, phases are formed only at higher concentrations of A(OH),. The distorted tridymite lattice retains the A2+ ions within the open channel which is along the pseudo-hexad axis. Compounds of general formula H15nm Fig.3 TEM micrographs of (a) as prepared powders of Ba-ferrite with particle size of the order of 3-10nm and (h) the selected area diffraction pattern of (u) A,A1203+n (n< 1) have been reported by hydrothermal prep- arations.22 However, under hydrothermal conditions, the pre- dominant stable phase has n = 1, i.e. AA1,0,. This phase does not show any tendency to take up Mg2+ ions, possibly because of the large size of the open channels, whereas the incorporation of Mg2+ ions in hexa-aluminate derivatives, e.g.BaMgAlloO1,, does take place. The changes in the X-ray diffraction pattern between the end-member SrA1,,0,,, and BaMgAlloO,, are shown in the Fig. 5. The crystallite size of the aluminates lie in the range 10-25 nm. Boric oxide is usually added in minor concentrations (< 2 wt.%) for attaining better crystallinity through particle size enhancement for al~rninates.~~Hence small amounts of B203 were introduced in the reaction mixture during the synthesis. By this method, improved crystallinity is observed for the aluminates (particle size 30-1 10 nm). The most significant feature of the aluminates prepared by G-C conversion is their ability to retain rare- earth-metal ions which act as luminescent hosts for indepen- dent ionic luminescent centres.This is typically illustrated with Eu2+ in SrA1,,0,, and by multiple dopants, e.g. Ce3++ Mn2+ +Tb3+ in BaAl12019 after reduction in an N,-H, atmosphere at 950 "C. The single-band luminescence J. MATER. CHEM., 1994. VOL. 4 (b1 I I1 1 I I1 I1 I I1 I1 I Ill 1 108642024 6810 velocity/m m-' Fig. 4 Mossbauer spectra of Ba-ferrite (a) as prepared powder and (b)heat treated at 950 "C spectrum of Eu2+ in SrAll,Ol9 is shown in Fig. 6. The excitation band is maximum around 290 nm and the emission band around 398 nm. The (4f)6(5d)' excited state of Eu2+ shows considerable relaxation effects in hexa-aluminates, as indicated by the large Stokes shift. In the case of other aluminates, AAln03 + n, Eu2+ luminescence is dominated by the green band and the 398 nm band.22 Ce3+ acts as a good sensitizer in most aluminates.This is illustrated with BaAl120,, co-doped with Ce3+, Mn2+ and Tb3+ in Fig. 7. This material has multiple excitation bands corresponding to the 5d-4f state of Ce3+ with various ionic pairs such as Ce3+-Ce3', Ce3+-Mn2+(Tb3+).The Ce3+-sensitized lumin- esence (i,,,= 270 nm) gives rise to a strong green band around 520nm. This emission arises from Mn2+ in the tetrahedral site. Along with the Mn2+ emission band, those of Tb3+ can also be observed. However, when the ibex,is shifted to longer wavelength (310 or 340 nm), the emission band due to Tb3+ alone is observed.These results show that efficient luminescent phosphor materials can be prepared by the G-C conversion route. Chromates The chromates listed in Table 2 were also synthesized through the G-C conversion route. They were found to be crystalline, with orthorhombic symmetry.20 The particle size of the chro- mates ranged from 20 to 150 nm. Discussion The present results show that G-C conversion is a general technique for the preparation of multicomponent oxides as nanosized particles, provided the gel is reactive. Results from various systems indicate that the first phase formed, is in general, metastable because of the retention of the OH-in the structure. Metastability is observed even in the case of the ferrites wherein the as-prepared powders exhibit superpara- magnetic behaviour.The basic reactions involved in hydro- thermal and G-C conversion are the same, except that J. MATER. CHEM.. 1994, VOL. 4 Cu-Ka I-$ I I I I I I 1 I 1 I 24.2 38.4 52.6 66.8 28Ydegrees Fig. 5 X-Ray diffraction pattern of (a)SrA1,,0,, and (b)BaMgAl,,O,, 100 N+100 i ii 80 80;;i 3 h .-5.z 60 E 60 C0, Q,c c C .-C .-.-P .-9 c c. aQ3 40 5 40 2020 0 200 300 400 500 6000 200 300 400 500 Unm NVnm Fig. 7 Excitation and emission spectra (300 K) of BaAl,,O,, doped Fig.6 (u) Excitation and (b)emission spectra (300 K) of SrAlI2Ol9 with Ce3+, Mn2-and Tb3+.(u) i.,,=520 ntn (Mn”); (b) &,= 398 nm and i.,.=290 nm 270 nm; and (c) Ewe, =3 10nmdoped uith Eu”; iem= m~nocrystallites~"~~are produced in the former route whereas the latter method gives rise to polycrystallites.Metal hydroxide gels are in general polymeric chains, forming an entangled network in which the solvent is entrapped. The stability of the gel is dependent on three factors:26( 1) polymer-polymer affinity; (2) rubber elasticity; and (3) the ionic pressure. The sum of these three components is the osmotic pressure which determines whether the gel will take up fluid or expel it. If any one of the factors is altered, the gel becomes unstable and the gel network collapses irreversibly. In the present context, it is the ionic imbalance that contributes to the collapse of the gel network. The polymer network tends to ionise, giving rise to H+ or OH- depending upon the nature of the chemical species in the network.Whether H+ or OH-is dominant will depend on the pH at which the gel stabilises. For B"+O,,,.xH,O gels, ionisation leads to OH- ions, which are able to move around the entrapped solvent within the gel and contribute a pressure similar to that of the randomly moving gas molecules. Electrically they are neutralised by the positive charge retained by the polymer network. The rapid influx of aliovalent ions such as A2+ into the gel cavities containing entrapped solvent can upset the charge balance. Hence the system has to rearrange itself to maintain neutrality. Since there is a continu- ous influx of ions, the interactive stability between the network and the solvent breaks down.The imbalance in the electrical charge has to be countered by an appropriate amount of H+ or OH-. Under such a continuous influx of A2+ ions it becomes difficult to maintain the charge balance and hence the gel collapses. The collapse of a gel can be brought about at constant temperature and constant solvent composition by changing the pH of the solution or adding a salt. Both of these factors can alter the effective ionisation of the polymer network. The influx of A2+ can couple with the ionic charges on the framework. The presence of anionic contaminants, OH J. MATER. CHEM.. 1994, VOL. 4 such as SO4,-, NO,-or C1-, can impede this reaction. These anions may try to couple with the H+ as well as A*+ and maintain neutrality. If this happens, the gel persists.Hence the presence of such ions impedes G-C conversion. Divalent ions in low concentration are more effective in bringing about gel collapse than monovalent ions. Splitting of bridging groups such as B-(0H)-B and B-0-B, accompanied by chemical rearrangements are possible as the gel shrinks. This causes the splitting of the coarser gel into finer particles so that the reaction rate is increased by the influx of A:+ ions. This results in polyhedra that are free of solvent, giving rise to a stable multicomponent oxide lattice. The process is illustrated in the case of TiO, .xH,O gel which is converted directly into ATiO, crystallites by the influx of A2+ ions (Fig. 8).It is inevitable that OH-ions are retained randomly in the place of 0,-sites. Bridging hydroxy groups escape the crystallites only after heat treatment. The basic mechanism involved in the formation of the multicomponent oxides may be the de- olation of the bridging groups such as B-(OH)-B and B-0-B (because of the increase in pH within the gel) followed by oxolation, leading to the formation of Bn+O,,,, which are charge compensated by the A2+ ions to form a stable multicomponent oxide lattice. The presence of hydrophilic solvent such as an alcohol or acetone can bring about a faster reaction because of their affinity for H,O, so that the water molecules from the gel do not get back to the gel network. The stabilit? of the gel is solvent dependent.However, this is not the prime factor in the present case, since the presence of such solvents without A(OH), does not lead to the disintegration of the gel network. Conclusions A disordered framework can give rise to an ordered crystalline lattice because of the decrease in the free energy of the system. Fig.8 Reaction mechanism involved in the formation of ATiO,, where A=Ba2+ or Sr2+; 0,H,O; 0,OH-; 0,Ti4+:0,A2+ J. MATER. CHEM., 1994, VOL. 4 However, it tends to result in metastable phases. Results from various systems show that G-C conversion can be easily adapted for the preparation of multicomponent oxide systems. References 1 P. P. Phule and S. H. Risbud, J. Muter. Sci., 1990,25, 1169. 2 A. D. Hilton and R.Frost, in Electronic Ceramic Materials, ed. J. Nowotny, Trans Tech Publications, Switzerland, 1992, p.145. 3 J. Twu and P. K. Gallagher in Properties and Applications of Perovskite-type Oxides, ed. L. G. Tejuca and J. L. G. Fierro, Marcel Dekker, New York, 1993, p.1. 4 L. L. Hench and D. R. Ulrich, Ultrastructure Processing of Ceramics, Glasses and Composites, Wiley-Interscience, New York, 1984, p.43. 5 C. W. Turner, Am. Ceram. Bull., 1991,70, 1487. 6 C. J. Brinker and G. W. Scherer, J. Non Cryst. Solids, 1985,70,31. 7 G. Tomandl, H. Rosch and A. 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Goodwin, Acta Crystallogr., Sect. B, 1975, 31,2940. 20 F. P. Glasser and L. S. Dent Glasser, J. Am. Ceram. Yoc., 1963, 46, 377. 21 H. D. Megaw, Crystal Structures, A Working Approuch, W. B. Saunders, London, 1973, p.312. 22 T. R. N. Kutty, R. Jagannathan and R. P. Rao, Muter. Res. Bull., 1990,25, 1355. 23 B. Smets, J. Rutten, G. Hoeks and J. Venlijsdonk, J. Ektrochem. Soc., 1989, 136, 2119. 24 T. R. N. Kutty, R. Vivekanandan and S. Philip, J. Rlater. Sci., 1991,25,3649. 25 T. R. N. Kutty, R. Vivekanandan and P. Murugaraj. Muter. Chem. Phys., 1988, 19, 533. 26 T. Tanaka, Sci. Am., 1981,244, 110. Paper 4/01268B; Receizjed 2nd March, 1994