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 as-

prepared precursor were carried out by a Netzsch thermal analyzer STA 449 F1 and a Perkin-

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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

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Supplementary Figure S1: Thermogravimetry and differential scanning calorimetry of

precursor powder prepared by citrate-based sol-gel method.

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Supplementary Figure S2: X-ray diffraction patterns of BiFeO3 nanoparticles around 2 = 32o

showing the merging of characteristic double peak (104) & (110) as a function of size.

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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.

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(a) (c)

(b) (d)

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.

Supplementary Figure S5: Room temperature M-H hysteresis curves of (a) BiFeO3

nanoparticles for 1 T (b) BFO-230 and BFO-580 samples

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