Factors controlling pure-phase multiferroic BiFeO3 powders synthesized by chemical co-precipitation

Article abstract of DOI:10.1016/j.jallcom.2010.09.213

A chemical co-precipitation method is proposed for the synthesis of nano-sized multiferroic BiFeO₃ powders. Pure phase can be obtained by controlling the chemical co-precipitation process, pH level, Bi content, and calcination temperature. The evolutions of phase constitution and structural characteristic of BiFeO₃ powders were investigated by XRD, FT-IR, SEM, and TEM techniques. The slight excess in Bi content plays an important role in the synthesis. The resultant BiFeO₃ powders are of pure phase and have the R3c crystal structure under optimized conditions. The grain size of the BiFeO₃ powders with weak magnetism ranges from 30-40 nm.

Full text of DOI:10.1016/j.jallcom.2010.09.213

Journal of Alloys and Compounds 509 (2011) 2192–2197
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Factors controlling pure-phase multiferroic BiFeO3 powders synthesized by
chemical co-precipitation
Hua Kea,∗, Wen Wanga, Yuanbin Wanga, Jiahuan Xua, Dechang Jiaa, Zhe Lub, Yu Zhoua
a Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, PR China
b Department of Physics, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2010
Received in revised form 6 September 2010
Accepted 8 September 2010
Available online 9 November 2010
A chemical co-precipitation method is proposed for the synthesis of nano-sized multiferroic BiFeO3 pow-
ders. Pure phase can be obtained by controlling the chemical co-precipitation process, pH level, Bi content,
and calcination temperature. The evolutions of phase constitution and structural characteristic of BiFeO3
powders were investigated by XRD, FT-IR, SEM, and TEM techniques. The slight excess in Bi content plays
an important role in the synthesis. The resultant BiFeO3 powders are of pure phase and have the R3c crys-
tal structure under optimized conditions. The grain size of the BiFeO3 powders with weak magnetism
ranges from 30–40 nm.
Keywords:
Chemical co-precipitation
Multiferroic
© 2010 Elsevier B.V. All rights reserved.
BiFeO3
1. Introduction
thesis temperature (300–500 ◦C), reducing impurities to a great
extent. Using an oxygen-enriched atmosphere can keep the Fe ion
In recent years, considerable attention has been drawn to mul-
tiferroic materials because of their fascinating applications in novel
electronic devices [1]. Single-phase BiFeO3 holds much poten-
tial as a multiferroic candidate because magneto-electric coupling
occurs even at room temperature—an attribute that endows inte-
grated devices with technological advantage [2]. The use of BiFeO3,
however, presents a few problems. Avoiding impurities is one
of the primary concerns regarding the synthesis of such single-
phase multiferroic materials [3]. The impurities are thought to arise
from three different causes: First, the evaporation of Bi compo-
nent occurs easily at the onset of synthesis because of the low
decompounded temperature of bismuth salts; this Bi2O3 compo-
nent appears again in the final production as an impurity. Second,
the chemical valence of Fe ion varies in an oxygen-deficient atmo-
sphere; the charge defects with respect to Fe2+ ions produced in
the synthesis are usually related to the large leakage current in
BiFeO3 [4,5]. Third, the synthesis area of single-phase BiFeO3 in the
phase diagram of Bi2O3–Fe2O3 is very narrow from the view point
of thermodynamics, in which two kinds of impurities (Bi2Fe4O9 and
Bi25FeO39) are the usual substitutions for BiFeO3 [6,7].
in Fe3+ form, which minimizes the leakage current of BiFeO3. For
the preparation of ceramics, liquid phase sintering completed in
the solid-state route with a high heating rate (100 ◦C) can inhibit
the production of Bi2Fe4O9 and Bi25FeO39 [11].
In this paper, a simple chemical co-precipitation method is pro-
posed for the synthesis of pure single-phase BiFeO3 powders. The
production of impurities can be avoided by controlling the tech-
nical parameters using the laws of thermodynamics as bases. The
crystallization process of BiFeO3 is also discussed.
2. Experimental
The reaction precursors were Bi(NO3)3·5H2O and Fe(NO3)3·9H2O powders dis-
solved in nitric acid. To precisely control the chemical precipitation process, we
kept pH level constant through synchronized dropping of ammonia (2.5 M) and
nitrate solution (2 M). Fig. 1 illustrates this modified method. Fig. 1(a) shows a tradi-
tional chemical precipitation process, and Fig. 1(b) shows the modified precipitation
process in which a constant pH level is maintained. This modification can ensure
homogenous precipitation in the entire process. All of the chemical reagents used
were of analytical grade. The final suspension was continually washed until a pH
of 7 was obtained. The powders were then filtered, dried, and weighed. Finally, the
powders were calcined at various heating schedules.
Thermogravimetric-differential thermal analysis (TG-DTA, STA449C) was con-
ducted with a heating rate of 10 ◦C/min to investigate the thermal reactions of the
powder. The phase structure of the powder was identified using an X-ray diffrac-
tometer (Rigaku-D/max-cB, Japan) with CuK␣ radiation. Fourier transform infrared
(FT-IR) spectra were recorded using a Bruker IFS66 v/S spectrometer. The morphol-
ogy and size distribution of the BiFeO3 powders were examined by transmission
electron microscopy (TEM, CM12). The room-temperature magnetization was mea-
sured using a vibrating sample magnetometer (VSM, Lakeshore 7410).
The wet-chemical route is feasible for the pure-phase synthe-
sis of BiFeO3 considering the aforementioned factors [8–10]. For
instance, the sol-gel method is able to substantially lower syn-
∗
Corresponding author.
E-mail address: hua ke@hit.edu.cn (H. Ke).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.09.213

H. Ke et al. / Journal of Alloys and Compounds 509 (2011) 2192–2197
2193
Fig. 1. Schematic of the chemical co-precipitation preparation for BiFeO3 powder;
(a) a traditional co-precipitation process; (b) a homogenous co-precipitation.
Fig. 3. XRD patterns of the resultant powders prepared at different pH levels (atomic
ratio Bi/Fe = 1:1).
3. Results and discussion
3.2. Effect of Bi content
3.1. Effect of pH
To further decrease impurities, Bi content (molar ratio) was con-
sidered as a means of controlling the co-precipitation process. Fig. 4
illustrates the XRD patterns of the BiFeO3 powders with differ-
ent amounts of excess Bi at pH 9.3 and calcination temperature
of 550 ◦C. The BiFeO3 powders are of pure phase and have the R3c
crystal structure when the Bi content has an excess of 3%. The impu-
rities also appear when excess Bi content is larger or smaller than
3% (Fig. 4).
Fig. 2 shows the TG-DTA curves of the powder prepared by a
typical co-precipitation process. The peak near 510 ◦C corresponds
to the crystallization of the BiFeO3 powder. Thus, to investigate
the yield of BiFeO3 powders as a function of pH, the calcination
temperature was set to 550 ◦C; this was high enough for the crys-
tallization of BiFeO3 powder in the experiment. Fig. 3 illustrates
the XRD patterns of the resultant powders prepared at different pH
levels. Two kinds of impurities (Bi2Fe4O9 and Bi2O3) are found in
the products. The yield of BiFeO3 powders increases with decreas-
ing pH level. The weight percent of impurities is estimated to be
12% in the sample at pH 9.3. When the solution is at a lower pH
level, impurities are slightly reduced and precipitation slowly pro-
gresses. This is an indication that homogenous powders of uniform
size can be obtained when the solution is at a moderate pH level
in the precipitation process. This is also important for the comple-
tion of the chemical reaction in the succeeding calcination process
(Section 3.4). The influence of pH on BiFeO3 yield may be related to
the evolution of the surface activity of the powders.
3.3. Crystallization
3.3.1. TG-DTA curve
Fig. 5 shows the TG-DTA curves of the resultant powders pre-
pared under the optimized technical parameters (pH 9.3, Bi content
excess of 3%). The endothermic peaks located at 228.5 and 270 ◦C
in the DTA curve are attributed to the decomposition of hydrates
and nitrates, which is accompanied by a large weight loss in this
stage. A chemical reaction occurs at 481 ◦C, which corresponds to
the crystallization of BiFeO3 powders. The crystallization temper-
Fig. 2. TG-DTA curves of BiFeO3 powders prepared by a typical co-precipitation process.

2194
H. Ke et al. / Journal of Alloys and Compounds 509 (2011) 2192–2197
Fig. 4. XRD patterns of BiFeO3 powders with different amounts of excess Bi at pH 9.3 and calcination temperature of 550 ◦C.
ature of this optimized BiFeO3 powder is 30 ◦C lower than that of
the powder prepared without optimization. Crystallization tem-
perature is dominated by this chemical reaction, which can be
controlled by the pH level and Bi content in the preceding chemical
co-precipitation. The small weight loss at 270–481 ◦C is considered
the decomposition of remnant nitrates. A ferroelectric–paraelectric
phase transition without weight loss is expected to appear at 829 ◦C
in the BiFeO3 powders, as shown in the TG-DTA curve [6,12,13].
the calcination temperature increases, the organic bands disappear
except for a small remnant of NO3 in the 300 and 400 ◦C-calcined
−
samples; this is consistent with the TG-DTA results. The broad
bands located at 600–400 cm−1 are generated by the vibration of
Fe–O and Bi–O in the amorphous samples. After crystallization, the
BiFeO3 powders show the vibrational Fe–O bands, which stem from
the appearance of A1 and E phonon modes [14]. The peaks at 555
and 420 cm−1 respectively correspond to the bending vibration of
O–Fe–O and stretching vibration of Fe–O, a typical characteristic of
BiFeO3 perovskite with the R3c structure.
3.3.2. FT-IR spectra
Fig. 6 presents the FT-IR spectra of BiFeO3 powders calcined at
different temperatures for 1 h. The spectra agree well with the TG-
DTA curves. The peak at 3420 cm−1 is attributed to the stretching
of –OH. Both in the original co-precipitation and 200 ◦C-calcined
samples, the peak located at 1630 cm−1 indicates the presence of
hydrates. In the original co-precipitation sample, the strong peak
near 1380 cm−1 can be attributed to the vibration of NO3−. When
3.3.3. Phase constitutions
The XRD patterns of the BiFeO3 powders calcined at differ-
ent temperatures for 1 h are shown in Fig. 7. Below 450 ◦C, the
diffraction patterns are from the amorphous phase. The pow-
ders crystallize when the annealing temperature is higher than
450 ◦C [Fig. 7(b)], but have remnant Bi2O3 both in 450 and 600 ◦C-
Fig. 5. TG-DTA curves of BiFeO3 powders prepared by a homogenous co-precipitation process at pH 9.3 and 3% excess Bi.

H. Ke et al. / Journal of Alloys and Compounds 509 (2011) 2192–2197
2195
Fig. 6. FT-IR spectra of BiFeO3 powders calcined at different temperatures for 1 h.
Fig. 7. XRD patterns of BiFeO3 powders calcined at different temperatures for 1 h (a) at room temperature ∼425 ◦C; (b) at 450–600 ◦C, the Rietveld analysis of the 550 ◦C-
calcined sample is shown in the figure; the inset (right) shows the patterns of other samples.

2196
H. Ke et al. / Journal of Alloys and Compounds 509 (2011) 2192–2197
Fig. 9. Typical M–H loop of BiFeO3 powder at room temperature. The inset is an
enlarged part of the loop between −2 kOe and 2 kOe.
Fig. 8. TEM image of BiFeO3 powder.
calcined samples. XRD Rietveld analysis indicates that no secondary
phase is observed in 500 and 550 ◦C-calcined samples. After
subtracting background contribution, the crystallinity of BiFeO3
powders calcined at high temperatures can be calculated as follows
[15]:
antiferromagnet at room temperature and does not exhibit mag-
netism in the M–H curve. The weak magnetism can be attributed to
the size effect of the nano-sized BiFeO3 powder, which differs from
the linear M–H relationship in BiFeO3 ceramics [17,18].
3.4. Discussion
∞
ꢀ
ICcorrr(2ꢀ) sin2(ꢀ) cos(ꢀ)d(ꢀ)
The nano-sized powder prepared by homogenous precipitation
was subjected to the nucleation and growth of nano-sized parti-
cles. The pH level of the Bi–Fe solution plays an important role in
the “homogenous” nucleation and growth of Bi(OH)3 and Fe(OH)3
particles. The corresponding chemical reactions are as follows:
0
Cr =
(1)
∞
ꢀ
(ICcorrr(2ꢀ) + Iacmorr(2ꢀ)) sin2(ꢀ) cos(ꢀ)d(ꢀ)
0
Bi3+ + 3OH− → Bi(OH)3↓; Fe3+ + 3OH− → Fe(OH)3↓
(3)
corr
where ꢀ is the diffraction angle, I
is the corrected scattering
Cr
intensity from the crystalline phase, and Iacmorr denotes the corrected
scattering intensity from the amorphous phase. In the 500 ◦C-
calcined sample, the crystallinity of the BiFeO3 powder is estimated
as 88.5% according to Eq. (1). The BiFeO3 powder prepared at 550 ◦C
is of pure phase and has the R3c crystal structure.
The grain size of the BiFeO3 powder was determined from the
integrated width of the diffraction peak using the Scherrer equation
[16].
In this study, “homogenous” has two connotations: the balance
between the nucleation and growth of Bi(OH)3 and Fe(OH)3 for
maintaining sufficiently small precipitated particles, which should
keep the pH level low; and the precipitation of Bi(OH)3 proceeding
slowly to keep pace with that of Fe(OH)3, which should keep the pH
at a constant level. The modified co-precipitation route proposed in
this study is a feasible solution for achieving this homogenous pro-
cess. Through this homogenous co-precipitation, the precipitated
powders exhibit phase stability and have no secondary phase in
the calcination process, as shown in Fig. 7(a). The structural char-
acteristic of BiFeO3 in the amorphous phase can be observed from
the original precipitated powder in the 425 ◦C-calcined sample. The
peak broadening in these samples is generated by the diffraction of
amorphous-phase BiFeO3. No substantial changes are observed in
the broadened peaks except for the width and intensity. This phase
stability can be considered essential to inhibiting the production of
impurities during the calcination process at low temperatures.
The phase constitution varies with further increase in calci-
nation temperature. The appearance of very small amounts of
Bi2O3 and Bi2Fe4O9 in the 450 ◦C-calcined sample [Fig. 7 (b)] indi-
cates that the powder undergoes a solid-state reaction between
Bi2O3 and Fe2O3, as shown by the exothermic peak in Fig. 5.
This chemical reaction also marks the crystallization of BiFeO3.
The 500 and 550 ◦C-calcined samples are phase-pure and possess
the R3c structure. As Carvalho and Tavares pointed out, however,
the decomposition of BiFeO3 shows one-dimensional kinetics, and
BiFeO3 is not stable even at low temperatures (500–600 ◦C) [19]. In
this study, two aspects can be analyzed as the causes of impurity
reduction: First, the excess Bi content can effectively compensate
for the evaporation of Bi component in the chemical reaction during
the calcination process. Second, from a thermodynamics perspec-
ꢁ
ꢂ
ꢁ
ꢂ
2
ꢃ
ꢄ
2
ı
Kꢂ
ı
ꢃd
dhkl
=
+ 16
(2)
Lhkl
tan ꢁ
tan ꢁ sin ꢁ
where ı is the integrated width of diffraction peak, K is a constant
(usually equal to 1), Lhkl denotes the size of (h k l) normal direc-
tion, ꢂ is the wavelength, ꢁ represents the diffraction angle, dhkl is
the interplanar distance, and ꢃd is the average deviation caused
by stress. The value of Kꢂ/Lhkl provides the grain size of nanopar-
ticles. We used the (0 1 0) and (1 1 0) peaks to calculate grain size.
Two factors, i.e., “apparatus width” and the K␣2 line of the target,
must be taken into account to extract the integrated widths of the
diffraction peaks. The calculated grain sizes of the BiFeO3 pow-
ders prepared at 500 and 550 ◦C are 31 and 38 nm, respectively.
The TEM image of the BiFeO3 powder with an irregular morphol-
ogy is shown in Fig. 8. The size of the BiFeO3 powder obtained
from the TEM image is slightly larger than that obtained from
the XRD calculations; this is due to the soft agglomeration of the
powder.
3.3.4. Magnetism
Fig. 9 shows the typical M–H loop of the BiFeO3; weak mag-
netism can be observed in the powder. Bulk BiFeO3 is a G-type

H. Ke et al. / Journal of Alloys and Compounds 509 (2011) 2192–2197
2197
tive, the fast chemical reaction between Bi2O3 and Fe2O3 needs
References
excess Bi to serve as the driving force because the R3c phase of
BiFeO3 has no solvus line in the Bi2O3–Fe2O3 phase diagram. Fur-
thermore, the phase diagram shows that the slight excess in Bi
content can reduce the production of Bi2Fe4O9 because a chem-
ical reaction between the Bi-rich phase and Bi2Fe4O9 may occur.
Thus, the slight excess in Bi content may play a fundamental role
in the preparation of pure-phase nano-sized BiFeO3 powders.
When the calcination temperature was increased further,
BiFeO3 began to decompose because of its unstable thermody-
namic character. Small amounts of Bi25FeO39 can be observed in
the 600 ◦C-calcined sample.
[1] J.F. Scott, Nat. Mater. 6 (2007) 256–257.
[2] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland,
V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M.
Wuttig, R. Ramesh, Science 299 (2003) 1719–1722.
[3] M.M. Kumar, V.R. Palkar, K. Srinivas, S.V. Suryanarayana, Appl. Phys. Lett. 76
(2000) 2764–2766.
[4] S.J. Clark, J. Robertson, Appl. Phys. Lett. 94 (2009) 022902.
[5] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 224103.
[6] R. Palai, R.S. Katiyar, H. Schmid, P. Tissot, S.J. Clark, J. Robertson, S.A.T. Redfern,
G. Catalan, J.F. Scott, Phys. Rev. B 77 (2008) 014110.
[7] J. Yu, N. Koshikawa, Y. Arai, S. Yoda, H. Saitou, J. Cryst. Growth 231 (2001)
568–576.
[8] J.K. Kim, S.S. Kim, W.-J. Kim, Mater. Lett. 59 (2005) 4006–4009.
[9] S.M. Selbach, M.-A. Einarsrud, T. Tybell, T. Grande, J. Am. Ceram. Soc. 90 (2007)
3430–3434.
[10] S. Ghosh, S. Dasgupta, A. Sen, H.S. Maiti, J. Am. Ceram. Soc. 88 (2005) 1349–1352.
[11] Y.P. Wang, L. Zhou, M.F. Zhang, X.Y. Chen, J.-M. Liu, Z.G. Liu, Appl. Phys. Lett. 84
(2004) 1731–1733.
[12] V.V. Shvartsman, W. Kleemann, R. Haumont, J. Kreisel, Appl. Phys. Lett. 90
(2007) 172115.
[13] I.A. Kornev, S. Lisenkov, R. Haumont, B. Dkhil, L. Bellaiche, Phys. Rev. Lett. 99
(2007) 227602.
4. Conclusions
Multiferroic BiFeO3 powders were synthesized through a simple
chemical co-precipitation route. The slight excess in Bi plays an
important role in the chemical co-precipitation synthesis of nano-
sized BiFeO3 powders. The resultant BiFeO3 powders with weak
magnetism are of pure phase and have the R3c crystal structure.
[14] E. Kroumova, M.I. Aroyo, J.M. Perez-Mato, A. Kirov, C. Capillas, S. Ivantchev, H.
Wondratschek, Phase Trans. 76 (2003) 155–170.
[15] W. Ruland, Acta Cryst. 14 (1961) 1180–1185.
[16] W.L. Zhong, B. Jiang, P.L. Zhang, J.M. Ma, H.M. Cheng, Z.H. Yang, J. Phys.: Condens.
Matter 5 (1993) 2619.
Acknowledgments
[17] S.T. Zhang, M.H. Lu, D. Wu, Y.F. Chen, N.B. Ming, Appl. Phys. Lett. 87 (2005)
262907.
[18] A.K. Pradhan, K. Zhang, D. Hunter, J.B. Dadson, G.B. Loiutts, P. Bhattacharya,
R. Katiyar, J. Zhang, D.J. Sellmyer, U.N. Roy, Y. Cui, A. Burger, J. Appl. Phys. 97
(2005) 093903.
This work is supported by the Chinese National Natural Sci-
ence Foundation under grant No. 50902033, Natural Scientific
Research Innovation Foundation in HIT (HIT. NSRIF. 2008.35), and
Heilongjiang Postdoctoral Fund (No. LBH-Z06100).
[19] T.T. Carvalho, P.B. Tavares, Mater. Lett. 62 (2008) 3984–3986.