Effect of Pr substitution on structural and electrical properties of BiFeO3 ceramics

Materials Chemistry and Physics 143 (2014) 629e636

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Effect of Pr substitution on structural and electrical properties of BiFeO3 ceramics Poorva Sharma a, Dinesh Varshney a, *, S. Satapathy b, P.K. Gupta b a b

School of Physics, Vigyan Bhawan, Devi Ahilya University, Khandwa Road Campus, Indore 452001, India Laser Material Development and Devices Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


XRD patterns infer triclinic (P1) of Prdoped BiFeO3 (0.15  x  0.25).
Raman spectra reveal suppression of ferroelectric behavior due to Pr doping.
Changes in Raman normal modes are noticed with increasing doping concentration.
Room temperature ε0 values are y65, 82 and 95 for x ¼ 0, 0.15, 0.25 at 1 MHz.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 May 2012 Received in revised form 14 August 2013 Accepted 29 September 2013

Investigation on structural, vibrational, dielectric and ferroelectric properties of Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) ceramic samples has been carried out. Room temperature Rietveld-refined X-ray diffraction pattern shows the crystal structure of Bi1xPrxFeO3 is rhombohedral for x ¼ 0 and triclinic for x ¼ 0.15, 0.25. The changes in Raman normal modes with increasing doping concentration infer the structural transformation is due to Pr substitution at A-site in BiFeO3. Raman spectra also reveal suppression of ferroelectric behavior due to Pr doping. The dielectric parameters, namely, dielectric permittivity (ε0 ) and loss tangent (tan (d)) were evaluated as a function of frequency at room temperature. The ferroelectric polarization reduces in Pr doped bulk BFO samples due to structural change. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: A. Ceramics C. Powder diffraction C. Raman spectroscopy and scattering D. Dielectric properties

1. Introduction Bismuth ferrite (BFO) based oxides are promising materials not only to understand the mechanism but also for possible device applications and their properties are reviewed at great length [1,2]. Pristine BFO is fascinated due to its high ferroelectric transition temperature (TC w 1103 K [3]) and antiferromagnetic Neel temperature (TN w 643 K [4]) which are well above room temperature. BFO posses ABO3 distorted perovskite-type material at

* Corresponding author. Tel./fax: þ91 731 2467028. E-mail address: [email protected] (D. Varshney). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.045

room temperature and crystallize in a rhombohedral system (space group R3c). This structure permits the development of a spontaneous polarization along the pseudocubic (111)C, because the 6s2 lone pair of Bi3þ ion introduces an off-center distortion results in a spontaneous polarization in BFO, whereas super exchange interactions between Fe3þ ions determine antiferromagnetic ordering [5]. Due to high conductivity, antiferromagnetic ordering and high leakage current, device application based on BFO is difficult. The partial substitution of Bi3þ ions with rare-earth (RE) ions was made to seek any possible structural transition and also to improve the physical properties of the BFO [6e12]. It is noticed that beyond certain percentage of doping, the crystal structure of BFO is

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changed. A structural phase transition from rhombohedral to orthorhombic is reported in Bi1xLaxFeO3 (x ¼ 0.30) [6]. A structural transformation from rhombohedral structure to triclinic structure is reported for Bi1xNdxFeO3 (x ¼ 0.05e0.15), but further Nd doping leads to a centrosymmetric tetragonal (P4/mmm) structure and the existence of ferroelectricity is ruled out for doping range (0.175  x  0.2) of Nd in BFO [7]. The co-doped Bi0.9xLa0.1NdxFeO3 (x ¼ 0.05e0.1) ceramics showed an induced phase transition from rhombohedral R3c to triclinic and monoclinic on subsequent Nd and La doping [8]. The crystal structure of co-doped BFO like Bi0.8Gd0.1Ba0.1FeO3 is characterized by the coexistence of Pnma and R3c phases [9]. Similarly it has been noticed that for 15% Sm dopant the BFO retain its rhombohedral (R3c) structure, while the structure was changed to orthorhombic Pnma with higher percentage of doping (>15%) [10]. Due to substitution at A-site in BFO by rare earth dopants (Tb, La, Yb, Gd) the ferroelectric, conductivity and dielectric properties of BFO were improved [11e13] but for enhanced doping (>20%) of La, Gd, Nd, the electric properties gets deteriorated [6,14,15]. It is reported that the Pr substitution at Bi site as Bi0.9xLa0.1PrxFeO3 (BLPFO x ¼ 0, 0.1 and 0.2) eliminate small impurity phase in BFO and stabilized the crystal structure which enhanced the dielectric properties [16]. For Pr doping (0  x  0.15), there is a decrease in dielectric loss but it is insufficient to sustain the electric field more than 15 kV cm1 as observed from ferroelectric polarization measurement [17]. Several groups have thus reported the structural, and electrical properties for RE doping as well co doping at Bi site in BFO (x  15%) [6e17]. To the best of our knowledge no systematic efforts have been made to investigate structural, dielectric and ferroelectric properties of Pr doped BFO (x  25%). It is well known that Pr, like La, belongs to the lanthanides and the ionic radii of Pr3þ (0.99  A) and Pr4þ (0.85  A) are smaller than that of Bi3þ (1.17  A). The substitution of Bi with Pr (having small ionic radius) in bulk BFO might distort the unit cell and change the dielectric and ferroelectric properties of BFO. Therefore, it might be interesting to prepare and investigate Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) with an objective to seek the structural and electrical behavior of Pr doped BFO. In this paper, we report the structure, dielectric and ferroelectric properties of Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) ceramics as prepared by solid-state reaction route. A substitution-induced structural phase transition from rhombohedral (x ¼ 0) to triclinic for x ¼ 0.15, 0.25 is observed. The structural deformations were further supported from Raman scattering results. The effect of Pr substitution at Bi-site in BFO on dielectric and ferroelectric properties were also investigated. For Pr doped samples (x ¼ 0.25) the dielectric loss increases sharply at low frequencies confirms an increase in conductivity.

Rigaku X-ray diffractometer with CuKa1 (1.5406  A) radiation. Later on, the Rietveld refinements have been performed using FullPROF program to determine the diffraction parameters. The Raman measurements on as synthesized samples were carried out on LABRAM HR800 spectrometer with a 632.8 nm excitation source equipped with a Peltier cooled CCD detector (1024  256 pixels of 26 microns) and the laser beam was focused on the sample by a 50 lens to give a spot size of 1 mm; the resolution was better than 2 cm1. Dielectric studies were performed as a function of frequency in the range of 100 Hze1 MHz on Novocontrol alpha-ANB impedance analyzer. Ferroelectric (PeE) measurement of sintered pellets was carried out using a ferroelectric loop tracer based on Sawyer-Tower circuit. 3. Results and discussion 3.1. Structural analysis The XRD pattern of parent and Pr doped perovskites having a concentration of Bi1xPrxFeO3 (x ¼ 0, 0.15 and 0.25) are shown in Fig. 1. The XRD pattern of pristine BFO is indexed in the rhombohedral system (space group R3c) with a very small amount of nonperovskite Bi2Fe4O9 impurity phase (marked by * in Fig. 1). The occurrence of Bi2Fe4O9 secondary phase peaks is generally observed in pure BFO due to the kinetics of phase formation and the high volatility of Bi2O3. We didn’t obtain any impurity phase in BPFO-15 and BPFO-25. The peak position at around 22.2 gets splitted (as shown in inset of Fig. 1) with Pr doping concentration. This splitting of the diffraction peak might be due to structural phase transition. In order to determine the structure the diffraction pattern were Rietveld refined using FullPROF 2000 program [20]. The program allowed us to reproduce all observed reflections and gave all identical reliability factors. The profile fits for the Rietveld refinement of Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) samples are shown in Fig. 2. It is noticed that the simulated XRD pattern agrees well with the measured data. The XRD pattern of parent BFO were refined with rhombohedral R3c space group with lattice parameters a w5.5798  A and c w13.867  A whereas, the XRD pattern of BPFO-15 and BPFO-25 refined in triclinic P1 space group with lattice parameter (a ¼ 3.9288  A, b ¼ 3.9467  A, c ¼ 3.9420  A) and (a ¼ 3.9539  A, b ¼ 3.9629  A, c ¼ 3.9621  A). The obtained lattice parameter matches well with the earlier reported data [17,21].

2. Experimental details Polycrystalline pure BFO and doped Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) samples, were prepared using solid-state reaction route [18]. Henceforth, Bi1xPrxFeO3 (x ¼ 0.0, 0.15 and 0.25) samples were designated as BFO, BPFO-15 and BPFO-25, respectively. The dried Bi2O3, Fe2O3 and Pr6O11 reagents in the desired stoichiometric ratios were wet mixed together. The powder was doubly calcined consecutively at 650  C for 1 h and 830  C for 1.3 h with intermediate grinding in between to achieve the desired phase. The calcined powders were leached with dilute HNO3 followed by multiple washing with deionized water [19]. The leached powders were mechanically pressed at 8e10 tonnes to form 2 mm thick and 15 mm diameter pellets. X-ray powder diffraction was carried out to identify the crystal structure and to detect any impurity phase in the sample using

Fig. 1. The X-ray diffraction pattern for Bi1xPrxFeO3 samples for x ¼ 0.0, 0.15, 0.25.

P. Sharma et al. / Materials Chemistry and Physics 143 (2014) 629e636

BiFeO3

* Bi2 Fe4 O9

(a)

631

Yobs Ycal Yobs - Ycal Bragg position

*

Bi0.85 Pr0.15 FeO3

Intensity (a.u.)

(b)

(c)

20

Bi 0.75 Pr0.25 FeO3

30

40

50

60

70

2θ (deg.) Fig. 2. Rietveld refinement XRD pattern of Bi1xPrxFeO3 samples for x ¼ 0.0, 0.15, 0.25.

We may add that as compared to parent BFO the volume of BPFO-15 decreases but volume of BPFO-25 increases compare to volume of BPFO-15. It may possible that for enhanced doping (0.15  x  0.25) more Pr3þ replaces Bi3þ compared to Pr4þ which reflects an increase in volume at higher percentage of Pr doping as the radius of Pr3þ (0.99  A) is larger than Pr4þ (0.85  A). The calculated parameters of Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) after refinement are listed in Table 1. We have also refined BPFO-15 and BPFO25 using R3c and C2 space group, but Rwp and Rp is lowest for triclinic (P1) rather than for rhombohedral and monoclinic structures. Thus, the structure of BPFO-15 and BPFO-25 is triclinic (P1). The crystal structure of BFO and BPFO is developed by FpStudio Ver-2.0 program as shown in Fig. 3. The figures shown are

perspective views of three-dimensional abc-axis. These schematic figures represent the structure transform from R3c to P1 with Pr substitution in BiFeO3. 3.2. Raman analysis The measured Raman scattering spectra of BFO, BPFO-15 and BPFO-25 perovskites are shown in Fig. 4. By fitting the observed data and decomposing the fitted curves into individual Lorentzian components, the peak position of each component, i.e., the natural frequency of each Raman active mode is thus obtained. The Raman active modes of the structure can be summarized using the irreducible representation GRaman ¼ 4A1 þ 9E [22]. The parent BFO

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P. Sharma et al. / Materials Chemistry and Physics 143 (2014) 629e636 Table 1 Structural parameter for Bi1xPrxFeO3 (x ¼ 0.0, 0.15 and 0.25) obtained by Rietveld refinement of the XRD patterns at room temperature. Bi1xPrxFe O3

Crystal structure

Lattice parameters ( A) Volume ( A3)

Atomic positions

R factors (%)

x ¼ 0.0

Rhombohedral (R3c)

a ¼ 5.5798 c ¼ 13.867 V ¼ 373.76

Bi (0.0, 0.0, 0.0) Fe (0.0, 0.0, 0.2163) O (0.5638, 0.0197, 0.9475)

c2 ¼ 2.786

Bi/Pr (0.0, 0.0, 0.0) Fe (0.4502, 0.4555, 0.546) O1 (0.5655, 0.4155, 0.3545) O2 (0.5345, 0.0644, 0.6843) O3 (0.4338, 0.4702, 0.0166) Bi/Pr (0.0, 0.0, 0.0) Fe (0.4502, 0.4555, 0.546) O1 (0.5548, 0.4245, 0.3611) O2 (0.5505, 0.0442, 0.6843) O3 (0.4338, 0.4702, 0.0166)

x ¼ 0.15

Triclinic (P1)

a ¼ 3.9288 b ¼ 3.9467 c ¼ 3.9420 V ¼ 61.55

x ¼ 0.25

Triclinic (P1)

a ¼ 3.9539 b ¼ 3.9629 c ¼ 3.9621 V ¼ 62.07

perovskites are characterized by rhombohedral symmetry with the space group R3c. The Raman peak position of BFO is illustrated in Table 2 to comprise with as synthesized samples. From the Raman spectra of BPFO-15 and BPFO-25, we have observed A1 (i.e. A1-1, A12, A1-3, and A1-4) modes and E phonon modes, which are mentioned in Table 2. In the present BPFO-15 and BPFO-25 samples, the A1 modes have strong intensities as compared to E modes. Raman spectroscopy is a powerful tool to probe the structural and vibrational property of a material and gives the useful information about cationic substitution and ferroelectric polarization with increasing doping concentration. In present study, for pure BFO, there are nine Raman active phonon modes including A1-1, A1-2, A1-3, E-2, E-5, E-6, E-7, E-8 and E-9 modes at 125, 167, 215, 259, 318, 363, 420, 491 and 613 cm1 are observed and in good agreement with earlier reported data [23,24]. The lattice distortion caused due to lower ionic radii Pr substituted at higher ionic radii Bi essentially leads to the following: a) the active phonon modes in Raman spectra documents a blue shift, b) the Raman modes: A1-1, A1-2, and A1-3 are broadened with reduced intensity and c) an additional A1-4 mode has been induced in the Raman spectra. The presence of additional Raman mode has also

Rwp ¼ 17.8 RF ¼ 14.4 Rexp ¼ 9.16 c2 ¼ 3.24 Rwp ¼ 27.1 RF ¼ 16.3 Rexp ¼ 9.56

c2 ¼ 3.45

Rwp ¼ 24.2 RF ¼ 13.9 Rexp ¼ 8.3

been noticed in rare earth Nd doped Bi1xNdxFeO3 (x ¼ 0.0e0.2) samples [25]. Usually, the stereochemical activity of the Bi lone electron pair plays the main role in changing both BieO covalent bonds as well six characteristic modes as E-1, A1-1, A1-2, A1-3, A1-4, and E-2. These modes are supposedly believed to be responsible for the ferroelectric nature of the BFO samples. The lattice distortion caused due to Pr substitution at Bi site results a change in BieO covalent bonds causes the decline in the stereochemical activity of the Bi lone electron pair and thus in long range ferroelectric order with increased Pr doping concentration. The peak positions of Raman active phonon modes are influenced by oxygen stoichiometry and the internal stress within the compound [8]. The crystal symmetry varies from rhombohedral R3c to triclinic P1 on subsequent Pr doping in the BFO samples. This change in crystal structure is due to the A-site disorder created by Pr substitution, which leads to the shift of Raman modes at higher frequencies and also the attrition of the prominent modes: A1-1, A1-2 responsible for stereochemical activity. The strong characteristic modes: A1-1, A1-2, and A1-3 corresponding to BieO covalent bonds shift toward a higher wave

Fig. 3. Representations of room temperature (a) rhombohedral for BFO and (b) triclinic structures of Bi1xPrxFeO3. The FeO6 are shown as octahedra, the Fe atom being visible within them; the yellow and blue spheres outside these octahedra represent Bi and O respectively. The figures shown are perspective views along the abc-axis. These schematic figures represent the structure transform from R3c to P1 with Pr doping concentration in BiFeO3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

P. Sharma et al. / Materials Chemistry and Physics 143 (2014) 629e636

(c)

633

1

E9

E7

-4 1

A

E6

E4

-7

-9 E

A1-2

BiFeO 3

100

200

300

400

500

E9

E5 E6 E7

E8

E2

1

A

-3

1

A

-1

(a)

E

-8 E

E

E

-6

-4 E

-5

E

1

E

A

-1

-3

-3

1

A

-2

1

A

Intennsity (arb. unit)

Bi 0.85 Pr0.15 FeO 3

-1

(b)

1

A

-3

E1

A

-2

1

A

-1

Bi 0.75 Pr0.25 FeO 3

600

700

-1

Raman Shift (cm ) Fig. 4. Room temperature Raman spectra of Bi1xPrxFeO3 samples for x ¼ 0.0, 0.15, 0.25.

number with increasing Pr content for BPFO-15 and BPFO-25 as compared to pure BFO. These modes frequencies are governed by local factors, such as the force constant, ionic mass of the doping elements and electric polarization. Thus, it will be proportional

Raman modes (cm1)

A1-1 A1-2 A1-3 A1-4 E-1 E-2 E-3 E-4 E-5 E-6 E-7 E-8 E-9

Bi1xPrxFeO3 x ¼ 0.0

x ¼ 0.15

x ¼ 0.25

125 167 215 e e 259 e e 318 363 420 491 613

141.45 173.95 228.27 474.18 103.77 e 275.64 336.27 375.33 430.18 528.67 e 631.37

142.28 176.04 236.82 495.33 105.99 e e 307.20 e 410.80 532.59 e 618.51

Dielectric Constant

Table 2 Observed Raman modes (cm1) for Bi1xPrxFeO3 (x ¼ 0.0, 0.15 and 0.25) samples.

120

BiFeO

115

Bi

Pr

FeO

110

Bi

Pr

FeO

105 100 95 90 85 80 75 70 65 100

1k

10k

100k

1M

Frequency (Hz) Fig. 5. Variation of real part of dielectric constant (ε0 ) of the Bi1xPrxFeO3 samples for x ¼ 0.0, 0.15, 0.25, with frequency (100 Hze1 MHz) at room temperature.

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P. Sharma et al. / Materials Chemistry and Physics 143 (2014) 629e636

0.40

BiFeO3 Bi0.85Pr0.15FeO3 Bi0.75Pr0.25FeO3

0.35 0.30

tan

0.25 0.20 0.15 0.10 0.05 0.00 100

1k

10k

100k

1M

Frequency (Hz) Fig. 6. Variation of tan d of the Bi1xPrxFeO3 samples for x ¼ 0.0, 0.15, 0.25, with frequency (100 Hze1 MHz) at room temperature.

to (k/M)1/2, where, k is the force constant and M is the reduced mass [26]. As the mass of Pr3þ (140.90 g) ion is less than that of Bi3þ (208.98 g) ion, relatively lighter Pr3þ substitution may cause an increase in mode frequency. The intensities of A1-1 and A1-2, modes for Pr doped BiFeO3 samples gradually decreased whereas; E-2, E-3 and E-5 modes are suppressed completely, which confirms the structural transition from rhombohedral (R3c) to triclinic (P1). The disappearance of some peaks corresponding to E phonon modes and the decrease in intensities of A1 modes attributed that Pr substitution affects Bi-site in BFO. In a true sense, the Raman scattering spectra is sensitive to atomic displacements and the appearance of Raman active modes provides substantial information’s about the lattice distortion caused by the Pr substitution in BFO ceramics. 3.3. Dielectric analysis The frequencies dependence real part of permittivity (ε0 ) and loss tangent (tan d) of BPFO perovskites at room temperature is represented in Figs. 5 and 6. The room temperature values of ε0 are about 65, 82 and 95 for Bi1xPrxFeO3 (x ¼ 0, 0.15, 0.25) respectively at 1 MHz. For BPFO-15 and BPFO-25, it is noticed that the dielectric constant decreases with increasing frequency. The higher value of dielectric constant at low frequencies (w100 Hz) could be explained by the phenomenon of dipole relaxation wherein at low frequencies the dipoles are able to follow the frequencies of applied field. The dielectric dispersion in lower frequency region may be attributed to interfacial polarization (space charge polarization, MaxwelleWagner or defect relaxation). At room temperature in all frequency ranges the dielectric constant increases with increasing doping concentration of Pr in BFO as BPFO-15 and BPFO-25. The above described results are comparable with the previous reported bulk pure BFO (w70), Bi0.9xLa0.1NdxFeO3 (x ¼ 0.05, 0.07, 0.1) (w350, 500, 500) [8], and pure BFO (w115), Bi0.875Sm0.125FeO3 (w118) [27]. Some authors have described high value of ε0 in pristine BFO (w250), Bi0.8La0.1Pr0.1FeO3 (w2000) [16] and BiFe0.75Ti0.25O3 (w378) [28]. We note that Bi1xPrxFeO3 (x ¼ 0, 0.15, 0.25) samples possess very small value of tan d (0.04) at 1 MHz frequency comparable to reported value [16,17].

At low frequencies the dielectric loss of BPFO-15 and BPFO25 increases consistent with earlier reports on Pr doped BFO [16,17]. It is earlier argued that the loss enhancement is due to increase in defect concentration leading to enhance conductivity [16,17]. The effective permittivity of bulk materials is determined by dielectric polarization mechanisms and in low frequency region; the space charge polarization, defects and conductivity in bulk samples mostly influence permittivity. We shall further discuss it in connection with ferroelectric loop measurement. At high frequencies, we observe an increase in permittivity and minor variations in loss tangent for BPFO-25 as compared to BPFO-15 and BFO (please see Figs. 5 and 6). Higher values of permittivity in the frequency range (0e1 MHz) are earlier reported for Pr doped BFO [17]. We note that an increase in dielectric polarization of bulk (grains in ceramic) leads to enhance permittivity. The variations in dielectric polarization might be due to change in structure as a consequence of Pr doping: BFO (rhombohedral R3c space group) / BPFO-15 and BPFO-25 (triclinic P1 space group). The conductivity values of BFO, BPFO-15 and BPFO25 at 100 kHz are 3.23  108, 4.3  106 and 4.9  106 U1 cm1 respectively. 3.4. Ferroelectric (PeE loop) analysis The polarizationeelectric field (PeE) hysteresis loop of the Bi1xPrxFeO3 (x ¼ 0.0, 0.15, 0.25) samples at room temperature with different applied field is shown in Fig. 7. For parent BFO, the spontaneous polarization mainly comes from the hybridization between 6s2 lone electron pair and the 6p empty orbital of Bi3þ ions, which induce the non-centric-symmetric distortion of the electron cloud and result in the ferroelectricity [29]. Under an applied field of about 160 kV cm1, the remnant polarization (2Pr) of the BFO was found to be 18.4 mC cm2 at 100 Hz. It is noticed that for BPFO-15 doping the hysteresis loop is improper due to increase in leakage current of the sample at low frequencies. We note from dielectric measurements that at low frequency the loss in BPFO-15 and BPFO-25 is very large as compared to BFO (please see Fig. 6). Hence, BPFO-15 and BPFO25 sustain small field w10 kV cm1 at 100 Hz, which is in agreement with the reported data i.e. the BPFO samples sustain upto 6e15 kV cm1 [16,17]. 4. Conclusions The polycrystalline single-phase Bi1xPrxFeO3 (x ¼ 0, 0.15, 0.25) [BFO, BPFO-15 and BPFO-25] samples were successfully prepared by using solid-state reaction route. The effect of RE ion Pr at A-site in BFO compound on its structural, vibrational and dielectric behavior has been studied. The structure of BFO is rhombohedral (R3c) with lattice parameters a z 5.5798  A and c z 13.8670  A. The doping of rare earth Pr causes a systematic change in structure of BFO from rhombohedral (R3c) to triclinic (P1). Raman scattering spectra reveal active phonon modes for all of the synthesized samples and show the structural transition from rhombohedral to triclinic crystal structure due to enhanced doping concentration. Raman spectra also reveal suppression of ferroelectric behavior due to Pr doping. At low frequencies the dielectric loss of BPFO-15 and BPFO25 increases. At high frequencies, an increase in permittivity and minor variations in loss tangent is evident for BPFO-25 as compared to BPFO-15 and BFO. Remarkably, the dielectric loss remains very small at 1 MHz around room temperature. The ferroelectric polarization reduction in Pr doped bulk BFO samples are attributed to structural change.

P. Sharma et al. / Materials Chemistry and Physics 143 (2014) 629e636

20

635

(a)

BiFeO3

15

2

P ( C/cm )

10 5 0 -5 -10 -15 -20 -250

4

-200

-150

-100

-50

0

50

100

150

200

250

(b)

Bi 0.85 Pr0.15 FeO3

2

P( C/cm )

2

0

-2

-4 -15

0.2

-10

-5

0

5

10

15

(c)

Bi 0.75 Pr0.25 FeO3

P( C/cm2 )

0.1

0.0

-0.1

-0.2 -10

-8

-6

-4

-2

0

2

4

6

8

10

E(kV/cm) Fig. 7. Typical ferroelectric hysteresis loops of BFO (a), BPFO-15 (b), BPFO-25 (c) at room temperature.

Acknowledgments The authors acknowledge UGC, New Delhi, for financial assistance. Authors are thankful to UGC-DAE CSR, Indore for providing characterization facilities. Useful discussions and support from Dr. V. G. Sathe is also acknowledged. References [1] [2] [3] [4]

G. Catalan, J.F. Scott, Adv. Mater. 21 (2009) 2463. N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. J.R. Teague, R. Gerson, W.J. James, Solid State Commun. 8 (1970) 1073. P. Fischer, M. Polomska, I. Sosnowska, M. Szymanski, J. Phys. C: Solid State Phys. 13 (1980) 1931. [5] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694e6709.

[6] S.-T. Zhang, Y. Zhang, M.-H. Lu, C.-L. Du, Y.-F. Chen, Z.-G. Liu, Y.-Y. Zhu, N.B. Ming, Appl. Phys. Lett. 88 (2006) 162901. [7] G.L. Yuan, S.W. Or, J.M. Liu, Z.G. Liu, Appl. Phys. Lett. 89 (2006) 052905. [8] P. Pandit, S. Satapathy, P.K. Gupta, V.G. Sathe, J. Appl. Phys. 106 (2009) 114105. [9] V.A. Khomchenko, D.A. Kiselev, J.M. Vieira, A.L. Kholkin, Appl. Phys. Lett. 90 (2007) 242901. V.A. Khomchenko, V.V. Shvartsman, P. Borisov, W. Kleemann, D.A. Kiselev, I.K. Bdikin, J.M. Vieira and A.L. Kholkin, J. Phys. D: Appl. Phys. 42 (2009) 045418. [10] Y.-J. Wu, X.-K. Chen, J. Zhang, X.-J. Chen, Phys. B 411 (2013) 106e109. [11] V.R. Palkar, D.C. Kundaliya, S.K. Malik, S. Bhattacharya, Phys. Rev. B 69 (2004) 212102. [12] Z. Yan, K.F. Wang, J.F. Qu, Y. Wang, Z.T. Song, S.L. Feng, Appl. Phys. Lett. 91 (2007) 082906. [13] Z. Chen, C. Wang, T. Li, J. Hao, J. Zhang, J. Supercond. Novel Magn. 23 (2010) 527. [14] V.A. Khomchenko, D.A. Kiselev, I.K. Bdikin, V.V. Shvartsman, P. Borisov, W. Kleemann, J.M. Vieira, A.L. Kholkin, Appl. Phys. Lett. 93 (2008) 262905. [15] A. Kumar, D. Varshney, Ceram. Int. 38 (2012) 3935. [16] P. Uniyal, K.L. Yadav, J. Phys. Condens. Matter 21 (2009) 405901.

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[17] N. Kumar, N. Panwar, B. Gahtori, N. Singh, H. Kishan, V.P.S. Awana, J. Alloys Compd. 501 (2010) L29eL32. [18] P. Pandit, S. Satapathy, P. Sharma, P.K. Gupta, S.M. Yusuf, V.G. Sathe, Bull. Mater. Sci. 34 (2011) 899. [19] M.M. Kumar, A. Srinivas, S.V. Suryanarayan, J. Appl. Phys. 87 (2000) 855. [20] J. Rodríguez-Carvajal, Phys. B 192 (1993) 55. [21] Dinesh Varshney, A. Kumar, J. Mol. Struct. 1038 (2013) 242e249; J. Alloys Compd. 509 (2011) 8421–8426; Poorva Sharma, Dinesh Varshney, Adv. Mater. Lett. (2013), http://dx.doi.org/10.5185/amlett.2013.fdm.10; Dinesh Varshney, Poorva Sharma, S. Satapathy, P.K. Gupta, J. Alloys Compd. 584 (2014) 232– 239; Mater. Res. Bull. 49 (2014) 345–351. [22] M.K. Singh, H.M. Jang, S. Ryu, M.H. Jo, Appl. Phys. Lett. 88 (2006) 042907.

[23] X. Zheng, Q. Xu, Z. Wen, X. Lang, D. Wu, T. Qiu, M.X. Xu, J. Alloys Compd. 499 (2011) 108. [24] D. Kothari, V.R. Reddy, V.G. Sathe, A. Gupta, A. Banerjee, A.M. Awasthi, J. Magn. Magn. Mater. 320 (2008) 548. [25] G.L. Yuan, S.W. Or, H.L.W. Chan, J. Appl. Phys. 101 (2007) 064101. [26] P.J. Klar, T. Rentschlerb, Solid State Commun. 103 (1997) 341. [27] G.L. Yuan, S.W. Or, Y.P. Wang, Z.G. Liu, J.M. Liu, Solid State Commun. 138 (2006) 76e81. G.L. Yuan, S.W. Or, J. Appl. Phys. 100 (2006) 024109. [28] M. Kumar, K.L. Yadav, J. Appl. Phys. 100 (2006) 074111. [29] S. Yuan, H. Zu-Fei, M. Xing, W.C. Zhong, C. Gang, F.H. Gang, Acta Phys. Sin. 58 (2009) 193.