Synthesis, characterization and dielectric properties of nanometer-

sized barium strontium titanates prepared by the polymeric citrate

precursor method

Padam R. Arya, Pika Jha and Ashok K. Ganguli*

Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India.E-mail: ashok@chemistry.iitd.ernet.in

Received 27th May 2002, Accepted 28th November 2002

First published as an Advance Article on the web 11th December 2002

Barium strontium titanate (BST) powders of the formula Ba12 xSrxTiO3 (0 ¡ x ¡ 1) have been prepared for

the first time by the polymeric precursor route using citric acid and ethylene glycol. Pure BSTs were obtained

at 500 uC. These oxides were found to have the cubic structure, which is retained even after heating at

800 uC. Detailed X-ray studies on samples sintered at 1100 uC show weak tetragonal distortion for BaTiO3,

while the other BSTs retain their cubic structure. The particle size of the sintered oxides increases from 55 nm

for BaTiO3 to 88 nm for SrTiO3, from X-ray line-broadening studies. The nano-sized grains are reasonably

stable to sintering (the particle size for BaTiO3 changes from 25 nm at 500 uC to 55 nm at 1100 uC). Thedielectric constant of the sintered oxides decreases from 510 for BaTiO3 (x ~ 0) to 190 for SrTiO3 (x ~ 1) at

100 kHz. The dielectric loss decreases from 0.05 for BaTiO3 to 0.001 for SrTiO3 at 100 kHz. No ferroelectric

transition was observed in either the dielectric studies or by differential scanning calorimetry.

Introduction

Barium titanate and related compounds have been extensivelyused in the preparation of high dielectric constant capacitors,PTC resistors, transducers and ferroelectric memories.1 BaTiO3

is seldom used in its pure form, but rather combined withspecial additives to enhance the dielectric properties. Sr dopingin BaTiO3 has been one of the important areas of research inelectronic materials over the last few years. It has been foundthat barium strontium titanates (BSTs) have applications intunable filters, oscillators and phase elements for directionalphase array antennas.2 Due to their high dielectric constants,BSTs have the potential to be included in very large scaleintegrated circuits, such as dynamic and non-volatile randomaccess memories (DRAM). As a result, BST compounds arenow being investigated with regard to various electronicapplications.Traditionally, BSTs are prepared by the ceramic method, in

which the carbonates of barium and strontium, and titaniumoxide are calcined at high temperature (1200 uC).3 However,the ceramic method is not very useful for the preparationof high performance ceramics because the materials havevery large particle sizes, are non-homogeneous and have highimpurity contents. BST powders can be prepared by variouslow temperature methods. Among others, the sol–gel synthesis4

and oxalate precursor5 methods provide good alternatives tothe conventional methods. The preparation of Ba12xSrxTiO3

(x ~ 0.0–0.3) powder with particle sizes of 10–40 nm has beenreported by Noh et al.6 They prepared BSTs by addition of anaqueous solution of titanyl oxalate to a mixed aqueous solutionof barium and strontium nitrates. Roeder and Slamovich7

prepared Ba12 xSrxTiO3 powders at low temperature byreacting nano-sized TiO2 powder in an alkaline, aqueoussolution of BaCl2, SrCl2 and NaOH. They observed that theBST powders that were prepared with a large initial excess ofbarium and strontium over titanium yielded a monophasicsolid solution. However, those that were processed with a smallexcess of barium and strontium over titanium gave a biphasic

product that corresponded to separate barium-rich andstrontium-rich phases. Recently, Sharma et al.8 reported thepreparation of barium strontium titanate from the Ba(acac)–Sr(acac)–Ti(OC3H7)4 triethanolamine–iPrOH–water system(acac ~ 2,4-pentanedionate). Apart from the above low tem-perature routes, other methods involving metallo-organics,9

metal alkoxides10,11 and hydrothermal methods12,13 have alsobeen employed for the preparation of BaTiO3.The citrate precursor method has previously been used by

Cho14 and Arima et al.15 to prepare BaTiO3. However, thepreparation of BSTs using the citrate method has not beenattempted. Although there have been several previous reportspertaining to the low temperature synthesis of BaTiO3 andBSTs, details of the dielectric properties of these materials arenot included (see Table 1). Most of the low temperature routesyield powders with nano-sized grains and it is becomingincreasingly important to understand their properties in thisage of integrated thin layer dielectrics.In this paper, we report the synthesis of barium strontium

titanates by the polymerized citrate method, based on thePechini-type reaction route.16 The oxides obtained have beenanalyzed by chemical and spectrophotometric methods toascertain their stoichiometries. We carried out detailed X-raydiffraction, thermal (TGA, DSC) and FT-IR studies of thegel and precursors obtained. In addition, we report powderX-ray diffraction, transmission electron microscopy (TEM)and differential scanning calorimetry (DSC) studies, and thedielectric properties of the barium strontium titanates sinteredat 1100 uC. The dielectric properties were studied in thefrequency range 50 to 500 kHz and in the temperature range30 to 300 uC.

Experimental

Preparation of ethylene glycol–titanium tetraisopropoxide–citricacid solution

Titanium tetraisopropoxide (Acros, 981%) was added toethylene glycol (Qualigens, SQ grade) under nitrogen. The

DOI: 10.1039/b205087k J. Mater. Chem., 2003, 13, 415–423 415

This journal is # The Royal Society of Chemistry 2003

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online / Journal Homepage / Table of Contents for this issue

mixture was stirred on a magnetic stirrer for 10 min to obtain aclear transparent solution. To this solution was addeddried citric acid (Qualigens, SQ grade, 99.5%). The ethyleneglycol : titanium isopropoxide : citric acid ratio used was10 : 1 : 40. A whitish precipitate was observed immediatelyafter adding citric acid to the solution, but it dissolved afterstirring the solution for 5–6 min. The contents were stirred atroom temperature for about 3.5 h until all the citric aciddissolved and a clear solution was obtained. Barium carbonate(Loba Chemie, GR grade, 99%) and strontium carbonate(Fluka) were dried in an oven at 110 uC for 2 h and groundto a powder. Stoichiometric amounts of these carbonateswere weighed out and added to the ethylene glycol–titaniumtetraisopropoxide–citric acid solution. The mixture was stirredon a magnetic stirrer for 4 to 5 h until the carbonates dissolvedand a clear transparent solution without any visible impuritieswas obtained. This pale yellow solution was stirred further at55 ¡ 5 uC for 2 h. The solution was then kept in an oven at135 ¡ 5 uC for 20 h to evaporate the solvent and to promotepolymerization. The solution became a black viscous resin.This resin was charred in an electrically heated furnace for 2 hat 300 uC and then cooled to room temperature. The resinturned into a black mass, which was ground to a powder in anagate mortar. This ground black mass is henceforth referred toas the precursor. White BST powders were obtained by heatingthis precursor at 500 uC for 20 h and then at 800 uC for 8 h. Theproduct was compacted into pellets at a pressure of 4 ton andthen sintered at 1100 uC for 3 h.

Estimation of barium and strontium contents

Barium and strontium contents in Ba12 xSrxTiO3 (x~ 0 to 1.0)powders were estimated as BaSO4 and SrSO4, and determinedas the sum of barium and strontium sulfates.Wet chemical analysis was carried out for all barium

strontium titanate powders calcined at 800 uC for 8 h. About0.05 g of each powder was dissolved in 10 ml of concentrated

sulfuric acid on a hot plate. The solution was cooled anddiluted with distilled water to 50 ml. A white precipitate wasformed on dilution of the solution. The solution was furtherheated on a hot plate for 10 min and kept undisturbedovernight to allow for complete precipitation. The solution wasthen filtered through a Whatman 42 filter paper and the filtratecollected in a beaker to allow the titanium content to beestimated. The precipitate was washed with 20 ml of hot waterand then three times with 20 ml of distilled water. The filterpaper was allowed to stand for an hour at room temperatureand then placed in a silica crucible, which was covered with alid and kept in an electric muffle furnace for 2 h at 600 uC. Thecrucible was then kept in a dessicator and the weight monitoreduntil it became constant. The increase in weight correspondsto the weight of the sulfate. The percentage of barium andstrontium was calculated from the weight of the sulfate.

Estimation of titanium content

The filtrate obtained above was used to estimate the titaniumcontent. Acidic titanium(IV) solution gives a yellow color withhydrogen peroxide. The intensity of the color depends on theconcentration of titanium. Standard solutions with concentra-tions of 10, 20, 30 and 40 ppm were made by dissolving TiO2

(CDH, 99.99%) in concentrated sulfuric acid, and then dilutingto 100 ml with distilled water. 20 ml of each sample was takenin 50 ml volumetric flask and 5 ml of 3% H2O2 was added toit, the volume was then made up to 50 ml with distilled water.The absorbances of the standard and sample solutions wasmeasured using an ECIL GS 5701V UV-Visible spectro-photometer at lmax 410 nm. A calibration graph of concentra-tion vs. absorbance was plotted for the standard solutions andthe concentration of the sample solution was evaluated fromthe calibration curve. There is excellent agreement between thetheoretical and observed metal contents for the BST powdersgiven in Table 2.

Table 1 Details of some of the studies on BaTiO3 and SrTiO3 prepared by low temperature routes

Method of synthesis(composition)

Treatmenttemperature/uC Particle size/nm Dielectric constanta Dielectric lossa Reference

Oxalate precursor(BaTiO3/SrTiO3)

1100 (1 h, sintering) 210–300 363, 428 (SrTiO3); 1620,1965 (BaTiO3) (10 kHz)

— 5

Sol–gel (Ba12 xSrxTiO3) 900 (0.5 h, sintering) v100 80 0.03–0.05 81300 (0.5 h, sintering) 600–800 700–900 (1 MHz)

Sol–gel BaTiO3 1300 (sintering) 5000 2630 (1 kHz)b 0.02 11Sol–gel BaTiO3 (200 nm film) 750 (10 min) 25 220 (1 kHz) 0.01 24Monomeric metallo-organicprecursor (BaTiO3)

600 10 — — 91350 (sintering) 1100

Hydrothermal (BaTiO3) 240 (synthesis) 30–90 — — 12Hydrothermal (BaTiO3) 650 (15 h, sintering) 100 — — 13Citrate precursor (BaTiO3) 900 (8 h, sintering) 20 — — 14Citrate precursor (BaTiO3) 1260 (10 h, sintering) 1000 w5500 (1 kHz)b — 34Citrate precursor (BaTiO3) 1100 (3 h, sintering) 55 510 (100 kHz ) 0.05 This studyCitrate precursor (Ba0.5Sr0.5TiO3) 1100 (3 h, sintering) 72 268 (100 kHz ) 0.009 This studyCitrate precursor (SrTiO3) 1100 (3 h, sintering) 88 190 (100 kHz ) 0.001 This studyaAt room temperature. bFerroelectric transition (y120 uC).

Table 2 Chemical analysis of BST powders

Composition

Experimental value Calculated Error

Ba 1 Sr (%) Ti (%) Ba 1 Sr (%) Ti (%) Ba 1 Sr (%) Ti (%)

BaTiO3 58.5 20.1 58.9 20.5 0.6 1.9Ba0.75Sr0.25TiO3 55.6 21.1 56.6 21.7 1.6 2.7Ba0.5Sr0.5TiO3 53.2 22.8 54.0 23.0 1.3 0.8Ba0.25Sr0.75TiO3 50.2 23.8 51.1 24.4 1.6 2.3SrTiO3 47.1 25.7 47.7 26.1 1.2 1.5

416 J. Mater. Chem., 2003, 13, 415–423

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

Characterization

The various phases in the BST powders were characterized bypowder X-ray diffraction using a Bruker D8 Advance X-raydiffractometer with Ni-filtered Cu-Ka radiation. Scan rates of2u min21 were used for normal scans. For more detailed X-raystudies (observation of weak distortion or grain size) slow scanswith a step size of 0.005u in 2h and a step time of 2 s wereemployed. In addition, the ka2 stripping procedure was used.To obtain the grain size from X-ray line broadening, we usedthe TOPAS software (Bruker AXS) for the refinement of the(111) reflection of the BSTs, using the raw data withoutany stripping. Both Ka1 and Ka2 peaks were refined. In thisprocedure, the crystallite size was determined using a formulasimilar to that of Scherrer. The instrumental broadening wascalculated by the fundamental parameter approach and takeninto account. In the TOPAS software, the crystallite size is afitting parameter and is used directly with the instrumentalparameters (which are fixed) to fit the peak. With thisconvolution-based method, axial divergency, wavelength dis-tribution (emission profile of the copper anode), etc., are takencare of. We have also calculated the grain sizes from the line-broadening studies using the raw data (with no stripping ofKa2), applying Scherrer’s formula (t ~ 0.9l/B cosh), where t isthe diameter of the grain, l is the wavelength (for Cu-Ka, l ~1.5418 A) and B ~ d(BM

2 2 BS2) (BM is the full width at half-

maximum of the sample and BS is that of a standard grain sizeof around 2 mm). The standard used was quartz and was sochosen such that the peak of the sample and the standard havesimilar 2h values [quartz gives a reflection at 2h ~ 40.27u,which is near the (111) reflection of the BSTs]. The (111)reflection of the observed X-ray data was chosen for calculatingthe grain size of the BSTs. Both the above methods give verysimilar grain sizes.The FT-IR spectra were collected on a Nicolet 460 Protege

spectrophotometer in the range 4000–225 cm21 after every stepin the synthesis to 800 uC. FT-IR spectra of barium strontiumtitanate gels were recorded from CsBr disks and those of thecalcined powders and precursor from KBr disks. Thermo-gravimetric analysis and differential scanning calorimetryof precursors were carried out on a Netzsch SimultaneousThermal Analyzer Model 409 EP, with a heating rate10 uC min21 in air. Transmission electron microscopy ofpowders dispersed on carbon grids was carried out using aJEOL 200CX electron microscope. Dielectric properties weremeasured on sintered disks coated with silver paste (dried at90 uC for 6 h) using a Hewlett Packard HP 4284L multi-frequency LCR meter in the frequency range 50–500 kHz.Temperature variation studies of the dielectric constant anddielectric loss were carried out in the temperature range 30–300 uC. The densities of the sintered disks were obtained bythe Archimedes method using CCl4. The disks were soaked inthe organic medium for a sufficiently long time and the weightmonitored until it became constant. For consistency, threedifferent density measurements were carried out for eachsample. The densities were found to vary between 95 to 96% ofthe theoretical densities.

Results and discussion

Oxides of the type Ba12 xSrxTiO3, were obtained after heatingthe precursors (see Experimental section) at 500 uC. Thepowder X-ray diffraction patterns were indexed on the basis ofa cubic cell, as for cubic BaTiO3. Pure phases without anyimpurity peaks were obtained. The lattice parameters obtainedare plotted in Fig. 1. It can be seen that there is a decrease in thea lattice parameter with increasing strontium substitution,which is expected, since Sr21 has a smaller ionic radius thanBa21. Fig. 2 shows the powder X-ray patterns of Ba0.5Sr0.5TiO3

at various stages of synthesis. The reflections become narrowerwith higher temperatures, as can be seen clearly from the highangle (211) reflection. The gel formed (after 135 uC) and theprecursor (after 300 uC heating) show amorphous nature,as observed from the powder X-ray diffraction studies (notshown in Fig. 2). An earlier report17 on the synthesis of BaTiO3

through the citrate precursor route finds very weak reflectionsof the cubic BaTiO3 phase at 550 uC, in contrast to the sharpreflections observed by us. In addition, they report the presenceof an oxycarbonate phase (Ba2Ti2O5.CO3) up to 550 uC in thepowder XRD patterns. Our studies [Fig. 2(a)] do not showany evidence for this oxycarbonate phase in samples heatedat 500 uC for 20 h. This is true for all the compositions inthe Ba12xSrxTiO3 family heated at 500 uC. The cubic phase

Fig. 1 Plot of the variation of the cubic lattice parameter withcomposition in Ba12xSrxTiO3.

Fig. 2 Powder XRD patterns for Ba0.5Sr0.5TiO3 precursor heated at (a)500 uC for 20 h, (b) 800 uC for 8 h (c) 900 uC for 12 h and (d) 1100 uC for3 h.

J. Mater. Chem., 2003, 13, 415–423 417

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

formed at 500 uC is observed after all successive stages ofheating (800, 900 and even at 1100 uC), as shown forBa0.5Sr0.5TiO3 (Fig. 2).Fig. 3 shows the X-ray diffraction patterns of all the

Ba12xSrxTiO3 compositions sintered at 1100 uC for 3 h. Atfirst glance, the X-ray diffraction patterns appear to belong tothe cubic phase. The decrease in the intensity of the (100)reflection with Sr substitution can be seen clearly. For SrTiO3,this (100) reflection is nearly absent. The (100) reflection isreported with an intensity of 3.6% in the JCPDS data for cubicstrontium titanate. This is due the difference in atomic numbers(dZ) between Ba (Z ~ 56) and Sr (Z ~ 38). The (111) and the(211) reflections also decrease in intensity with Sr substitution.These X-ray diffraction patterns were obtained at a scan speedof 2u min21. However, using a slow scan with a step time of 2 sand a step size of 0.005u, we observed a very weak (100)reflection. Detailed X-ray analysis (slow scan with 0.005u stepsize, a step time of 2 s and with Ka2 stripping) of the 1100 uCsintered samples (Fig. 4) shows the presence of weak tetragonaldistortion in the x ~ 0 composition (BaTiO3). The reflections(200) and (002) overlap partially to give an asymmetric peak(Fig. 4). However, for the x ~ 0.25, 0.5 and 0.75 samples, wedo not see any splitting of the (200) reflection, even under theslow scan conditions, and hence they were indexed on a cubiccell. The lattice parameters obtained are a ~ 3.999(3), b ~4.0101(4) A for BaTiO3 and a ~ 3.948(3) A for the 0.5

composition, Ba0.5Sr0.5TiO3. The reported18 lattice parameters

for BaTiO3 are a ~ 3.9945 and c ~ 4.0335 A. The XRDpattern of the x~ 1.0 composition (SrTiO3) sintered at 1100 uCappears to be cubic with weak additional reflections veryclose to the major phase (Fig. 5). On careful analysis of thediffraction pattern, these additional weak reflections weresatisfactorily indexed to the higher homologue, Sr4Ti3O10

(Fig. 5, inset). It should be noted that no additional reflectionswere observed for the samples of SrTiO3 heated at 500 or800 uC, even when studied with the slow scan rate. Severalattempts were made to obtain well sintered SrTiO3 by thepolymeric precursor route. However, the pure SrTiO3 phase (asobserved till 800 uC) was found to partially disproportionate at1100 uC. The amount of the minor impurity phase was around

Fig. 3 XRD patterns for oxides of the Ba12xSrxTiO3 system sinteredat 1100 uC for 3 h.

Fig. 4 XRD pattern (slow scan) showing weak tetragonal distortion inBaTiO3.

Fig. 5 Powder XRD pattern for SrTiO3 sintered at 1100 uC for 3 h. Theinset shows a slow scan of the (200) reflection, showing the additionalweak reflection.

418 J. Mater. Chem., 2003, 13, 415–423

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

5–10%. The percentage of impurity was determined using theformula,

[A107 (Sr4Ti3O10) 6 100]/[A107(Sr4Ti3O10) 1 A110(SrTiO3)]

where Ahkl is the area under the curve of the hkl reflection. Notethat 110 and 107 are the most intense reflections for SrTiO3 andof Sr4Ti3O10.FT-IR studies of various compositions of strontium-doped

barium titanate have been obtained after each step of thesynthesis. In Fig. 6, the IR spectra of the compound with x ~

0.5 is shown. In the case of the polymeric gel heated at 135 uC,we find IR bands due to OH stretch (3400 cm21), carboxylateanion (1378 and 1646 cm21), and the ester (y1735 cm21).CO3

22 bands are also present (1378, 1080, 882 and 644 cm21)at this temperature, along with the characteristic carboxylateband, the C–O stretches of citric acid and ethylene glycol, andthe metal carboxylate interaction, give rise to bands in theregion 1500–1400 cm21 [Fig. 6(a)]. On further heating at300 uC, the organic matrix is decomposed and, as a result, theabsorption between 1000 and 1500 cm21 decreases in intensity.The characteristic carboxylate band at 1558 cm21 is present(analogous result reported earlier17), showing that all thecarboxylate is not decomposed at this temperature. Probably,the metal ion is coordinated with the carboxylate groupsand loses these ligands at temperatures above 300 uC (around

340 uC).19 We also find a broad peak pertaining to adsorbedwater, which is stretched out to 2000 cm21. This is probablydue to the black color of the precursor obtained at this stage,which inhibits proper IR absorption. The adsorbed water bandarises as a result of water being released as a decompositionproduct and later getting adsorbed. The C–OH stretch at1080 cm21 disappears. The band at 1717 cm21 is due to theCLO stretch of the ester linkage formed during polymerization,and is observed at this temperature because the polymer doesnot wholly decompose at 300 uC. For the sample heated at500 uC [Fig. 6(c)], the OH stretching band is present, but itdisappears in the spectrum of the sample heated at 800 uC[Fig. 6(d)]. The broad Ti–O bands (as normally observed inBaTiO3) in the region 200–400 and 540–700 cm21 are clearlyvisible in the spectrum of the sample heated at 500 uC[Fig. 6(c)]. This also corroborates our XRD studies, where weobserved the pure cubic phase after heating at 500 uC. Afterheating at 800 uC, the Ti–O band becomes more prominent.However, even at this temperature, the O–H stretch isobserved. This indicates that some water may be present inthe structure. Similar observations were reported by Yen et al.20

We have also carried out thermogravimetric analysis ofthe precursors, as shown in Fig. 7. It can be seen that thetemperature at which the precursor decomposes varies withcomposition, and increases from 250 uC for the compositionwith x ~ 0 to 300 uC for that with x ~ 1. The higher

Fig. 6 FT-IR spectra of Ba0.5Sr0.5TiO3 after various heat treatments:(a) polymeric gel heated at 135 uC; (b) after heating at 300 uC; (c) afterheating at 500 uC; (d) after heating at 800 uC.

Fig. 7 Thermogravimetric analysis plots for Ba12 xSrxTiO3 samples:(a) x ~ 0.0; (b) x ~ 0.25; (c) x ~ 0.50; (d) x ~ 0.75; (e) x ~ 1.0.

J. Mater. Chem., 2003, 13, 415–423 419

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

decomposition temperature of the strontium precursor (x ~ 1)compared the barium precursor (x ~ 0) may be explained bylooking at the heat of fomation of metal–carboxylate bondsbetween the metal ions and the carboxylate groups of citricacid. The strontium–carboxylate bond appears to be stronger(higher or more negative heat of formation is reported21 forstrontium oxalate compared to barium oxalate) than thebarium–carboxylate bond. Hence, the decomposition tempera-ture of the barium-containing precursor is lower than that ofthe strontium-containing precursor.The decompositions are complete at temperatures ranging

from 580 to 640 uC, depending on the composition. Thisexplains the presence of the CO3

22 band in the IR spectrum ofthe sample heated at 500 uC [Fig. 6(c)]. Thus, the crystallinephase was formed at a temperature lower than that of thefinal decomposition step. This observation is similar to thosereported by Mondelaers et al.19 and Van Werde et al.22 fromstudies of the thermal behavior of zinc citrate precursor, wherethe crystalline phase was obtained at a lower temperature thanthe final decomposition step.The differential scanning calorimetric studies of the pre-

cursors of the BST materials are shown in Fig. 8. Thedecomposition is accompanied by an exothermic peak. Thedecomposition temperature varies linearly with increasingstrontium content between the x values of 0.25 and 1.0, from526 to 540.2 uC (see Fig. 8, inset). However, the pure bariumtitanate shows a much lower decomposition temperature(465 uC). Careful inspection of the DSC curves reveals asym-metry in the exotherms, which indicates that there are twodecomposition temperatures close together.It should be noted that our XRD studies on samples after

heating at 500 uC did not show the presence of any metalcarbonates. Rationalizing the IR and DSC data, we can thensay that there may be decomposition of the carbonates starting

from around 250–300 uC and complete around 480–550 uC,depending on the composition. So around 500 uC, there maybe some amorphous metal carbonates present that are notobserved in the XRD pattern. It should also be noted that thegel (obtained at 135 uC) and the precursor (obtained at 300 uC)prepared by the citrate precursor route were amorphous, asshown by X-ray powder patterns.Apart from the powder XRD studies carried out for

structural characterization (discussed earlier), we have alsocarried out detailed line-broadening studies using X-raydiffraction to evaluate the grain size of the BSTs. The grainsize of the samples sintered 1100 uC was calculated from X-raystudies (Fig. 9) using the TOPAS software, which uses grainsize as a fitting parameter. The average grain size wascalculated to be around 55 nm for BaTiO3 and 88 nm forSrTiO3. Scherrer’s formula, using the FWHM (see Experi-mental section for details), was also used directly and theaverage grain size was calculated to be around 62 nm forBaTiO3 and 93 nm for SrTiO3. The grain size systematicallyincreases with Sr concentration (Fig. 10). It should be notedthat at 500 uC, the grain size of these compositions appears tobe lower, that for BaTiO3 being around 25 nm. Transmissionelectron microscopy studies (Fig. 11) of the samples heated at500 uC show a distribution of grain sizes, with most sizes in therange 25 to 50 nm for BaTiO3 and SrTiO3. The grains areagglomerated at 500 uC, as observed by the TEM studies. Thus,the increase in grain size on sintering appears to be marginal

Fig. 8 DSC plots for Ba12xSrxTiO3 samples: (a) x~ 0.0; (b) x~ 0.25;(c) x~ 0.50; (d) x~ 0.75; (e) x~ 1.0. The inset shows the variation ofthe decomposition temperature of the precursor with composition.

Fig. 9 The raw data and the fits obtained by refinement of the (111)reflections used to calculate the grain sizes of (a) BaTiO3, (b)Ba0.5Sr0.5TiO3 and (c) SrTiO3 sintered at 1100 uC.

420 J. Mater. Chem., 2003, 13, 415–423

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

and the grains are still nano-sized (60–90 nm) after sinteringat 1100 uC. The stability of nano-sized grains during sintering isof immense importance in the area of dielectric ceramics.Sintering normally involves consolidation of grains, leading tograin growth and, hence, higher grain sizes. Since sintering is anunavoidable step during bulk synthesis of dielectric ceramics,any process which maintains the stability of the nano-sizedgrains is of significance. There have been very interestingstudies23–25 on prevention of coarsening of grains on sinteringin nanostructured polycrystalline materials. These studiesdiscuss the importance of a minor secondary phase in slowingdown the migrating grain boundaries and, hence, preventinggrain growth.An important aspect in the study of nano-sized BaTiO3

powders is the observation of a size-dependent cubic totetragonal phase transition. There have been several earlierreports on the grain size dependence of the cubic to tetragonaltransition in BaTiO3. Uchino et al.26 suggested a size of0.12 mm, below which the cubic structure of BaTiO3 could be

stabilized at room temperature. Begg et al. have reported27 agrain size of 0.19 mm for the cubic to tetragonal transition.However, Asiaie et al.12 found tetragonal distortion, even forgrains of 90 nm, and have shown that the distortion decreaseswith particle size (c/a ~ 1.0078 for the 90 nm grains, while it is1.0105 for 300 nm grains). Yen et al.20 reported the critical sizeto be 30 nm. In our studies using high quality X-ray data, wefind tetragonal BaTiO3 at an average grain size of 55 nm. Thisis in accordance with the findings of Yen et al.20 The tetragonaldistortion observed by us (c/a ~ 1.0027) is much smaller thanthat found by Asiaie et al.12 for 90 nm grains, which is alsoexpected, since the distortion should decrease with decreasinggrain size.The tetragonal distortion has also been examined in recent

years by Raman spectroscopy along with X-ray analysis tolook for distortion in the sub-micron region. It is reported20,28

that the critical size for the cubic to tetragonal distortion mayvary and is highly dependent on the strain between the grains,which is itself dependent on the synthetic method employed.This may explain the variation in the critical sizes obtained bydifferent groups. In our studies, as discussed above, we find atetragonal distortion in BaTiO3, even at a grain size of 55 nm.We have studied the dielectric constant and the dielectric loss

of sintered disks of the barium strontium titanates as a functionof composition and temperature. In Fig. 12, the variation of thedielectric constant as a function of x (composition) at 100 kHzis shown, measured at room temperature. The dielectric con-stant varies from 510 for BaTiO3 to 190 for SrTiO3.Thesesamples were sintered at 1100 uC and the density of the diskswas 95–96%. Earlier reports on BaTiO3 prepared by thesol–gel8 method give dielectric constants of 500–650 and700–900 (at 1 MHz) for samples sintered at 1200 and 1300 uC(for 30 min), respectively. Another report on BaTiO3 preparedby the oxalate precursor route5 gave a dielectric constant of1620 for BaTiO3 and 363 for SrTiO3. Thus, it appears that themethod of synthesis and the sintering temperature affect thedielectric constant to a great extent (see Table 1). The dielectricconstant of BaTiO3 prepared by our method and sintered at1100 uC shows very little change with temperature up to 150 uC,beyond which it shows a decrease [Fig. 13(a)]. For Ba0.5Sr0.5-TiO3, the dielectric constant decreases in a regular fashionfrom room temperature to 300 uC [Fig. 13(b)]. The dielectricconstant decreases slightly with frequency for BaTiO3 up to10 kHz, beyond which it remains almost constant [Fig. 14(a)].This behavior persists, even at higher temperatures (up to300 uC). The dielectric loss also shows a gradual decreasefrom 0.75 (at 0.4 kHz) to 0.002 (at 100 kHz). At higherfrequencies (300 to 500 kHz), the loss increases and it appearsthat beyond 500 kHz there is a loss peak, normally seen dueto a change in the polarization mechanism. Ba0.5Sr0.5TiO3

sintered at 1100 uC shows negligible variation in dielectricconstant and dielectric loss in the frequency range 100 to500 kHz at room temperature, as shown in Fig. 14(b). Thedielectric constant shows negligible variation with frequency,even at high temperature (up to 300 uC), in the above frequencyrange

Fig. 11 Transmission electron micrographs of (a) BaTiO3 and (b)SrTiO3 heated at 500 uC.

Fig. 12 Variation of the dielectric constant (e) and dielectric loss (D)with composition for the Ba12 xSrxTiO3 system.

Fig. 10 Plot of the variation of grain size with composition afterheating at 1100 uC.

J. Mater. Chem., 2003, 13, 415–423 421

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

In the above studies on BaTiO3, the dielectric constant doesnot show any peak in the temperature region 50–150 uC [datataken at intervals of 10 uC; see Fig. 13(a), inset]. DSC studiescarried out in this temperature range (Fig. 8) also confirm theabsence of the ferroelectric transition. This absence of theferroelectric transition in BaTiO3 is expected, due to the smallparticle size of the grains (55 nm) obtained using our synthesisprocedure. Earlier studies29 on a 200 nm thin layer ofBaTiO3 with 25 nm grain size have also revealed a lack offerroelectricity.It has been pointed out previously that the cubic structure

and absence of ferroelectricity in BaTiO3 occurs for small grainsizes. Ginzburg-Landau’s mean -field theory has been appliedby Binder30 to show that as the grain size decreases, thepolarization decreases (the bulk has ordered dipoles, whereasthe surface has more disorder) and, hence, the ferroelectricity islost. The increase in depolarization field with decreasing grainsize has also been suggested to destroy the ferroelectricstate.31,32 Calculations by Wang et al.33 led to a critical grainsize of 44 nm, below which the ferroelectric state is unstable.These ceramics, made up of such small grains, would alsohave much lower dielectric constants. A very weak ferroelectrictransition is seen for 90 nm BaTiO3 particles in DSC mea-surements.12 However Frey and Payne28 found no phasetransition in DSC studies of 35 nm BaTiO3 grains. All thesamples discussed above not showing ferroelectricity arereported to be cubic. It should be noted that the BaTiO3

particles obtained by us with a grain size of 55 nm show a weaktetragonal distortion (c/a ~ 1.0027).

Conclusions

We have demonstrated the possibility of obtaining nano-sizedoxides for the entire range of compositions of Ba12xSrxTiO3,as single phases crystallizing in the cubic structure at 500 uCusing the polymeric citrate precursor route. Sintering at 1100 uCleads to weak tetragonal distortion in BaTiO3 only, whichwas observed after detailed X-ray studies, while all Sr-dopedcompositions remain cubic. These materials have grain sizesranging from 55 to 88 nm. The nano-sized grains prepared bythe citrate precursor route appear to be reasonably stable onsintering. Oxides sintered at 1100 uC show a decrease in thedielectric constant with increasing strontium substitution.The dielectric loss of SrTiO3 is much lower (nearly 1/50th)than that of the BaTiO3. In addition, Sr-doped sampleshave better temperature stability of the dielectric constant. The

Fig. 13 Variation of the dielectric constant (e) and dielectric loss (D) with temperature for (a) BaTiO3 [the inset shows the variation of the dielectricconstant with temperature in the region 50 to 140 uC (scanning for a ferroelectric transition)] and (b) Ba0.5Sr0.5TiO3. All measurements were carriedout at 100 kHz.

Fig. 14 Variation of the dielectric constant (e) and dielectric loss (D)with frequency for (a) BaTiO3 and (b) Ba0.5Sr0.5TiO3.

422 J. Mater. Chem., 2003, 13, 415–423

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online

ferroelectric transition normally observed in micron-sizedBaTiO3 is absent in the above oxides with nano-sized grains.

Acknowledgement

The authors thank Dr G. N. Subbanna, MRC, IISc,Bangalore, for the TEM measurements.

References

1 G. H. Hertling, J. Am. Ceram. Soc., 1999, 82, 797.2 F. Selimi, S. Komarneni, V. K. Varadan and V. V. Varadan,

Mater. Lett., 1990, 10, 235.3 A. Bauger, J. C. Moutin and J. C. Niepce, J. Mater. Sci., 1983, 18,

3041.4 P. P. Phule and S. H. Risbud, J. Mater. Sci., 1990, 25, 2571.5 M. A. Sekhar, G. Dhanraj, H. L. Bhat and K. C. Patil, J. Mater.

Sci.: Mater. Electron., 1992, 3, 237.6 T. Noh, S. Kim and C. Lee, Bull. Korean Chem. Soc., 1995, 16,

1180.7 R.K.RoederandE.B.Slamovich,J.Am.Ceram.Soc., 1995,82, 1665.8 P. K. Sharma, V. V. Varadan and V. K. Varadan, Chem. Mater.,

2000, 12, 2590.9 H. J. Glasel, E. Hartmann, D. Hirsch, R. Bottcher, C. Klimm,

D. Michel, H. Semmelhack, J. Hormes and H. Rumpf, J. Mater.Sci., 1991, 34, 2319.

10 T. Fukui, C. Sakurai andM. Okuyama, J.Mater. Sci., 1997, 32, 189.11 M. H. Frey and D. A. Payne, Chem. Mater., 1995, 7, 123.12 R. Asiaie, W. Zhu, S. A. Akbar and P. K. Dutta, Chem. Mater.,

1996, 8, 226.13 J. O. Eckert Jr, C. C. Hung-Houston, B. L. Gersten, M.M. Lencka

and R. E. Riman, J. Am. Ceram. Soc., 1996, 79, 2929.14 W. S. Cho, J. Phys. Chem. Solids, 1998, 59, 659.15 M. Arima, M. Kakihana, Y. Nakamura, M. Yashima and

M. Yoshimura, J. Am. Ceram. Soc., 1996, 79, 2847.

16 M. P. Pechini, US Pat., 330697, 1967; M. Kakihana, M. Arima,Y. Nakamura, M. Yashima and M. Yoshimura, Chem. Mater.,1998, 11, 438.

17 P. Duran, D. Gutierrez, J. Tartaj, M. A. Banares and C. Moure,J. Eur. Ceram. Soc., 2002, 22, 797.

18 R.Waesche, W. Denner andH. Schulz,Mater. Res. Bull., 1981, 16,497.

19 D. Mondelaers, G. Vanhoyland, H. Van den Rul, J. D’Haen,M. K. Van Bael, J. Mullens and L. C. Van Poucke, Mater. Res.Bull., 2002, 37, 901.

20 F. S. Yen, H. I. Hsiang and Y. H. Chang, Jpn. J. Appl. Phys., 1995,34, 6149.

21 CRC Handbook of Chemistry and Physics, CRC Press, BocaRaton, FL, 68th edn., 1987–88.

22 K. Van Werde, D. Mondelaers, D. Nelis, G. Vanhoyland,M. K. Van Bael, B. Van der Veken, H. O. Desseyn, J. Mullensand L. C. Van Poucke, J. Mater. Sci., 2002, 37, 81.

23 J. Weismuller, Nanostruct. Mater., 1993, 4, 261.24 H. Gleiter, Acta Mater., 2000, 48, 1.25 I. W. Chen and X. H. Wang, Nature, 2001, 404, 168.26 K. Uchino, E. Sadanaga and T. Hirose, J. Am. Ceram. Soc., 1989,

72, 1555.27 B. Begg, E. R. Vance and J. Nowotny, J. Am. Ceram. Soc., 1994,

77, 3186.28 M. H. Frey and D. A. Payne, Phys. Rev. B, 1996, 54, 3158.29 M. H. Frey and D. A. Payne, Appl. Phys. Lett., 1993, 63, 2753.30 K. Binder, Ferroelectrics, 1981, 35, 99.31 (a) I. P. Batra, P. Wurfel and B. D. Silverman, Phys. Rev. Lett.,

1973, 30, 384; (b) I. P. Batra, P. Wurfel and B. D. Silverman, Phys.Rev., 1973, 8, 3257.

32 W. Y. Shih, W. H. Shih and I. A. Aksay, Phys. Rev. B, 1994, 50,15575.

33 Y. G. Wang, W. L. Zhong and P. L. Zhang, Solid State Commun.,1994, 90, 329.

34 P. Duran, D. Gutierrez, J. Tartaj and C. Moure, Ceram. Int., 2002,28, 283.

J. Mater. Chem., 2003, 13, 415–423 423

Publ

ishe

d on

11

Dec

embe

r 20

02. D

ownl

oade

d by

Van

derb

ilt U

nive

rsity

on

23/0

5/20

13 1

6:45

:52.

View Article Online