Synthesis of BiFeO3 by Wet Chemical Methods

Sverre M. Selbach,z Mari-Ann Einarsrud,*,z Thomas Tybell,y,z and Tor Grande*,w,z

zDepartment of Materials Science and Engineering, Norwegian University of Science and Technology, 7491 Trondheim,Norway

yDepartment of Electronics and Telecommunications, Norwegian University of Science and Technology, 7491Trondheim, Norway

zNTNU Nanolab, Norwegian University of Science and Technology, 7491 Trondheim, Norway

The synthesis of phase-pure BiFeO3 has been demonstrated by achemical synthesis route as well as the solid-state method at8251C. Polymeric BiFeO3 precursors were obtained from aque-ous solutions of nitrate salts and carboxylic acids with and with-out ethylene glycol added as a polymerization agent. Thepolymeric precursors were shown to decompose above 2001Cwith successive nucleation and growth of BiFeO3 above 4001C.The phase purity of the product was shown to depend on the typeof carboxylic acid used, and tartaric, malic, and maleic acidsresulted in nanocrystalline phase-pure BiFeO3. The unit cell andNeel temperature of the bulk materials obtained by the twomethods were in accord with previous reports.

I. Introduction

THE BiFeO3 is termed multiferroic12 due to the coexistence

of ferroelectricity (TC,bulk5 8301C)3,4 and antiferromagnet-

ism (TN,bulk5 3701C).5 The material has received considerable

attention in the last few years due to the potential applications indata storage, sensors, and devices for spintronics, and reports ofgreatly enhanced ferroelectricity in epitaxially strained thinlms.67 Bulk BiFeO3 is a rhombohedrally distorted perovskitebelonging to the space group R3c, with lattice parametersa5 5.63 A and a5 59.351, or equivalently a5 5.58 andc5 13.87 A in the hexagonal representation.811 The ferroelec-tric displacement of Bi31 and Fe31 is with respect to the [111]direction in the rhombohedral unit cell.12 Unpaired electrons inthe d5 HS ion Fe31 are the origin of the G-type antiferromag-netism, with weak ferromagnetic ordering due to a canted spinstructure.1314

BiFeO3 is an incongruently melting compound that meltsperitectically at 9341C.4,15 Bi25FeO40 and Bi2Fe4O9 are the stablecompounds on each side of BiFeO3 in the phase diagram,

4,15

and the formation of these phases during synthesis is challengingaccording to previous studies.16 An early approach using tradi-tional solid-state synthesis by ring a mixture of Bi2O3 andFe2O3 was to apply an excess of Bi2O3 and leach away excess Bi-rich compounds with diluted nitric acid after ring.16 A tech-nique termed rapid liquid phase sintering3,1719 has recentlybeen reported to be successful, where an equimolar mixture ofprecursor oxides is rapidly heated and red for a short while at atemperature above the melting temperature of Bi2O3(Tfus5 8171C). Volatile Bi2O3 limits the temperature and time,which can be utilized without losing control of stoichiometry.

In wet chemical synthesis routes, much lower temperaturesare required, eliminating the potential evaporation of Bi2O3, andhence chemical homogenity at the atomic level can be achieved.A nonaqueous route20 and co-precipitation synthesis21 havebeen reported to produce BiFeO3, but neither succeeded inyielding phase-pure powder, without subsequent leaching withdiluted nitric acid.21 Ghosh et al. have prepared phase-pure neBiFeO3 powders using nitrates as metal presursors and tartaricacid22 and oxalic acid23 as complexing agents. The classicPechini method24 with citric acid as a complexing agent andethylene glycol (EG) as a polymerizer was reported not to yieldphase-pure BiFeO3.

21 Successful hydrothermal synthesis ofBiFeO3 has also been reported recently.

25,26

In this work, we present a simple wet chemical route forobtaining phase-pure BiFeO3 powders. A modied Pechinimethod where citric acid was substituted with tartaric acid,maleic acid, and malic acid resulted in phase-pure BiFeO3 pow-ders at temperatures from 4251 to 5001C. Necessary require-ments for the complexing agentpolymerizer system arediscussed. The possibility of using different carboxylic acids ascomplexing agents, with or without various amounts of EG aspolymerizing agents, can create opportunities to tailor the mor-phology and size of as-prepared BiFeO3 particles. We nallydemonstrate that the traditional solidstate reaction methodyields phase-pure BiFeO3 if careful measures with respect tostoichiometry and homogeneity are taken.

II. Experimental Procedure

(1) Synthesis

Wet chemical syntheses were performed with aqueous nitratesolutions as metal precursors. A bismuth nitrate precursor so-lution was prepared by dissolving Bi(NO3)3 5 H2O (Fluka,Seelze, Germany, 499%) in distilled water with addition ofHNO3 (Merck, Darmstadt, Germany, 65%) to pH 12. An ironnitrate precursor solution was prepared by dissolving anhydrousFe(NO3)3 (Merck, 499%) in distilled water with addition ofHNO3 (Merck, 65%) to pH 01. The concentrations of metalcations in the nitrate solutions were determined by thermogravi-metrical analysis (TGA) to be 0.317 and 1.628 mmol g1 forbismuth and iron, respectively. The solutions were heat treatedfor 24 h at 6501 and 10001C for bismuth and iron nitrates, re-spectively. A relatively low annealing temperature for the bis-muth solution minimized the risk of loss of volatile bismuthoxide. A scheme of the synthesis route is shown in Fig. 1. Car-boxylic acid (0.03 mol) was dissolved in distilled water (30 mL)in a pyrex beaker with a magnetic stirrer on a hot plate byholding at 501C. The carboxylic acids used were DL-tartaric acid(Acros Organics, Geel, Belgium, 99.5%), DL-malic acid (Aldrich,St. Louis, MO, 99%), succinic acid (Merck,499%), maleic acid(Merck, 499%), and malonic acid (Fluka, 99%). The metalnitrate precursor solutions (0.015 moles of each) were weighed,mixed, and then poured slowly into the beaker under stirring toensure complexing of metal cations. All the solutions displayed a

L. Kleincontributing editor

*Member, American Ceramic Society.This work was nancially supported by the Norwegian University of Science and

Technology, and the Research Council of Norway (NANOMAT, grant no. 158518/431,and SUP, grant no. 140553/I30).

wAuthor to whom correspondence should be addressed. e-mail: tor.grande@material.ntnu.no

Manuscript No. 22988. Received March 27, 2007; approved June 21, 2007.

Journal

J. Am. Ceram. Soc., 90 [11] 34303434 (2007)

DOI: 10.1111/j.1551-2916.2007.01937.x

r 2007 The American Ceramic Society

3430

yellow to orange color after addition of the nitrates, and turneddarker upon further heating. EG (Acros Organics,499.9%) ina molar ratio to the respective carboxylic acid of 1:1 was nallyadded to half of the syntheses as a polymerizing agent. Homog-enous solutions were heated under stirring with the hot plate byholding at 1501C. The solutions were nally transferred to crys-tallization jars on the same hot plate to evaporate water to pre-pare a dry polymeric precursor. After the precursor wascompletely dried, it was ground into a ne powder in an Agatmortar. Ground powders were calcined in air at a heating rate of4001C/h, and heat treated for 2 h at various temperatures.Bulk BiFeO3 powder was prepared by a solid-state reaction

between Bi2O3 (Aldrich,499.9%) and Fe2O3 (Merck,499%).Precursor powders were dried for 8 h at 6001C and carefullyweighed to ensure control of stoichiometry. Stoichiometricamounts of precursor powders were ball milled in ethanol for24 h with yttria-stabilized zirconia balls. Dried homogenousmixtures of precursor powder were pressed into pellets, placedon an alumina disk, and red at 8251C for 8 h.

(2) Characterization

X-ray diffraction (XRD) characterization was performed on ayy Bruker AXS D8 ADVANCE diffractometer (Karlsruhe,Germany) with a VANTEC-1 detector, CuKa radiation, and asecondary monochromator. Data for phase identication werecollected with a step size of 0.0161 and a count time of 0.2 s overthe 2y range 101601, and data for Rietveld renements were col-lected using the same step size and a count time of 1 s over the 2yrange 101901.Applying the Scherrer equation, dXRD5Kl/b cos y, for es-

timating crystallite sizes, K was set to 1.0, l to be a weightedaverage of 1.5418 A according to the emission prole for theCuK radiation source, with b being the FWHM of the peak.The (024) peak was chosen due to the high-relative intensity andthe absence of adjacent peaks. Using a Cauchy prole, b wascorrected for instrumental broadening with a NIST StandardReference Material

s

660a LaB6 powder (National Institute ofStandards and Technology, Gaithersburg, MD).

Lattice parameters were rened by the Rietveld27 methodusing the program TOPAS R (Bruker AXS v. 2.1). The back-ground was rened using a fth-order Chebichev function. Amodied ThompsonCoxHastings pseudo-Voigt function pro-le function (PV TCHZ) was used to rene the shapes of theBiFeO3 peaks. The space group R3c (No. 161) was used as amodel, with the starting values of lattice parameters and atomicpositions adopted from Kubel and Schmid11; a5 5.57874 A,c5 13.8688 A, Bi31 (0,0,0), Fe31 (0,0,0.22), and O2

(0.44,0.02,0.95). Peak shapes, lattice parameters, and scalewere rened simultaneously. After convergence, atomic posi-tions and nally isotropic temperature factors were included inthe renement.

Thermogravimetrical analysis of the precursors wasperformed with a Netzsch STA 449 C Jupiter (Selb, Germany)in synthetic air using a 101C/min heating rate up to 6001C. Sur-face area measurements (BET) were performed with a Micro-meritics Tristar Surface Area and Porosity Analyzer (Norcross,GA) and a Micromeritics VacPrep 061 Sample Degas System.The particle size, dBET, was calculated from the measured sur-face area using the relation dBET5 6(rcryst ABET)1, where rcrystis the crystallographic density of BiFeO3 (8.34 g/cm from TableII) and ABET is the surface area according to the BET iso-therm.28 Differential scanning calorimetry was performed with aPerkin Elmer DSC 7 (Waltham, MA), PE thermal analysis con-troller TAC 7/DX, and Pyris v. 3.81 software. Samples of 2050mg were encapsulated in aluminum sample pans, and the mea-surements were performed at 101C/min heating and coolingrates in the temperature region 2515501C.

III. Results

An overview of the syntheses and products identied by XRDafter annealing at 6001C is presented in Table I; (X) indicatesthat only trace amounts of the phase were present in the XRDpatterns, which were compared with JCPDS Card No. 86-1518(BiFeO3), 72-1832 (Bi2Fe4O9), and 46-0416 (Bi25FeO40). Syn-theses 1 (c), 2 (a), 2 (b), and 4 (b) are new wet chemical routesyielding phase-pure BiFeO3; synthesis 1 (a) was reproduced afterGhosh et al.22 for comparison.

All the carboxylic acids in Table I complexed the metalcations, as no precipitates were observed before the evapora-tion. Wet chemical syntheses, which yielded phase-pure BiFeO3,displayed more porous precursors after drying than those thatresulted in multiphase samples. Syntheses with EG as a poly-merizer produced more porous powder than those without.Powders from syntheses 1 (a), 1 (c), 2 (b), 3 (a), and 5 (a) dis-played completely amorphous XRD patterns before calcination,while XRD of the other as-synthesized powders showed someweak unidentied Bragg reections. All these weak reections

BiFeO nanoparticles

Bi-Fe-O precursors gel

Bi- and Fe-acid complexes

Bi-Fe-nitrate solution

Bi(NO ) 5 H O (s)

Bi nitrate (aq)

Fe(NO ) 9 H2O (s)

Fe nitrate (aq)

Distilled water

Carb. Acid (aq)

TGA

Distilled waterHNO (65 %)

Stirring and heating

Carboxylic acid Stirring at 50 C

Heating at hot plate holding 150 C

EG added to syntheses marked b) and c), Table 1

Calcination for 2 h

Fig. 1. Scheme of wet chemical synthesis routes.

Table I. Syntheses and Product Phases Identied by XRDafter Annealing at 6001C for 2 h

Synthesis Carboxylic acid Polymerizer BiFeO3 Bi2Fe4O9 Bi25FeO40 Unknown

1 (a) Tartaric X (X)J

1 (b) Tartaric EG X X1 (c) Tartaric EG X2 (a) Malic X2 (b) Malic EG X3 (a) Succinic X X X3 (b) Succinic EG X (X) (X)4 (a) Maleic X X X X4 (b) Maleic EG X5 (a) Malonic X X X5 (b) Malonic EG X (X) (X)

JPrecipitation of white crystals, possibly Bi(NO3)3, on the walls of the beaker at

the liquidair interface was observed during evaporation of the solution, and may

explain the trace amounts of Bi2Fe4O9 after calcination. XRD, X-ray diffraction;

EG, ethylene glycol.

November 2007 Synthesis of BiFeO3 3431

disappeared upon heat treatment due to the decomposition ofthe organics. The phase composition of the nal calcined BiFeO3powders was not inuenced by traces of crystalline material insome of the as-prepared powders. The solution with tartaric acidin synthesis 1 (a) wetted the surface of the beaker and white crys-tals precipitated on the walls of the beaker during evaporation ofthe solution. Synthesis 1 (b) with tartaric acid and EG ignitedduring TGA at 2101C, and probably also during calcination. Thepowder from the identical synthesis 1 (c) was then subjected tointermediate 2-h calcinations at 2001C and subsequently 2501C toavoid ignition. In synthesis 4 (a), using maleic acid, the precursorgel ignited during drying on the hot plate.

XRD patterns of as-prepared powders and after calcinationsat various temperatures from synthesis 2 (a) with malic acid andsynthesis 4 (b) with maleic acid and EG are shown in Figs. 2(a)

and (b), respectively. XRD patterns of powders calcined at6001C from the other syntheses are shown in Fig. 2(c). Crystal-lization and crystallite growth of BiFeO3 was evidenced by theXRD patterns recorded after increasing the calcination temper-ature (Fig. 2). The crystallization rate or temperature showsminor variations for the different syntheses as for synthesis 2 (a),the powder was amorphous after calcination at 4001C, while forsynthesis 4 (b), the powder was partly crystallized.

TGA of the powder from synthesis 4 (b) in Fig. 3 demon-strates that the precursor powder is completely decomposed at4501C when heated at 101C/min. Precursor powders from theother syntheses do not differ significantly, except the powderfrom synthesis 1 (b), which was shown to ignite at approximate-ly 2001C. The low weight loss above 4001C was also conrmedin separate subsequent calcinations at 4001, 5001, and 6001C.

Powder from synthesis 2 (b) precalcined for 2 h at 4001C wascompletely amorphous by XRD analysis, and negligible weightloss after further calcinations at higher temperatures indicatedthat negligible amounts of organic residuals were present in theamorphous oxide. Crystallization of the amorphous powder wasobserved by DSC (Fig. 4). Two separate exothermic peaks areevident, and the peaks are shifted to higher temperatures withincreasing heating rate. As discussed later, the two exothermicevents correspond to the nucleation and growth of crystallites.

The ordering temperature, TN, for the antiferromagnetic toparamagnetic second-order phase transition was determinedfrom the maximum of the endothermic peaks by numerical dif-ferentiation of DSC traces (Fig. 5). The values of 372171.51Cobtained are in very good agreement with previous reports.5

Inte

nsity

a.u

. (lin)

A

Inte

nsity

a.u

. (lin) B

2 /20 30 40 50 60

2 /20 30 40 50 60

2 /20 30 40 50 60

Inte

nsity

a.u

. (lin)

C

1 a)1 b)1 c) 2 b)3 a)3 b)4 a)5 a)5 b)

Fig. 2. X-ray diffraction patterns of powders from syntheses 2 (a)(A) and 4 (b) (B), respectively; labels refer to calcination temperatures(2 h). All syntheses except 2 (a) and 4 (b) after calcinations at 6001Cfor 2 h (C).

Temperature /C100 200 300 400 500 600

Res

idua

l mas

s %

40

50

60

70

80

90

100

Heat flow a.u. (Endo up)Synthesis 4 b)

Synthesis 4 b)Synthesis 1 b)

Synthesis 1 b)

Fig. 3. Thermogravimetrical analysis of powders from syntheses 1 (b)and 4 (b).

Temperature /C350 400 450 500 550

Hea

t Flo

w a

.u. (E

ndo u

p)

2 C min1

10 C min1

Fig. 4. Crystallization of amorphous powder observed by DSC. Minorpeaks due to noise are present.

3432 Journal of the American Ceramic SocietySelbach et al. Vol. 90, No. 11

The mean crystallite sizes estimated fromXRD data of phase-pure samples are shown in Fig. 6. Particle sizes calculated fromthe surface area for powders from synthesis 4 (b) are shown forcomparison. The particle size is significantly larger than thecrystallite size in line with expectations. An increase in crystallitesize with increasing calcination temperature is evident. Compar-ing dXRD with dBET, the increasing difference with increasing

calcination temperature can be attributed to an increased coars-ening of the powders with increasing temperature. Lattice pa-rameters rened by the Rietveld method are given in Table II,and the values were in accordance with the literature values forbulk BiFeO3.

811

XRD conrmed that phase-pure BiFeO3 was formed by asolid-state reaction between Bi2O3 and Fe2O3 at 8251C. Noevident peaks from residual reactants or secondary phases werepresent in the XRD pattern shown in Fig. 7.

IV. Discussion

Four new aqueous metal nitrate-based routes for the prepara-tion of phase-pure BiFeO3 have been demonstrated by syntheses1 (c), 2 (a), 2 (b), and 4 (b) in Table I. The syntheses without EGwere successful for tartaric acid and malic acid, but not for

Temperature /C300 325 350 375 400 425 450

Hea

t Flo

w a

.u. (E

ndo u

p)Heating

Cooling

Bulk, SSMPMAFM

Fig. 5. DSC traces of powders calcined for 2 h at 6001C (labels refer tosyntheses in Table I) and from a solid-state method (SSM). The verticaldotted line is a guide to the eye, separating the antiferromagnetic (AFM)and paramagnetic (PM) regions.

Calcination Temp. C400 450 500 550 600

Crys

tallit

esi

ze, d

[nm]

0

20

40

60

80

240

Fig. 6. Crystallite sizes from X-ray diffraction for different calcinationtemperatures (2-h soaking time) for powders prepared by wet chemicalsyntheses; labels refer to Table I. Typical error estimates for the Scherrerequation are shown for crystallites from synthesis 4 (b). One series withparticle sizes measured by BET, from synthesis 4 (b), is also shown.

2Th Degrees9080706050403020

Lin.

Cou

nts

70 000

65 000

60 000

55 000

50 000

45 000

40 000

35 000

30 000

25 000

20 000

15 000

10 000

5 000

0

5 000

10 000Difference

Fig. 7. Calculated and recorded X-ray diffraction patterns of bulk powder, from TOPAS.

Table II. Rened Lattice Parameters for Powders fromSynthesis 4 (b) and the SSM

Sample a (A) c (A) Rwp Rexp Rp

Single-crystal data4 5.57874 13.8688Bulk powder from SSM 5.578(5) 13.868(5) 7.06 1.67 4.674 (b), dXRD5 70 nm 5.578(9) 13.866(9) 4.43 1.35 3.15

SSM, solid state method; XRD, X-ray diffraction.

November 2007 Synthesis of BiFeO3 3433

succinic, maleic, or malonic acid. A requirement for the forma-tion of a homogenous polyester precursor without segregationof cations is thus proposed to be COOH groups for complexingBi31 and Fe31, and OH groups for polyesterication withCOOH groups. Bi31 has been reported to form nine-coordinat-ed complexes with tartaric and maleic acid.29 The addition ofequimolar amounts of EG with respect to the carboxylic acidsalso led to the synthesis with maleic acid to yield phase-pureBiFeO3 as well, while the syntheses with succinic and malonicacid were not successful. Succinic and malonic acids lack OHgroups on the main branch, which also only contains singlebonds, allowing the COOH groups to rotate independently,while maleic acid displays a double bond between the carbonatoms in the main branch, xing the relative orientations of theCOOH groups as cis. A rigid 3D network can form by conden-sation polymerization of OH groups, on either EG or the mainbranch of the carboxylic acid molecule, and COOH groups.

Precursor powder from synthesis 1 (b) with equimolaramounts of tartaric acid and ethylene glycol ignited during cal-cination, while syntheses 2 (b) and 4 (b) with tartaric acid sub-stituted by malic and maleic acid, respectively, did not. The ratioof OH groups to NO3 groups may determine a critical heatingrate for ignition. We propose that Fe31 may catalyze the igni-tion. Detection of an iron-rich phase Bi2Fe4O9, but not Bi25-FeO40, in the XRD pattern from synthesis 1 (b) is attributed tohigh local temperatures and evaporization of volatile Bi2O3.

The two exothermic events during heating (Fig. 4), and thediffractogram of the powder from synthesis 4 (b) calcined at4001C (Fig. 2(b)), suggests that the amorphous BiFeO3 goesthrough a nucleation and growth process to form crystallineBiFeO3. The shift toward higher temperatures with a higherheating rate signies that the events are thermally activated.Crystallite growth is also evident from XRD and BET (Figs. 2and 6) in the same temperature region as the second peak in theDSC curves. The present ndings show that nanocrystallineBiFeO3 can be obtained below approximately 4501C by thepresent wet chemical route.

The solid-state method presented here was successful due tothe careful measures taken to avoid the formation of Bi25FeO40and Bi2Fe4O9 by controlling the molar ratio of Bi:Fe, both withrespect to overall stoichiometry and homogeneity. Ball millingensured a homogenous mixture of ne Fe2O3 and Bi2O3 parti-cles, allowing a lower ring temperature to be utilized than forrapid liquid phase sintering.3,1719 No addition of extra Bi2O3to compensate for evaporization was thus needed to avoid theformation of undesirable secondary phases.

V. Conclusion

Successful synthesis routes for phase-pure BiFeO3 powders bysolid-state and wet chemical methods have been demonstrated.Phase-pure materials by wet chemical syntheses were obtainedwith aqueous nitrates as metal precursors and tartaric and malicacid, with and without EG, and maleic acid with EG as com-plexing and polymerizing agents, respectively. A nucleation andgrowth mechanism for the formation of crystalline BiFeO3 wasevidenced by DSC and XRD. Wet chemical syntheses yieldedcrystallite sizes from 13 to 70 nm for calcination temperaturesfrom 4001 to 6001C. Neel temperatures obtained by solid-statemethod and wet chemistry were in agreement with previousreports.

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3434 Journal of the American Ceramic SocietySelbach et al. Vol. 90, No. 11