Supplementary Information:
Effect of microstrain on the magnetic properties of BiFeO3 nanoparticles
Pavana S.V. Mocherla,1 C. Karthik,2 R. Ubic,2 M.S. Ramachandra Rao,3 C. Sudakar1,*
1
Multifunctional Materials Laboratory, Department of Physics, Indian Institute of Technology
Madras, Chennai 600036, India.
2
Department of Materials Science and Engineering, Boise State University, 1910 University
Drive, Boise, ID 83725, USA
3
Department of Physics and Nano Functional Materials Technology Centre, Indian Institute of
Technology Madras, Chennai-600036, India
* Corresponding author email ID: csudakar@iitm.ac.in
BiFeO3 nanoparticles of sizes ranging from 5 to 500 nm are synthesized using a low
temperature citrate sol-gel process.17. The as-obtained precursor gel was further dried by heating
between 80 oC to 100 oC. Particle size was controlled by calcining the precursor in the
temperature range 350 oC to 750 oC for various durations. The average crystallite size was found
from x-ray diffraction (XRD) using Scherrer’s equation after correcting for the instrumental
broadening and displacement errors due to sample mounting. This crystallite size is found to be
close to the particle size observed from TEM images. In the following discussion, we use the
sample code ‘BFO-d’, where‘d’ refers to the average crystallite size in nanometers.
The rate of sol heating and the metal cation to citric acid ratio are the two controlling
factors in synthesizing phase-pure BiFeO3. Any deviations from optimum conditions lead to the
formation of a small fraction of Bi2O3, Bi2Fe4O9, and Bi25FeO40 (iso-structural to Bi2O3) as the
common impure phases. To understand the phase formation from the precursor powder,
thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) of the asprepared precursor were carried out by a Netzsch thermal analyzer STA 449 F1 and a Perkin1
Elmer DSC respectively, at a heating rate of 5 oC/min in air. The TGA and DSC curves of as
prepared BiFeO3 precursor show that phase formation takes place when calcined ~ 400 oC
(Supplementary Fig. S1). Three clear stages of weight losses are observed in TGA plot. A
majority of weight loss (~ 40%) takes place around 170 oC during the first stage. Between 200 to
300 oC, a second stage of ~10 to 15 % weight loss takes place. Finally, during the third stage
between 300 to 400 oC, a small change in weight corresponding to 3 to 4 % is discerned. This
clearly shows that when the precursor powder is calcined at 350 oC for 3 h under static
conditions, it completely converts into a Bi-Fe-O amorphous phase with all the decomposed
organics becoming completely volatile from the precursor powder. The DSC studies on the
BiFeO3 precursor powder show exothermic peaks around these three stages of weight loss. The
first peak corresponds to the decomposition of citric acid at 176 oC, second peak at 321 oC is due
to the (NO3)- decomposition and the third broad peak in the range 400 to 500 oC corresponds to
the crystallization of BiFeO3.
Phase identification in the as-prepared and calcined powders as well as average crystallite
size calculations were done on x-ray diffraction patterns (XRD) recorded using PANalytical
X’pert PRO diffractometer, with Cu-kα radiation (λ=1.5406 Å) (Fig.S2). Further, bright field and
high resolution transmission electron microscopy (HRTEM) with electron diffraction were
carried out with a JEOL, JEM 2010HR operating at an accelerating voltage of 200 kV (Fig.S3).
A detailed report on this is presented elsewhere.17
2
Supplementary Figure S1: Thermogravimetry and differential scanning calorimetry of
precursor powder prepared by citrate-based sol-gel method.
15
Exo
80
10
60
5
Heat flow (mW)
Weight (%)
100
40
0
600
Endo
100
200
300
400
T (oC)
500
3
Supplementary Figure S2: X-ray diffraction patterns of BiFeO3 nanoparticles around 2 = 32o
Intensity (a.u.)
showing the merging of characteristic double peak (104) & (110) as a function of size.
BFO-65
BFO-38
BFO-30
(110)
(104)
BFO-29
BFO-25
BFO-22
BFO-5
31.2
31.6
32.0
2degree)
32.4
32.8
4
Supplementary Figure S3: Bright field TEM images [left panels (a), (d) and (g)], HRTEM
images [top right panels (b), (e) and (h)] and corresponding SADPs [bottom right panels (c), (f)
and (i)] of
BiFeO3 precursor calcined at 350 oC (BFO-5), 500 oC (BFO-38) and 550 oC
(BFO-65) respectively, showing the evolution of nanoparticles from amorphous form.
5
Supplementary Figure S4: HRTEM images of BFO crystallites (a) BFO-38 and (b) BFO-65.
Inverse FFT images of (a) and (b) are shown in (c) and (d) respectively. The images show the
decreasing microstrain with increasing size.
(a)
(c)
(b)
(d)
6
Supplementary Figure S5: Room temperature M-H hysteresis curves of (a) BiFeO3
nanoparticles for  1 T (b) BFO-230 and BFO-580 samples
0.15
(a)
0.0
BFO-AP
M (emu/g)
M (emu/g)
BFO-230
BFO-580
0.10
0.1
0.05
0.00
-0.05
BFO-5
BFO-22
-0.1
-0.10
(b)
BFO-38
BFO-65
-0.10
-0.05
0.00
H (T)
0.05
0.10
-0.15
-2
-1
0
H (T)
1
2
7