Effect of Mn-substitution on structural and dielectric properties of Pb(Zr0.65−xMnxTi0.35)O3 ceramics

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Solid State Sciences 11 (2009) 219e223 www.elsevier.com/locate/ssscie

Effect of Mn-substitution on structural and dielectric properties of Pb(Zr0.65xMnxTi0.35)O3 ceramics B. Tiwari, R.N.P. Choudhary* Department of Physics and Meteorology, IIT, Kharagpur 721 302, India Received 23 November 2007; received in revised form 14 April 2008; accepted 23 April 2008 Available online 2 May 2008

Abstract The polycrystalline samples of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) (PZMT) were prepared by a high-temperature solid-state reaction technique. Structural properties of the compounds were examined using an X-ray diffraction (XRD) technique to confirm the formation of single-phase compounds (with perovskite structure) at room temperature. Microstructural analysis of the surface of the compounds by scanning electron microscopy (SEM) exhibits that there is a significant change in grain size on introduction, at the Zr-site, of Mn. Detailed studies of the dielectric properties of PZMT show a measurable shift in Tc, change in dielectric constant, and ac conductivity. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Polycrystalline; X-ray diffraction; Scanning electron microscopy (SEM); Dielectric properties

1. Introduction Lead zirconate titanate [Pb(Zr,Ti)O3, PZT] a solid solution of ferroelectric PbTiO3 (Tc ¼ 490  C) and anti-ferroelectric PbZrO3 (Tc ¼ 230  C) [1], has perovskite structure with a general formula ABO3 (A ¼ mono or divalent; B ¼ tri-hexa valent ions). Because of its high dielectric constant, piezoelectric and pyroelectric coefficients, spontaneous polarization and Curie temperature much above the room temperature, with different Zr/Ti ratios it has widely been used for various kinds of piezoelectric, pyroelectric and ferroelectric devices [2e8]. To obtain suitable materials for the above mentioned applications various attempts have been made to modify PZT either by substitution of suitable elements at the A/B sites or fabricating composites with it [9e11]. Lot of work has been done on different kinds of modifications of PZT [12e16] both in forms of thin films and bulk, including modifications of PZT by substitution of Mn at A/B sites [17,18]. Further Guiffard et al. [17] have reported conductivity mechanisms of doubly doped * Corresponding author. Tel.: þ91 3222 283814; fax: þ91 3222 255303. E-mail address: [email protected] (R.N.P. Choudhary). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.04.023

(Mn with Nb/F) PZT (Pb0.89(Ba,Sr)0.11(Zr0.52Ti0.48)O3). Klimov et al. [19] studied the structural and piezoelectric properties of (Pb0.96Sr0.04)(Zr0.5275Ti0.4675Mn0.005)O3 system. Hall and Cherdhirunkorn [20] studied the effects of sintering on phase and microstructures of Pb(Zr0.52Ti0.48)1xMnxO3yFy complex system. Park et al. [21] reported the piezoelectric properties of Nb and Nb/Mn co-doped Pb(Zr0.53Ti0.47)O3 ceramics. Detailed literature survey showed that not much work on structure and dielectric properties of Mn modified Pb(Zr0.65xMnxTi0.35)O3 has been reported. Though main objective of this work is to develop new multiferroic materials [22], in this paper we report structural and dielectric properties only. The novelty of our study is the structural, dielectric and electrical characterization and studies of Mn-substitution at B-site of PZT with 65/35 Zr/Ti ratio. This ratio of Zr0.65Ti0.35 forms a part of our study, which includes ratios at and around MPB. Most of the reported works have used Zr/Ti ratios different from ours and none have used Mn as a single dopant. Also the studies reported in all the above papers have reported different properties than structural-dielectric behavior of the systems that we are reporting.

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3. Results and discussion 3.1. Structural Fig. 1 compares the X-ray diffraction pattern of calcined powders of Pb(Zr0.65xMnxTi0.35)O3 with different

(310)

(300)

(220)

(211)

(210)

(200)

(111)

(101)

x = 0.15

Intensity (a.u.)

The polycrystalline samples of Pb(Zr0.65xMnxTi0.35)O3 (PZMT) (x ¼ 0, 0.05, 0.10, 0.15) were prepared by a hightemperature solid-state reaction technique using high purity (99.9%) ingredients; PbO (M/S. Loba Chemie Pvt. Ltd., India), ZrO2 (M/S. Sarabhai Chemicals, India), TiO2 (M/S. Loba Chemie Pvt. Ltd., India) and MnO2 (M/S. Loba Chemie Pvt. Ltd., India) in a suitable stoichiometry. An excess of 3% of PbO was added to all the compositions to compensate Pb loss during high-temperature calcinations/sintering [22]. The ingredient oxides were mixed thoroughly; first in an air atmosphere for 1 h, and then in alcohol (i.e., methanol) for 1 h. The mixtures were then calcined in high purity (99.9%) alumina crucibles. The process of grinding and calcination was repeated several times at 900  C for 8 h until the formation of single-phase compounds of desired materials was confirmed. Polyvinyl alcohol (PVA) was used to prepare the pellets, which was burnt out during high-temperature sintering. The calcined (with binder) powders were cold pressed into cylindrical pellets of approximately 10 mm diameter and 1e 2 mm of thickness at a pressure of 4  106 N m2 using a hydraulic press. The pellets were sintered at an optimized temperature (1100  C) and time (2 h) in an air atmosphere in an alumina crucible, and then cooled to room temperature at the rate of 2  C min1. The formation of compounds and their preliminary structural analysis were carried out on clacined powder by an X-ray diffraction (XRD) technique using an X-ray powder diffractometer ˚ ) in (Rigaku Miniflex, Japan) with CuKa radiation (l ¼ 1.5405 A a wide range of Bragg angles 2q (20  2q  80 ) with a scanning rate of 4 min1. The surface morphology of the sintered samples was studied at room temperature using a scanning electron microscope (SEM) (JEOL-JSM-5800). Before taking SEM micrographs of the samples, both the surfaces of the pellets were made flat and parallel. The microstructures were recorded with flat, clean gold-coated surface. For the dielectric and electrical characterizations, the pellets were polished and electroded with high-quality air-drying silver paste. The elecroded samples were fired at 150  C in air for 4 h and cooled to room temperature (30  C) before taking any measurements to remove the moisture, if any. The dielectric constant and loss tangent were obtained as a function of frequency (100 Hze1 MHz) at different temperatures (30e500  C) (in small temperature interval) using HIOKI 3532 LCR (Hi-TESTER) meter in conjunction with a laboratory-made sample holder and heating arrangement with an ac signal of 1.3 V. A chromelealumel thermocouple and digital multimeter (M/S. Electronic of India, DM 6108) were used to measure the temperatures.

(001)

2. Experimental

x = 0.10

x = 0.05

x = 0.00 20

30

40

50

60

70

80

2θ (degree) Fig. 1. Comparison of XRD patterns of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) calcined ceramic powders.

concentrations of Mn (0.0 < x < 0.15). The XRD patterns were found to be very sharp with distinct single diffraction peaks indicating good homogeneity and crystallization of the samples. The patterns clearly show that there is no structural change of Pb(Zr0.65xMnxTi0.35)O3 on substitution of small amount of Mn at the Zr-site. It can clearly be seen that the structure has perovskite phase with out any secondary phase, indicating the complete diffusion of Mn into PZT lattice to form a single-phase compound. All of the diffraction peaks were indexed using 2q value of each peak in different crystal systems (i.e., cubic, tetragonal, orthorhombic) and unit cell configurations/parameters were determined using a computer software ‘‘POWDMULT’’ [23]. Finally, rhombohedral unit cell was selected as per the best agreement between observed (obs) and calculated (cal) interplaner spacing d (i.e., SDd ¼ S(dobs  dcal) ¼ minimum) [24]. The unit cell parameters of rhombohedral system were refined using a least-squares sub routine of the above software for PZMT. The crystallite size (P) of the samples, calculated using the broadening (b1/2) of reflection peaks in the Scherrer’s equation: P ¼ 0.89l/b1/2 cos q ˚ ) [25] for PZMT is shown in Table 1. As is (l ¼ 1.5405 A evident from the Table 1 that except for the slight increase in crystallite size at 0.05% of Mn doping, crystallite size decreases with increase in Mn concentration. The SEM micrographs of the PZMT pellets are shown in Fig. 2. It was found that the grains of different sizes (Table 1)

Table 1 Structural information of Pb(Zr0.65xMnxTi0.35)O3 ceramics x

0.0

0.05

0.10

0.15

˚ ) (from XRD) P (A G (mm) (from SEM)

565 1.25

567 0.995

528 0.999

531 0.746

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are uniformly and densely distributed over the entire surface of the samples. Most of the grains of the samples were found to be spherical in nature. The shape, size and distribution of grains of the microstructures of all the samples confirmed the polycrystalline nature of the sample. As is evident from the SEM micrographs (Table 1) that except for the slight increase in grain size at 0.10% of Mn doping grain size shows a decreasing trend with increase in Mn concentration. The reason for the decrease of both the crystallite and the grain sizes can be attributed to segregation of Mn at grain boundaries [26]. The difference in structural parameters are because crystallites are different from grains and XRD is a bulk study where as SEM is a surface study. Therefore the XRD gives crystallite (particle) size and SEM gives the grain size, which consists of crystallites. Since XRD was taken on calcined powder and SEM of sintered pellets, so there might have been some grain growth. 3.2. Dielectric studies Fig. 3 shows variation of relative dielectric constant (3r) and loss tangent (tan d) of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) with frequency at room temperature. Both the parameters decrease slowly on increasing frequency, which is a normal behavior of dielectric/ferroelectric materials [27]. The higher value of 3r (Fig. 3(a)) at lower frequencies is due to the simultaneous presence of all types of polarizations (i.e., interfacial, ionic, dipolar, electronic, space charge, etc.) in the compounds. At higher frequencies (>1010 Hz),

221

the main contribution to 3r comes from electronic polarization, as some of the polarizations become ineffective and thus, the value of 3r decreases. Both the parameters (3r and tan d) increase on increasing Mn content in PZMT. Fig. 4 shows the variation of 3r and tan d with temperature of PZMT. The value of 3r increases gradually on increasing temperature to its maximum value (3max), and then decreases. It indicates that a phase transition (from ferroelectric to paraelectric phase) occurs at a particular temperature, usually called the Curie or transition temperature (Tc) [28]. On further increase of temperature, the relative dielectric constant decreases for all the compositions except for pure PZT, which still shows an increase in dielectric constant and this, may be due to the presence of space charge polarization in the material. The transition temperature and dielectric characteristics (values) of all the compositions at 10 kHz are compared in Table 2. The Tc first increases and then decreases with increase of Mn concentration. The value of 3max of Mn modified samples is more than that of pure PZT, and has higher value of 3max for 0.05 and 0.15. The value of tan d of all the compositions is almost invariant with temperature up to 350  C, and then shows a sharp rise (upto 500  C). The increasing trend in tan d at high temperatures may be due to the presence of space charge polarization. 3.3. Conductivity Fig. 5(a) shows the plot of ac conductivity of Pb(Zr0.65xMnxTi0.35)O3 for different xs against the inverse of

Fig. 2. SEM micrographs of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0 (a), 0.05 (b), 0.10 (c), 0.15 (d)) ceramics.

B. Tiwari, R.N.P. Choudhary / Solid State Sciences 11 (2009) 219e223

222

a

a 1600

x = 0.0 x = 0.05

1400

x = 0.15

6000 5000

1000

εr

εr

x = 0.0 x = 0.05 x = 0.10 x = 0.15

7000

x = 0.10

1200

8000

800

4000 3000

600

2000

400 1000

200 102

103

104

105

106

0 100

Frequency (Hz)

200

300

400

500

Temperature (oC)

b

x = 0.0

b

x = 0.05 2

10 x = 0.0 x = 0.05 x = 0.10 x = 0.15

x = 0.10 x = 0.15

8

tanδ

tanδ

6

1

4

2

0

0

102

103

104

105

106

100

200

300

400

500

Temperature (oC)

Frequency (Hz) Fig. 3. Variation of relative dielectric constant (a) and dielectric loss (b) of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) ceramics as a function of frequency at room temperature.

Fig. 4. Variation of relative dielectric constant (a) and dielectric loss (b) of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) ceramics at 10 kHz as a function of temperature.

absolute temperature (103/T ) at 10 kHz. A nature of variation of curves over a wide temperature range supports the temperature dependence of transport properties of the materials obeying Arrhenius equation: sac ¼ so exp(Ea/KBT ), where, so, Ea and KB are pre-exponential factor, activation energy and Boltzmann constant, respectively. It is observed that there is an increase in conductivity above Tc, and below that the ac conductivity of all the materials decreases with decrease in temperature. This increase in the conductivity is attributed to the increase in polarizability of a material around Tc. Above Tc, the conductivity data tend to fall onto a straight line. This is a typical behavior of the DC component of conductivity [29]. The value of activation energy of all the compositions is shown in Table 2. It can be seen that with increase of Mn doping the value of activation energy decreases indicating an increase in conducting nature of the doped PZT, which also confirms the increase in 3r due to Mn doping in PZT.

Fig. 5(b) shows the variation of ac conductivity with frequency at room temperature of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15). All the compositions show dispersion of ac conductivity in both the low and the high frequency regions. The ac conductivity shows an increase with increase in Mn concentration at all frequencies. The existence of low frequency dispersion indicates that the charge carriers may be either ionic or electronic in nature.

Table 2 Some dielectric and electrical properties of Pb(Zr0.65xMnxTi0.35)O3 ceramics at 10 kHz x

3RT

3max

Tc ( C)

Ea (eV)

0.00 0.05 0.10 0.15

311 549 622 1643

2527 6630 4657 7901

340 348 327 309

0.42 0.26 0.23 0.24

B. Tiwari, R.N.P. Choudhary / Solid State Sciences 11 (2009) 219e223

References

a x = 0.0 x = 0.05 x = 0.10

0.01

σac( Ωm)-1

x = 0.15

1E-3

1E-4

1E-5

1.5

2.0

2.5

3.0

1000 / T (K -1)

b x = 0.0 x = 0.05

1E-3

x = 0.10 x = 0.15

σ ac( Ωm)-1

223

1E-4

1E-5

1E-6

102

103

104

105

106

Frequency (Hz)

Fig. 5. Variation of ac conductivity of Pb(Zr0.65xMnxTi0.35)O3 (x ¼ 0, 0.05, 0.10, 0.15) ceramics (a) at 10 kHz as a function of inverse of temperature and (b) at room temperature as a function of frequency.

4. Conclusions It can be concluded that substitution of Mn (in small amounts) at the Zr-site of PZT doesn’t provide a change in its structure. Mn doping changes the particle density on the surface. Detailed studies of dielectric and electric properties show that Mn doping increases the Tc, 3r and tan d. The ac conductivity also showed an improvement in modified PZT.

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