Structural, ferroelectric and impedance spectroscopy properties of Y3+ modified Pb(Fe0.5Nb0.5)O3 ceramics

Physica B 406 (2011) 1660–1664

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Structural, ferroelectric and impedance spectroscopy properties of Y3 + modified Pb(Fe0.5Nb0.5)O3 ceramics Subhadarsani Sahoo n, R.N.P. Choudhary, B.K. Mathur Department of Physics and Meteorology, I.I.T Kharagpur, West Bengal 721302, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2010 Received in revised form 20 December 2010 Accepted 31 December 2010 Available online 15 February 2011

Some ceramic samples of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3 (PYFN) (0.00 r x r 0.08) were synthesized by a mixed oxide route. X-ray diffraction patterns of all the above samples confirm the formation of single phase material crystallizing in monoclinic structure. Dielectric properties (er and tan d) were analyzed in a wide temperature (30–350 1C) and frequency range (100 Hz–1 MHz). Ferroelectric properties of these compounds were confirmed from polarization (P–E hysteresis loop) measurements at room temperature. All the room temperature hysteresis loops of PYFN ceramics were well simulated using the ferroelectric capacitor model. Yttrium substitution resulted in notable enlargement of room temperature remnant polarization (2Pr). The 2Pr of PYFN (x ¼ 0.02) reaches to a large value (23 mC/cm2), which is nearly 5 times greater than that of PFN ceramic (4.6 mC/cm2). All the compounds exhibits negative temperature coefficient of resistivity (NTCR) type behavior as that of semiconductors. Dc conductivity (estimated via bulk resistivity) variation with temperature of all the samples follows Arrhenius type of electrical conductivity. & 2011 Elsevier B.V. All rights reserved.

Keywords: Ferroelectrics Impedance spectroscopy NTCR Polarization

1. Introduction Lead iron niobate Pb(Fe1/2Nb1/2)O3 (PFN) is an important member of the perovskite family of a general formula ABO3 (A ¼mono or divalent, B ¼tri to hexavalent ions). It has high dielectric constant, diffuse phase transition [1,2] and magnetoelectric (ME) effect [3], which is very useful for device applications [4–6]. PFN and other lead based magnetoelectric materials have high-leakage current and low remnant polarization, which limits the materials to be used for any meaningful devices with higher sensitivity [7–9]. It has already been established that a long range of complex compounds, obtained by single or multielement doping at the A and/or B site of the perovskite structure, are potentially suitable for applications [10–13]. In general, in order to obtain suitable materials for devices, several attempts have been made to design and develop new materials/compounds by substituting isovalence/nonisovalence elements/atoms at different sites [14]. Trivalent rare earth ion substitution has always been of considerable interest with respect to enhancement in the ferroelectric properties [15]. Though yttrium (Y3 + ), which is having a stable valency of + 3 is not a rare earth element but still it is of particular interest for enhancement of properties of lead based perovskite ceramics. Therefore, in this paper we report the

n

Corresponding author. Tel.: +91 3222 283814; fax: + 91 3222 255303. E-mail address: [email protected] (S. Sahoo).

0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.12.072

effect of Y3 + substitution on the ferroelectric and ac electrical properties of Pb(Fe0.5Nb0.5)O3 ceramics.

2. Experimental details The ceramic samples of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (0.00rx Z0.08) were prepared using analytical reagents (withZ99% purity): PbO, Y2O3, Fe2O3 and Nb2O5 (M/s Loba Chemicals, India). The reactive powders were mixed mechanically in an appropriate stoichiometric ratio with 3% excess PbO mass to balance the loss of lead at elevated temperatures. A mixed oxide or solid state reaction technique was used to achieve a homogeneous mixing of the constituents. The homogeneous mixed powders were calcined at optimized temperature (950 1C) and time 8 h. The sintering of PFN and PYFN pellets was carried out at 1000 and 1050 1C, respectively. Structural characterization of compounds was carried out using a Panalytical high-resolution XRD-I, PW 3040/60 with CuKa radiation (l ¼1.5405 ) in a wide range of Bragg angles (201 r2y r801) at a scan speed of 31 min  1. The capacitance and electrical impedance parameters were obtained using a computer-controlled impedance analyzer (PSM 1735) with a laboratory-fabricated sample holder at different temperature (30–350 1C) and frequency (100 Hz–1 MHz). The P–E hysteresis loops were obtained on the poled samples using a precision loop tracer workstation (M/s Radiant Technologies. Inc.).

S. Sahoo et al. / Physica B 406 (2011) 1660–1664

3.1. XRD studies Fig. 1 shows the high-resolution X-ray diffraction pattern (XRD) of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (x¼0.00, 0.02, 0.04, 0.06, 0.08). The diffraction patterns of all the samples were found to be stronger and sharper suggesting the formation of single phase compounds. The best agreement between observed (obs) and calculated (cal) interplanar spacing d (i.e., SDd¼ S(dobs–dcal ¼ minimum) was found in the monoclinic PYFN crystal system like that of the parent compound (with monoclinic unit cell and space group Cm). The cell parameters were refined using standard computer software (POWD) [16]. The least-squares refined cell parameters for different Y3 + concentration have been compared in Table 1. A minor shift in XRD peak position towards lower 2y (Bragg’s angle) side and systematic decrease in intensity are observed because of the change in concentration of yttrium. 3.2. Dielectric properties

(220)

(212)

(202)

(210)

(110)

(010)

Fig. 2(a) shows the variation of relative dielectric constant (er) as a function of temperature for PYFN ceramics at fixed frequency

Intensity (a.u)

x = 0.08

x = 0.06

x = 0.04

x = 0.02

x = 0.00 20

30

40 Bragg's angle (degree)

50

60

Fig. 1. Comparison of room temperature XRD pattern of Pb1 xYx(Fe0.5Nb0.5)1  x/4O3, (x¼ 0.00, 0.02, 0.04, 0.06, 0.08) compounds.

12000

x = 0.00 x = 0.02 x = 0.04 x = 0.06 x = 0.08

10000

εr

8000 6000

100 kHz with oscillation amplitude of 1 V. A sharp ferroelectric phase transition was observed for all the samples. It is observed that there is a minor shift ( r5 1C) of Curie transition temperature (Tc) towards low temperature side on substitution of yttrium at the Pb-site. Ferroelectric phase transition temperatures of all the samples are given in Table 1. The variation of tangent loss (ln tan d) at 100 kHz as a function of temperature for PYFN is shown in Fig. 2(b). Loss anomaly corresponding to phase transition in all Y modified PFN has been observed, which is in good agreement with the Kramers–Kroning relations. However for PFN ceramics this loss anomaly is not observed, which is in good agreement with the previous reports [17,18]. It is observed that for all the samples tan d increases on increasing temperature. The reason for this increase (in ceramics) can be attributed to the space charge polarization [19]. Rapid increase in tan d at high temperatures is attributed to increase in electrical conductivity. Room temperature P–E loops of the PYFN samples are shown in Fig. 3(a–d). A significant change in the area of P–E response of PFN on increasing Y3 + is observed. All the ceramic samples exhibit a typical P–E hysteresis loop under an electric field of about 15 kV cm  1 supporting the ferroelectric character of the materials. The electric field dependence of polarization (i.e., hysteresis loops) has been modeled using a ferroelectric capacitor model [20]. Using this model, electric field dependence saturated dipolar polarization (Peff) can be calculated using the following expressions: þ Peff ¼ Ps 

2Ps 1 þ expðCPs ðEEc ÞÞ

ð1Þ

 Peff ¼ Ps 

2Ps 1 þ expðCPs ðE þ Ec ÞÞ

ð2Þ

+  where Peff and Peff are the upper and lower branch of the

hysteresis loop, C ¼ 2=Ps Ec log ðPs þ Pr Þ=ðPs Pr Þ , Ps ¼saturation polarization, Pr ¼remnant polarization, Ec ¼coercivity. A good agreement between our experimental data and model prediction was found in Fig. 3. Different physical parameters were obtained after fitting are given in Table 1. It is observed that even a small amount of yttrium substitution (x ¼0.02) in PFN increases 2Pr. Therefore, the comprehensive ferroelectric property is obviously improved by Y substitution with appropriate amount. The values of 2Pr and Ec are compared in Table 1. Asymmetricity in the loop is observed for x Z0.02 (i.e., E( + c) and E(  c) are different). Similar observation was reported by different authors where incorporation of donor dopant has caused a slight or no change in the Curie transition temperature (Tc) but has improved

x = 0.00 x = 0.02 x = 0.04 x = 0.06 x = 0.08

1

ln (tanδ)

3. Result and discussions

1661

0.1

4000 2000 0.01 50

100

150

Temperature (°C)

200

250

50

100

150

200

Temperature (°C)

Fig. 2. Variation of (a) relative dielectric constant (er) and (b) tangent loss (tan d) as a function of temperature of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (x ¼0.00, 0.02, 0.04, 0.06, 0.08) compounds at different frequencies.

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Table 1 ˚ (the estimated standard deviations of all the parameters are in the parenthesis), Curie transition temperature (Tc), remnant Comparison of lattice parameters (A), polarization (2Pr) and coercivity (Ec) and activation energy for dc conduction (estimated via bulk resistance) of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (x¼ 0.00, 0.02, 0.04, 0.06, 0.08) compounds. Composition (x)

˚ a (A)

˚ b (A)

˚ c (A)

b (deg.)

Tc (1C)

2Pr (lC/cm2)

Ec (kV/cm) (ev)

Ea (dc) (ev)

0.00 0.02 0.04 0.06 0.08

5.6755 (7) 5.6758 (10) 5.6763 (12) 5.6765 (8) 5.6768 (15)

4.0170 (7) 4.0170 (10) 4.0170 (12) 4.0170 (8) 4.0170 (15)

5.6917 (7) 5.6917 (10) 5.6918 (12) 5.6918 (8) 5.6917 (15)

90.116 (9) 90.118 (5) 90.114 (12) 90.110 (8) 90.098 (10)

113 112 111 110 109

4.6 23 22 20 18

2.4 3.3 3.0 2.9 2.8

1.20 0.47 0.50 0.60 0.65

x = 0.00 Fitted

20 Polarization (μC/cm2)

Polarization (μC/cm2)

10

0

x = 0.02 Fitted

10 0 -10

-10 -20 -20

-15

-10

-5

0

5

10

15

-20

20

-15

-10

20

20 x = 0.04 Fitted

10 5 0 -5 -10

0

5

20

10

15

20

-5 -10

-20 -5

15

0

-15

-10

10

5

-20 -15

5

10

-15

-20

0

x = 0.06 Fitted

15 Polarization (μC/cm2)

Polarization (μC/cm2)

15

-5

Electric Field (kV/cm)

Electric Field (kV/cm)

10

15

-20

20

-15

-10

Electric Field (kV/cm)

-5

0

5

Electric Field (kV/cm)

20 x = 0.08 Fitted

Polarization (μC/cm2)

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

-15

-10

-5

0

5

10

15

20

Electric Field (kV/cm) Fig. 3. Room temperature P–E loops of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3 ceramics, (a) x¼ 0.00, (b) x¼ 0.02, (c) x¼ 0.04, (d) x ¼ 0.06 and (e) x¼ 0.08 measured at 20 Hz.

S. Sahoo et al. / Physica B 406 (2011) 1660–1664

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the polarization properties significantly [21,22]. Enhanced polarization may be due to the strain caused by difference in ionic radii ˚ and Pb2 + (1.33 A) ˚ at the A-site of PYFN between Y3 + (1.04 A) ceramics.

properties are often represented in terms of some complex e*(o)¼ e0 je00 , parameters like Z*(o) ¼Z0 jZ00 ¼Rs j/oCs, M*(o) ¼M0 +jM00 , Y*(o) ¼Y0 + jY00 ¼1/R+ joC (impedance Z, dielectric permittivity e, dielectric loss (tan d) and electrical modulus (M)). They are interrelated as follows:

3.3. Impedance spectroscopic studies

M ¼ 1=e ¼ joCo Z  ¼ joCo ð1=Y  Þ tan d ¼ euu=e0 ¼ Z 0 =Zuu ¼ M 0 =Muu

Complex impedance spectroscopy (CIS) is a powerful nondestructive technique to study the correlation between the microstructure and the electrical properties of ceramics. The electrical 250

30 x = 0.00

x = 0.02

275 °C 300 °C 325 °C 350 °C Fitted

150

275 °C 300 °C 325 °C 350 °C Fitted

25 20 Z'' (kΩ)

200

Z'' (kΩ)

ð3Þ

It is well known that a perfect single semicircular (i.e., Debyetype response) in complex plane represents a simple parallel RC

15

100 10 50

5

0

0 0

50

100 150 Z' (kΩ)

200

250

5

0

15

10

15 Z' (kΩ)

25

30

12

x = 0.04

275 °C

x = 0.06

275 °C

300 °C 12

300 °C

325 °C

325 °C

9

350 °C

350 °C

Fitted Z'' (kΩ)

9 Z'' (kΩ)

20

6

Fitted 6

3

3

0

0 0

3

6

9

12

15

3

0

6 Z' (kΩ)

Z' (kΩ)

9

12

12 x = 0.08

275 °C 300 °C 325 °C

9

Z'' (kΩ)

350 °C Fitted 6

3

0 0

3

6

9

12

Z' (kΩ) Fig. 4. Variation of real (Z0 ) and imaginary (Z00 ) part of impedance of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (x¼ 0.00, 0.02, 0.04, 0.06, 0.08) compounds.

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S. Sahoo et al. / Physica B 406 (2011) 1660–1664

The dc conductivity (sdc) provides insight in to the transport of charge carriers (i.e., electrons/holes or cations/anions), which predominates the conduction process. The dc conductivity is calculated via bulk resistance using the relation sdc ¼t/RbA where Rb is the bulk resistance, t is the sample thickness and A the area of the sample electrode. The value of the bulk resistance (Rb) is obtained by fitting the Nyquist plot data in the commercially available software ZSimp Win with the equivalent circuit given in Fig. 4(a) inset. The variation of sdc with temperature is plotted in Fig. 5. It has been observed that value of dc conductivity increases on increasing Y concentration, which is the signature of NTCR behavior of the materials. The sdc follows the Arrhenius law expressed as sdc ¼ s0 exp ( Ea/kBT). The activation energy (Ea) for the conduction is calculated from the slope of the linear portion of the sdc vs. 103/T plot.

σdc (Ω-1 m-1)

0.1

0.01

1E-3

1.45

0.00 0.02 0.04 0.06 0.08 1.50

1.55

1.60 103

/T (K)

1.65

1.70

1.75

-1

Fig. 5. Variation of dc conductivity (bulk) of Pb1  xYx(Fe0.5Nb0.5)1  x/4O3, (x¼ 0.00, 0.02, 0.04, 0.06, 0.08) compounds.

circuit. A minor deviation from semicircle (i.e., non-Debye-type response) reveals the presence of a new circuit other than simple RC circuit. In an ideal case both grain and grain boundary follows Debye-like behavior and shows two perfect semicircles in the Nyquist plot. As several equivalent circuits will fit to the same experimental data, while designing equivalent circuits the following points have to be kept in mind: (i) the equivalent circuit should remain the same for low temperature as well as high temperature data and (ii) careful examination of the experimental data by comparing it with the hand fitting data to check the consistency of the circuit. It is to be noted that if one considers the energy band diagram of polycrystalline system (such as PFN) where grain boundary is depicted having two successive grains the donor states and the trapping states are the potential states available below the conduction band. The trapping states are presumed to have a range in the band gap below the conduction band. Upon the application of the ac small signal voltage these states will sweep through the amplitude of the small signal. These sweeping states are not singular in nature because of the presence of the defect states in the polycrystalline materials. Therefore, the time constant t comprises the charge mobility and charge storage represented by the conductance and the capacitance for the defect states. This gives rise to the trapping contribution in the series event for the single charge that is being trapped and subsequently released to become mobile carrier. This event essentially gives the concept of R–C combination circuit [23]. Nyquist plots (Z0 vs. Z00 ) for PYFN (0.00rx Z0.08) ceramics at few temperatures are shown in Fig. 4. Nyquist plots of PYFN ceramics are well fitted with that of calculated plot based on the brick-layer circuit model [24] using a commercially available software ZSimp Win. The equivalent circuits have been given in Fig. 4(a–e). An excellent agreement between measured and calculated data confirms the contribution of both grain and grain boundary to the total resistivity of the materials. Each of the circuit elements in the equivalent circuit has their own physical significance. Generally, semicircle formation is dependent on the strength of the relaxation and available frequency range. All the compounds exhibits negative temperature coefficient of resistivity (NTCR) behavior as that of semiconductors. The tendency of formation of semicircle on increasing yttrium content in PYFN exhibits decrease in total resistivity or increase in conductivity.

4. Conclusion The Y3 + modified Pb(Fe0.5Nb0.5)O3 (PYFN) compounds were prepared by a high-temperature solid-state reaction technique. Preliminary structural studies suggest that the crystal system of parent compound remains same (in monoclinic crystal system) even upto 8% yttrium substitution. Yttrium substitution has insignificant effect on transition temperature (Tc). A notable increase in room temperature remnant polarization (2Pr) of PYFN (x ¼0.02) is observed, which is five times greater than that of PFN ceramic. Complex impedance spectra indicate about the possible contribution of the bulk and grain boundaries at higher temperatures and also about the temperature-dependent relaxation phenomena.

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