Materials Chemistry and Physics 77 (2002) 60–64

Synthesis of SrBi2Ta2O9 nanocrystalline powder by a modifiedsol–gel process using bismuth subnitrate as bismuth source

Zhou Yu, Wang Wen∗, Jia Dechang, Ye FengSchool of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, PR China

Received 10 May 2001; received in revised form 20 August 2001; accepted 31 August 2001

Abstract

Steady sol and gel of SrBi2Ta2O9 (SBT) was prepared using pentaethoxy tantalum, strontium acetate, and bismuth subnitrate as rawmaterials and ethylene glycol as a solvent. After a dried gel was pretreated at 350◦C for 30 min and then it was calcined at 800◦C for 1 h,the perovskite phase SBT ferroelectric powder was obtained. The formation mechanism of the sol and gel was proposed. In the bakingprocess perovskite phase SBT forms via a reaction between bismuth oxide (cubic phase) and strontium tantalate (orthorhombic phase).© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Sol–gel process; SrBi2Ta2O9; Perovskite phase

1. Introduction

In recent years, interest in ferroelectric materials for non-volatile random access memory applications has been inten-sified dramatically. Ferroelectric thin films have been appliedonto silicon integrated circuits to provide high speed, highread/write endurance, good radioresistance and low powerconsumption nonvolatile memories. Some of the candidatematerials for these applications are perovskite ferroelectricssuch as PbZr1−xTixO3 (PZT) or doped PZT chosen for theirhigh Curie temperatures and promising electrical propertiessuch as large remnant polarization and low switching field.However PZT ferroelectric system, in general, is known tosuffer from serious degradation problems such as fatigue,aging, and leakage current that significantly affect the life-time of the devices. Recently, alternate PZT ferroelectricoxides belonging to the family of layered perovskites, e.g.,SrBi2Ta2O9 (SBT), were identified as promising candidatesfor nonvolatile memory applications. SBT thin films werefound to exhibit no fatigue up to 1012 switching cycles [1],very good retention properties, and low leakage current den-sities on Pt electrodes.

To date, pulse laser decomposition (PLD) [2,3], metal-organic chemical vapor deposition (MOCVD) [4] andmetal-organic decomposition (MOD) [5] methods havebeen reported for the preparation of SBT thin films. Thereare many reports on the sol–gel method in a broad sense

∗ Corresponding author.E-mail address: ce921@hope.hit.edu.cn (W. Wen).

[6–8]. The sol–gel process offers many advantages overconventional methods, such as high purity, molecular homo-geneity, low-temperature processing, convenient fabricationprocedures, and easy control of stoichiometry. However, asstarting materials, alkoxides would make the whole fabri-cating process more susceptible to temperature and ambienthumidity, and must be handled under inert atmosphere.Therefore, the alkoxides of some metals such as Ta and Biwere coordinated using pyridine [9], alcohol–amine [10] andacetylacetone [11], and metal complex compound becamestable. But the addition of complex agent makes it difficultfor the control of the processes of gel polymerization. Inorder to overcome these problems, new raw materials andsolvent are used, thus stable SBT precursor solution can beprepared to allow their handling in air, which makes theoperation more convenient.

2. Experimental procedures

Pure SBT powder was fabricated via inorganic materialsinstead of alkoxides in a solo alkoxide system. Strontium ac-etate (Sr(Ac)2, purity 99.9%, Shanghai Chemical Reagent),pentaethoxy tantalum Ta(OCH2CH3)5 (self-made, concen-tration (261.6 mg ml−1) was detected using inductively cou-pled plasma (ICP)) and bismuth subnitrate (BiONO3, purity99.9%, Shanghai Kechang Chemical Reagent) were weighedand dissolved in ethylene glycol to prepare SBT precur-sor solution. Bismuth subnitrate rather than alkoxide of bis-muth was used as bismuth source to prepare stable SBT

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0254-0584(01)00583-1

Z. Yu et al. / Materials Chemistry and Physics 77 (2002) 60–64 61

Fig. 1. Scheme for the preparation of SBT powder using bismuthsubnitrate.

sol. Thus, the hydrolysis and polymerization reaction iseasier to control. Ethylene glycol rather than ethanol andmethoxyethanol glycol was used as the solvent because ethy-lene glycol with higher boiling point is not easy to volatilize.Fig. 1 shows the schematic diagram of the modified sol–gelprocess.

During the dissolving process, the temperature was keptat about 80◦C. After the raw materials were completelydissolved, transparent solution formed. The mixed precursorexhibited an obvious Tyndall phenomenon, indicating theformation of the sol. Then, the sol was laid up at 40◦C for 4 hto prepare transparent gel and the gel was dried at 100◦C for10 h. After a dried gel was pretreated at 350◦C for 30 min,the temperature was raised to 800◦C at a rate of 5◦C min−1,and then the dried gel was calcined at 800◦C for 1 h under airatmosphere; the perovskite phase SBT ferroelectric powderwas obtained at last.

The mechanism of the polymerization reactions occurredduring the synthesis of the sol is suggested according to theFourier transform infrared (FTIR) spectrum (Nicolet-5DX)analysis. To get FTIR, the sol was studied by putting adroplet between two KRS5 windows and gels treated atdifferent temperatures were studied as powders dispersedin a KBr pellet. Differential scanning calorimetry and

thermogravimetric analysis (DSC–TGA) (Netzsch-sta449C)were conducted with the heating rate of 10◦C min−1 toexamine the thermal reactions of the gel. The phase ofthe powder was examined by X-ray diffraction (XRD)(Rigaku Rotaflex D/max-II B) using a scanning speed of10◦C min−1, an acceleration voltage of 40 kV and an elec-tric current of 50 mA. Morphology of the SBT powderwas examined by transmission electron microscopy (TEM)(Philips CM-12) with an acceleration voltage of 120 kV,SBT powder was deconglomerated using ultrasonic vibra-tion in acetone solvent for 1 h.

3. Results and discussion

Figs. 2 and 3 show the TGA and DSC curves ofthe gel obtained from solution with a mole ratio ofstrontium:bismuth:tantalum= 1:2:2. The DSC of the gelrevealed two endothermic peaks at 100 and 766.1◦C, andthree exothermic peaks at 266.6, 337.8 and 689.9◦C. Thefirst endothermic peak is due to the volatilization of organiccompound (such as HOCH2CH2OH and n-C2H5ONO2),and the weight loss is about 7%; the second small endother-mic peak at 766.1◦C is relevant to the crystallization ofthe SBT powder. Exothermic peaks at 266.6 and 337.8◦C

Fig. 2. TG curve of the gel.

Fig. 3. DSC curve of the gel.

62 Z. Yu et al. / Materials Chemistry and Physics 77 (2002) 60–64

Fig. 4. FTIR spectra of the SBT sol and gel treated at different temperatures for 5 min: (a) sol; (b) 120◦C; (c) 200◦C; (d) 300◦C; (e) 400◦C; (f) 500◦C.

are due to the combustion of many ethoxy groups and afew acetate groups in the oligomer. These two kinds oforganic groups have different rates of weight loss. Weightloss of many ethoxy groups is 17.41% from 250 to 270◦C,and weight loss of a few acetate groups is 6% from 270to 450◦C. The weight loss of 3% during the temperaturerange of 600–750◦C indicates combustion of carbon in theframework. As a result, the exothermic peak at 689.9◦Ccorresponds to the formation of SrTa2O6 phase after thecombustion of carbon, which is consistent with the resultdetected by XRD at 700◦C.

Fig. 4(a) is the FTIR spectrum of the sol obtained fromSBT precursor solution; Fig. 4(b) is the FTIR spectrum ofthe gel dried at 120◦C; Fig. 4(c)–(f) are the FTIR spectraof the gel treated for 5 min at 200, 300, 400 and 500◦C,respectively, showing the evolution of the gel with increasingtemperature.

Sol–gel process normally involves hydrolysis and poly-merization of metal alkoxide and metal inorganic saltsin an alcoholic medium. When ethylene glycol is usedas solvent, the resulting polymers can be expressed as:–M–O–CH2CH2–O–M–O–CH2CH2–O–, where M refersto Ta, Bi or Sr. The metal–oxygen vibrations appear atlower frequencies. In Fig. 4(a) and (b), the bands at∼670,∼610 and∼580 cm−1 correspond toν(Sr–O), ν(Ta–O) andν(Bi–O), respectively. These vibrations transform to a broadabsorption peak with increasing temperature as shown inFig. 4(d)–(f). The bonds due to the stretching vibrations ofC–O attached to metal element in the polymers appear at∼1086 and∼1042 cm–1, which corresponds to [ν(C–O)]in ethylene glycol; with further increasing the temperature,the stretching vibration of C–O weaken gradually, but evenat 300◦C (see Fig. 2(d)), when ethylene glycol and other

organic substances have been evaporated completely, thebonds due to the vibration of C–O still exist. So, it can beconcluded that the bonds come from the vibrations of C–Oin the polymers. In other words, ethylene glycol can bondwith metallic ions as single chelated complex because ithas better ability of chelating [12]. In addition, the metallicions have no effect on absorption peak of chelated groupaccording to the previous work [13], so compounds withdifferent metallic ions with homologous chelated groupshave homologous absorption peaks in the infrared range(4000–400 cm−1).

When BiO(NO3) was used as the raw material in theexperiment, no vibrations of free nitrate ions are observed[14], but the vibrations of ethyl nitrate appeared in the FTIRspectrum of the sol. In Fig. 4, the vibrations due to N–O inRONO2 appear at∼1650 cm−1 (νas(N–O)) and∼1256 cm−1

(νs(N–O)) [15,16]. Besides, the bond due to O2N–O appearsat ∼862 cm−1 (νN–O). Thus, it is proposed that this is con-cerned with the following reactions:

Ta(OCH2CH3)5 + BiO(NO3) + HOCH2CH2OH

→ C2H5ONO2+Ta(OCH2CH3)4OCH2CH2OBiO+H2O

(1)

Ta(OCH2CH3)5 + BiO(NO3) + 2HOCH2CH2OH

→ Ta(OCH2CH3)4OCH2CH2ONO2

+HOCH2CH2OBiO + H2O (2)

Ta(OCH2CH3)4OCH2CH2OBiO + H2O

→ Ta(OCH2CH3)3(OCH2CH2OBiO)OH+HOCH2CH3

(3)

Z. Yu et al. / Materials Chemistry and Physics 77 (2002) 60–64 63

Fig. 5. XRD patterns of the SBT gels calcined at different temperatures for 1 h.

Ta(OCH2CH3)4OCH2CH2ONO2 + Ta(OCH2CH3)3

×(OCH2CH2OBiO)OH → Ta(OCH2CH3)3

×(OCH2CH2OBiO)OTa(CH3CH2O)3OCH2CH2ONO2

+CH3CH2OH (4)

The referred reactions (1) and (2) make the dissolving ofbismuth subnitrate easier in ethylene glycol. With increas-ing temperature, the band at∼1650 cm−1 due to aceticester does not weaken gradually, and the formation ofTa(OCH2CH3)4OCH2CH2ONO2 makes the ethyl nitratenot easy to evaporate even at 300◦C. During the preparation

Fig. 6. TEM morphology of the SBT powders obtained from the SBT gel calcined at different temperatures: (a) calcined at 500◦C; (b) 800◦C.

process of the sol and gel, all these reactions are possibleand finally lead to the formation of polymers.

Earlier studies [17] have shown that in the preparation ofPZT precursor solution (using lead acetate as lead source),one alkoxy group replaces one of the acetate group, resultingin the formation of either free acid or ester,13

6 C{11H} NMR

investigation of the lead precursor solution confirmed this[18,19]. In the solution of SBT the reactions of strontiumacetate can be expressed as follows:

Sr(OOCCH3)2 + HOCH2CH2OH

→ HOH2CH2COSrOOCCH3 + CH3COOH (5)

64 Z. Yu et al. / Materials Chemistry and Physics 77 (2002) 60–64

Ta(CH3CH2O)5 + HOH2CH2COSrOOCCH3

→ Ta(OCH2CH3)4OH2CH2COSrOOCCH3

+CH3CH2OH (6)

CH3COOH+ HOCH2CH2OH

→ H3CCOOCH2CH2OH + H2O (7)

Ta(CH3CH2O)5 + H3CCOOCH2CH2OH

→ H3COOCH2CH2OTa(CH3CH2O)4+CH3CH2OH (8)

In Fig. 4(a), the acetic ester bonds due to the stretch-ing vibrations (ν(C=O), νas(C–O–C)) appear at∼1720 and∼1220 cm−1, verifying the exactness of the aforemen-tioned reaction. With increasing temperature, acetic esterevaporates, and the bond at∼1720 cm−1 due to aceticester is submerged by which of nitrate ester appearing at∼1650 cm−1 (νas(N–O)). In addition, reactions (6) and (8)also contribute to control the structure and molecular weightof the polymers.

Fig. 5 shows the XRD patterns of the SBT gel calcinedat different temperatures for 1 h, from which the state ofphase composites can be determined. After the dried gel wascalcined at 500◦C, the cubic has formed besides a smallamount of orthorhombic SrTa2O6; at 600◦C the cubic Bi2O3and the SrTa2O6 with orthorhombic structure coexisted innearly same amount; at 700◦C the orthorhombic SrTa2O6becomes the dominating phase, meanwhile a little SBT hasformed. After the dried gel was calcined at 800◦C, SBTbecomes the only phase. Therefore, the SBT powder mostprobably be produced by the following reaction:

SrTa2O6 + Bi2O3 (cubic) → SrBi2Ta2O9

Fig. 6 shows typical TEM images of powders obtainedfrom the SBT gel calcined at 500 and 800◦C for 1 h,as shown, the particle sizes are 30–50 and 50–70 nm,respectively.

4. Summary

A modified sol–gel process is developed to prepare SBTpowder. Pure SBT powder was fabricated via inorganic ma-terials instead of alkoxides in a solo alkoxide system. Stron-tium acetate, pentaethoxy tantalum and bismuth subnitratewere used as the raw materials, and ethylene glycol wasused as the solvent. The ethyl nitrate and acetate ester werein-process products in hydrolysis and polymerization of pre-cursor solution. In the baking process perovskite phase SBTforms via a reaction between bismuth oxide (cubic phase)and strontium tantalate (orthorhombic phase).

Acknowledgements

The authors would like to thank the project supportedby the Scientific Research Foundation of Harbin Institute

of Technology (HIT.2001.67) and Chinese National NatureScience Foundation.

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