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Effect of Fe and Mn doping at B-site of PLZT ceramics on dielectric properties
Journal of Alloys and Compounds 487 (2009) 494–498
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Fe and Mn doping at B-site of PLZT ceramics on dielectric properties Radheshyam Rai a,∗ , Sikha Mishra b , N.K. Singh b a b
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Department of Physics, V.K.S. University, Ara 802301, India
a r t i c l e
i n f o
Article history: Received 6 July 2009 Received in revised form 28 July 2009 Accepted 30 July 2009 Available online 5 August 2009 Keywords: Powders Solid-state reaction Grain size Dielectric properties Electrical conductivity
a b s t r a c t Iron (Fe) and manganese (Mn) doped lead lanthanum zirconate titanate (PLZT) ceramic powders have been synthesized by high temperature solid-state reaction method. Preliminary X-ray structural analysis of the compounds shows the formation of tetragonal structure. Detailed dielectric studies of the compounds as a function of temperature (from 300K to 600 K) at frequency 10 kHz suggest that the compounds undergo a diffuse phase transition. The transition temperature shifts towards higher side with decreasing the Zr ratio. The activation energy (Ea) of the samples was calculated from the plot of AC conductivity vs inverse of absolute temperature. Diffusivity () study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ferroelectric materials with perovskite structures have received much attention due to their excellent functional properties, such as piezoelectricity, pyroelectricity, electrooptic effects, useful for devices. Since the discovery of lanthanum-modified lead zirconate titanate (PLZT) ceramics with high optical transparency and good electro optical and other characteristics, numerous interests have been generated on the materials for possible potential applications such as dynamical and volatile memory components, transducers, sensors and many other active and passive devices [1–3]. The perovskite family is created by the doping of different types of cations into the stoichiometry and/or introducing anion deficiency. In the ABO3 structure, the valences of the A (12-cordinated) and B (6cordinated) cations are usually 2+ and 4+ , respectively. Ferroelectric materials with diffuse phase transition (DPT) characteristics and/or relaxor properties have been extensively studied in the last few decades [4–6]. The diffuse phase transition in ferroelectrics is characterized by extending the phase transition in a wide temperature interval around the temperature (Tmax ) where the dielectric constant assumes its maximum value (εmax ). Recently, it has been found that a wide variety of composition of perovskite ferroelectric can be obtained by making suitable substitutions at A and/or B-sited [7] in the general formula satisfying the conditions: (i) charge neutrality and (ii) suitable tolerance factor t [8]. Kamiya et al. [9] and Hase et al. [10] observed that addition of
∗ Corresponding author. Tel.: +91 351 962782932; fax: +91 351 234425300. E-mail address: [email protected] (R. Rai). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.161
MnO2 tended to increase Qm and k31 but decreased the tetragonality. Moure et al. [11] reported the microstructure and piezoelectric properties of PZT doped with Fe3+ and Sb3+ . Haertling and Land [12] reported that the La3+ because of its comparable ionic size could substitute Pb2+ ions. It has been reported that substitution of Pb2+ by La3+ ion created vacancies in the A-site of perovskite ABO3 structured ferroelectric PZT ceramics and refined to improve the optical quality of PLZT [13–14]. Dutta et al. reported the impedance spectroscopy studies on Fe3+ ion modified PLZT ceramics [15]. Ma et al. [16] reported that the PLZT ceramics film on nickel foil capacitors fabricated by the chemical solution deposition process exhibited good dielectric properties. Optical properties of Er3+ /Yb3+ -codoped transparent PLZT ceramic study by Zheng et al. [17]. Favaretto et al. [18] and Chen et al. [19] reported the influence of the A-site and B-site doping on the structural, microstructural and optical properties. Ramam et al. [20] observed that the Ti-rich region called the tetragonal symmetry increased with the introduction of BaTiO3 due to the combination of binary solid solutions of two perovskites (BT and PLSZNT), respectively. They also reported that, in ABO3 (PZT) perovskite structure, the acceptor (hard) dopants such as K+ , Rb+ , Na+ (occupy A-site, i.e., Pb-site) and Sc3+ , Mg2+ , Fe3+ , Fe2+ , Co2+ , etc. (occupy B-site, i.e., Zr/Ti-site) on structure enhanced the dielectric properties of materials. When Pb2+ ions are replaced by two acceptor ions or two acceptor ions replaces Zr4+ /Ti4+ , one oxygen vacancy is formed which cannot be removed by sintering the ceramic in oxygen atmosphere [21]. It is well known that lanthanum oxide is more suitable candidate for substitution on PLZT because of (a) its ability to reduce the distortion (anisotropy) of the oxygen octahedral (ABO3 ) unit cell (b) its high solubility in the PLZT perovskite structure, thus producing an
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
extensive series of homogeneous solid solution compositions and (c) its ability to produce a significant number of lattice vacancies resulting in the enhancement of the densification process, control of the grain growth behavior and the promotion of a highly uniform microstructure [22–24]. It was observed that transition is smeared out over a certain temperature interval, resulting in a gradual change in physical properties in this temperature interval, and it is mainly attributed to the structural disorder and compositional fluctuation [25]. In the present work, we have synthesized Mn and Fe modified PLZT ceramics, with a general formula Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1−(x+y)}/2] Mn[{1−(x+y)}/2] O3 where x = 0.60, 0.50, 0.55
495
Table 1 The lattice parameters and the measured density of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1–(x+y)}/2] Mn[{1–(x+y)}/2] O3 sample. Concentration (y%)
0.40 0.30 0.25 o.20
System (tetragonal) a
c
c/a
Theoretical density (gm/cm3 )
3.9452 3.8552 4.2545 4.2215
4.2917 4.1917 4.6488 4.5430
1.08 1.08 1.09 1.07
6.21 7.20 6.76 6.52
Fig. 1. (a) X-ray diffraction patterns of Fe and Mn doped PLZT ceramics, (b) variation of lattice parameter with concentration (mol%) of different compositions and (c) plots of tetragonality and relative density vs concentration (mol%) of different compositions.
496
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
Fig. 1. (Continued).
and 0.60 or y = 0.40, 0.30, 0.25 and 0.20, respectively. The effect of the Fe and Mn on different Zr/Ti ratio on the structural and electrical properties of the samples have been studied for better understanding of the nature of phase transition present in them. 2. Experimental procedures Polycrystalline samples of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1−(x+y)}/2] Mn[{1−(x+y)}/2] O3 were synthesized from high purity oxides. PbO, Fe2 O3 , and ZrO2 (99.9% M/S Aldrich chemicals USA), TiO2 (99.9% M/S Sd. fine-chem Ltd.), La2 O3, MnO (99.99% M/S Indian Rare-Earth Ltd.) using high temperature solid-state reaction technique in air atmosphere. The constituent compounds in suitable stoichiometry were thoroughly mixed in an agate mortar for 4 h. The mixed materials were then calcined in alumina crucible at 800 ◦ C for 2 h. The calcined fine powder was cold pressed into cylindrical pellets of size 10 mm of diameter and 1–2 mm of thickness using a hydraulic press with a pressure of 6 × 107 kg m−2 . The pellets were sintered in an air atmosphere at 900 ◦ C for 2 h in presence of PbZrO3 powder in order to compensate for PbO loss. The formation and quality of compounds were checked by XRD technique. The X-ray diffraction pattern of the compounds was recorded at room temperature using X-ray powder diffractometer (Rigaku Minifiex Japan) with CuK␣ radiation ( = 1.5418 Å) in a wide range of Bragg angles 2 (20◦ ≤ 2 ≤ 60◦ ) at a scanning rate of 2◦ min−1 . The dielectric constant (ε) and loss tangent (tan ı) of the compounds were measured using a HP4623B LCR meter as a function of frequency at 298 K (room temperature, [RT]) and temperature (RT to 600 K) at 10 kHz. The dc resistivity of the compounds was measured using Keithley-617 programmable electrometer with laboratory fabricated experimental setup. The flat polished surface of sintered pellets were electroded with air drying silver paste and fired at 150 ◦ C for 2 h before taking any electrical measurement. For the dielectrics, electrical measurements, the samples were placed in a laboratory made three-terminal sample holder and heated at the rate of 2 ◦ C min−1 .
3. Results and discussion XRD patterns of the perovskite ceramics are shown in Fig. 1(a). All the reflection peaks were indexed using observed inter-planar spacing d, and lattice parameters of samples were determined by using least-squares refinement method with a computer program package POWD [26]. The calculated and observed d values of all diffraction lines (reflections) of Fe and Mn modified PLZT compounds suggest that there is no change in the basic crystal structure (tetragonal) as y changes from 0.20 to 0.40. The tetragonality of samples also varies with the composition (Table 1). The broadening and splitting of XRD (2 0 0) and (2 1 0) peaks clearly demonstrate the formation of tetragonal phase. Fig. 1(b) shows that the variation of lattice parameter of the samples. These ceramics showed high sintered densities of the samples (Fig. 1(c)). Relative density increases with decreases of y mol%. But tetragonality of samples was decreases with decreases of y upto 0.35 and after that it increases upto 0.25 and again decreases for 0.20. Fig. 2(a) and (b) shows the variation of ε and tan ı with frequency at room temperature for all compositions. It was found that, with
Fig. 2. (a) Variation of dielectric constant of Mn and Fe doped PLZT samples as a function of frequency. (b) Variation of tan ı of Mn and Fe doped PLZT samples as a function of frequency at room temperature.
the increase of frequency, ε slowly decreases following the logarithmic law [27–29] This type of relaxation could be related to the domain wall relaxation in a random media similar to that found in other ABO3 systems [30,31]. Damjanovic [32] observed frequencydependent piezoelectric effect in doped PZT ceramics and related it to the Rayleigh type dependence. The decrease of the dielectric constant simply stems from the fact that the polarization does not occur instantaneously with the application of the electric field as charges possess inertia. The delay in response as a function of driving electric field leads to loss and dielectric constant frequency relaxation. The loss tangent (tan ı) increases with increasing frequency. This is due to effect of Fe and Mn ions on lattice site. Fig. 3(a) shows the temperature dependence of ε for different concentration of Zr and Ti at the B-site of PLZT at 10 kHz. Like any normal ferroelectric the dielectric constant increases gradually with rise in temperature up to its maximum value εmax and then decreases, indicating the phase transition in samples between room temperature and ∼600 K. The broadened peaks indicate that the transition in all the cases is of diffused type, an important characteristic of a disordered perovskite structure. The broadening is attributed to the disorder [27–28] in the arrangement of cations at
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
497
Fig. 4. Variation of ln(1/ε − 1/εmax ) vs ln(T − Tmax ) in the paraelectric region at 10 kHz.
a ‘complete’ DPT, the following empirical power relation expression was proposed to parameterize the phase transition diffuseness [34] ε=
Fig. 3. (a) Variation of dielectric constant of Mn and La doped PLZT samples as a function of temperature at 10 kHz and (b) dielectric constant maximum and transition temperatures vs doping concentration (mol%) at 10 kHz.
A-site which is occupied here by Pb2+ with La3+ additives, and or B-site occupied by Fe2+ , Mn2+ , Zr4+ and/or Ti4+ with lattice vacancies, leading to a microscopic heterogeneity in the composition and thus result in the distribution of different local curie point. Fig. 3(b) shows that the variation of Tmax and εmax with concentration of y mol%. It is clear that with decreases of concentration the Tmax and εmax increase. In Kirilov and Isupov [33] model, the material compositional fluctuations give rise to the diffuse phase transition, considering micro-regions that present a slightly different phase transition temperature. A semi-quantitative treatment based on a Gaussian distribution of local polar micro-regions was evolved to parameterize the broadening of the phase transition. In this way, a phase transition diffuseness parameter can be determined from the fitting of the dielectric constant curve as a function of temperature with the equation ε=
εmax 1 + 1/2((T − Tmax )/ı)
2
εmax
(2)
1 + (T − Tmax ) /2ı2
In Eq. (2) the same parameters as for Eq. (1) are considered; however the exponent 2 is substituted by another adjustable parameter that is the diffuseness exponent of the phase transition, ı being only an adjustable parameter. In Eq. (2), = 1 should represent ‘normal’ Curie–Weiss behavior, while = 2 corresponds to a ‘complete’ diffuse phase transition. The degree of disorderness of the samples was evaluated using the expression (1/ε − 1/εmax ) ∝ (T − Tmax ), [35] where is a measure of diffuseness of the ferroelectric to paraelectric phase transition. The plots between the ln(1/ε − 1/εmax ) vs ln(T − Tmax ) is shown in Fig. 4. The values of (Table 2) are found to lie between 1.50 and 1.56, which confirms the diffuse phase transition in PLZT compounds. The conductivity of the samples was estimated from the dielectric parameters. As long as the pure charge transport mechanism is the major contributor to the loss mechanisms, the ac conductivity may be calculated using the relation = ωεε0 tan ı [36] in which ε0 is the vacuum dielectric constant and ω is the angular frequency. In reality, loss tangent is due to the culmination of a variety of loss mechanisms and separation of these is a formidable task. However, at higher temperature the loss tangents are relatively low and hence using the above expression in the evaluation of conductivity is justified. The conductivity for all the samples was estimated over certain temperature range. The plot of ln ac vs inverse absolute temperature is shown in Fig. 5(a). The activation energy (Ea ) of the samples was calculated at a temperature in the ferroelectric region near the Tmax , where the loss tangents are relatively low, using the relation = 0 exp(−Ea /kB T) [37–38] where kB is the Boltzmann constant. The values of activation energy Ea are also tabulated in Table 2. Fig. 5(b) shows the variation of activation energy and diffusivity of the sample with different concentration. With increase of
(1)
where εmax is the maximum of the dielectric constant, Tmax the temperature of maximum dielectric constant and ı the diffuseness parameter related to the peak broadening of the phase transition. In other words, ı indicates the degree of the DPT. However, the quadratic law proposed in Eq. (1) deals only with ferroelectric materials that are considered as ferroelectrics with a so-called ‘complete’ DPT. Where ferroelectric materials with diffuse phase transition and/or relaxor characteristics do not necessarily present
Table 2 Physical parameters of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1–(x+y)}/2] Mn[{1–(x+y)}/2] O3 samples. Physical parameters
εRT Tmax (K) at 10 kHz εmax at 10 kHz Ea (eV)
Concentration (y%) 0.40
0.30
0.25
0.20
915 481 3000 0.33 1.56
925 490 3500 0.32 1.55
935 507 4750 0.25 1.50
940 515 6000 0.29 1.51
498
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
of single-phase compounds with tetragonal structure. Mn and Fe pair doping in PLZT provide many interesting features, such as shift in transition temperature, diffuse phase transition and modification of dielectric constant. From the present study we can say that the doping of softener (La), with hardener Fe and Mn enhanced the properties. It also shows the dielectric constant initials increase and then decrease with increase of temperature. It is clear that with decreases of concentration (y mol%) the Tmax and εmax increase. References
Fig. 5. (a) Variation of ac conductivity (ln ac ) with the inverse of the absolute temperature and (b) variation of activation energy and diffusivity of Mn and La doped PLZT samples at 10 kHz, respectively.
concentration first the value of activation energy and diffusivity decreases and than again increases. 4. Conclusions Fe and Mn doped PLZT ceramics, using high temperatures solidstate reaction technique exhibit good homogeneity, and formation
[1] T. Negas, G. Yeager, S. Bell, N. Coat, Am. Ceram. Soc. Bull. 72 (1993) 80. [2] H. Tamura, T. konolle, Y. Sakable, K. Wakino, J. Am. Ceram. Soc. 67 (1989) C59. [3] K. Wakino, K. Minal, H. Tamura, Am. Ceram. Soc. 67 (1984) 278. [4] S. Miga, K. wojcik, Ferroelectric 100 (1989) 167. [5] S.K. Shukla, A. Tiwari, G.K. Parashar, A.P. Mishra, G.C. Dubey, Talanta, doi:10.1016/j.talanta.2009.07.026, dated 13 July 2009. [6] L.E. Cross, Ferroelectrics 26 (1987) 241. [7] T.R. Shrout, A. Halliyal, Am. Ceram. Soc. Bull. 66 (1987) 704. [8] B. Jaffe, W.R. Cook Jr, H. Jaffe, Piezoelectric Ceramics, Academic, Press, New York, 1979. [9] T.K. Kamiya, T. Suzuki, T. Tasurumi, Jpn. J. Appl. Phys. 32 (1992) 3058. [10] K. Hase, M. Saitoh, T. Kittaka, K. Tomono, Jpn. J. Appl. Phys. 32 (1993) 4227. [11] C. Moure, M. Villegas, J.F. Fermandez, P. Duran, Ferroelectrics 127 (1992) 113. [12] G.H. Haertling, C.E. Land, J. Am. Ceram. Soc. 54 (1971) 1. [13] A. Oshima, K. Tashiosaki, S. Koizumi, Jpn. J. Appl. Phys. 33 (1994) 4940. [14] K. Oshima, K. Tsuzuki, Jpn. J. Appl. Phys. 33 (1994) 5389. [15] S. Dutta, R.N.P. Choudhary, P.K. Sinha, Ceram. Int. 33 (2007) 13. [16] B. Ma, D. Kwon, M. Narayanan, U. Balachandran, Mater. Lett. 62 (2008) 3573. [17] Z. Zheng, X. Li, J. Liu, Z. Feng, B. Li, J. Yang, K. Li, H. Jiang, X. Chen, J. Xie, H. Ming, Phys. B: Condens. Matter 403 (2008) 44. [18] R. Favaretto, D. Garcia, J.A. Eiras, J. Eur. Ceram. Soc. 27 (2007) 4037. [19] Y. Chen, S. Lin, S. Cheng, J. Alloys Compd. 449 (2008) 101. [20] K. Ramam, M. Lopez, J. Alloys Compd. 465 (2008) 446. [21] K. Ramam, M. Lopez, J. Alloys Compd. 466 (2008) 398. [22] R. Rai, S. Sharma, R.N.P. Choudhary, J. Mater. Sci. 41 (2006) 4259. [23] G.H. Heartling, Ferroelectrics 75 (1987) 25. [24] R. Rai, S. Sharma, Solid State Commun. 129 (2004) 305. [25] C.G.F. Stenger, A.J. Burggraaf, J. Phys. Chem. Solids 41 (1980) 25. [26] E. Wu, POWD, An interactive powder diffraction data interpretation and indexing program Ver2.1, School of Physical Science Flinders, University of South Australia, Bedford Park, SA, J042 AU. [27] R. Rai, S. Sharma, R.N.P. Choudhary, Mater. Lett. 59 (2005) 3921. [28] M.E. Lines, A.M. Glass, Principles and Application of Ferroelectrics and Related material, Oxford University Press, 1997. [29] A.K. Jonscher, J. Mater. Sci. 34 (1999) 3071. [30] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983. [31] A.K. Jonscher, Universal Relaxation Law, Chelsea Dielectrics Press, London, 1996. [32] D. Damjanovic, J. Appl. Phys. 82 (1997) 1788. [33] V.V. Kirilov, V.A. Isupov, Ferroelectrics 5 (1973) 3. [34] H.T. Martirena, J.C. Burfoot, Ferroelectrics 7 (1974) 151. [35] S.M. Pilgrim, A.E. Sutherland, S.R. Winzer, J. Am. Ceram. Soc. 73 (1990) 3122. [36] S. Miga, K. Wojcik, Ferroelectric 100 (1987) 167. [37] W.D. Kingery, Introduction to Ceramics, Wiley, New York, 1960. [38] V.M. Gurevich, Electrical conductivity of Ferroelectrics, Moskva, 1969.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of Fe and Mn doping at B-site of PLZT ceramics on dielectric properties Radheshyam Rai a,∗ , Sikha Mishra b , N.K. Singh b a b
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Department of Physics, V.K.S. University, Ara 802301, India
a r t i c l e
i n f o
Article history: Received 6 July 2009 Received in revised form 28 July 2009 Accepted 30 July 2009 Available online 5 August 2009 Keywords: Powders Solid-state reaction Grain size Dielectric properties Electrical conductivity
a b s t r a c t Iron (Fe) and manganese (Mn) doped lead lanthanum zirconate titanate (PLZT) ceramic powders have been synthesized by high temperature solid-state reaction method. Preliminary X-ray structural analysis of the compounds shows the formation of tetragonal structure. Detailed dielectric studies of the compounds as a function of temperature (from 300K to 600 K) at frequency 10 kHz suggest that the compounds undergo a diffuse phase transition. The transition temperature shifts towards higher side with decreasing the Zr ratio. The activation energy (Ea) of the samples was calculated from the plot of AC conductivity vs inverse of absolute temperature. Diffusivity () study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ferroelectric materials with perovskite structures have received much attention due to their excellent functional properties, such as piezoelectricity, pyroelectricity, electrooptic effects, useful for devices. Since the discovery of lanthanum-modified lead zirconate titanate (PLZT) ceramics with high optical transparency and good electro optical and other characteristics, numerous interests have been generated on the materials for possible potential applications such as dynamical and volatile memory components, transducers, sensors and many other active and passive devices [1–3]. The perovskite family is created by the doping of different types of cations into the stoichiometry and/or introducing anion deficiency. In the ABO3 structure, the valences of the A (12-cordinated) and B (6cordinated) cations are usually 2+ and 4+ , respectively. Ferroelectric materials with diffuse phase transition (DPT) characteristics and/or relaxor properties have been extensively studied in the last few decades [4–6]. The diffuse phase transition in ferroelectrics is characterized by extending the phase transition in a wide temperature interval around the temperature (Tmax ) where the dielectric constant assumes its maximum value (εmax ). Recently, it has been found that a wide variety of composition of perovskite ferroelectric can be obtained by making suitable substitutions at A and/or B-sited [7] in the general formula satisfying the conditions: (i) charge neutrality and (ii) suitable tolerance factor t [8]. Kamiya et al. [9] and Hase et al. [10] observed that addition of
∗ Corresponding author. Tel.: +91 351 962782932; fax: +91 351 234425300. E-mail address: [email protected] (R. Rai). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.161
MnO2 tended to increase Qm and k31 but decreased the tetragonality. Moure et al. [11] reported the microstructure and piezoelectric properties of PZT doped with Fe3+ and Sb3+ . Haertling and Land [12] reported that the La3+ because of its comparable ionic size could substitute Pb2+ ions. It has been reported that substitution of Pb2+ by La3+ ion created vacancies in the A-site of perovskite ABO3 structured ferroelectric PZT ceramics and refined to improve the optical quality of PLZT [13–14]. Dutta et al. reported the impedance spectroscopy studies on Fe3+ ion modified PLZT ceramics [15]. Ma et al. [16] reported that the PLZT ceramics film on nickel foil capacitors fabricated by the chemical solution deposition process exhibited good dielectric properties. Optical properties of Er3+ /Yb3+ -codoped transparent PLZT ceramic study by Zheng et al. [17]. Favaretto et al. [18] and Chen et al. [19] reported the influence of the A-site and B-site doping on the structural, microstructural and optical properties. Ramam et al. [20] observed that the Ti-rich region called the tetragonal symmetry increased with the introduction of BaTiO3 due to the combination of binary solid solutions of two perovskites (BT and PLSZNT), respectively. They also reported that, in ABO3 (PZT) perovskite structure, the acceptor (hard) dopants such as K+ , Rb+ , Na+ (occupy A-site, i.e., Pb-site) and Sc3+ , Mg2+ , Fe3+ , Fe2+ , Co2+ , etc. (occupy B-site, i.e., Zr/Ti-site) on structure enhanced the dielectric properties of materials. When Pb2+ ions are replaced by two acceptor ions or two acceptor ions replaces Zr4+ /Ti4+ , one oxygen vacancy is formed which cannot be removed by sintering the ceramic in oxygen atmosphere [21]. It is well known that lanthanum oxide is more suitable candidate for substitution on PLZT because of (a) its ability to reduce the distortion (anisotropy) of the oxygen octahedral (ABO3 ) unit cell (b) its high solubility in the PLZT perovskite structure, thus producing an
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
extensive series of homogeneous solid solution compositions and (c) its ability to produce a significant number of lattice vacancies resulting in the enhancement of the densification process, control of the grain growth behavior and the promotion of a highly uniform microstructure [22–24]. It was observed that transition is smeared out over a certain temperature interval, resulting in a gradual change in physical properties in this temperature interval, and it is mainly attributed to the structural disorder and compositional fluctuation [25]. In the present work, we have synthesized Mn and Fe modified PLZT ceramics, with a general formula Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1−(x+y)}/2] Mn[{1−(x+y)}/2] O3 where x = 0.60, 0.50, 0.55
495
Table 1 The lattice parameters and the measured density of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1–(x+y)}/2] Mn[{1–(x+y)}/2] O3 sample. Concentration (y%)
0.40 0.30 0.25 o.20
System (tetragonal) a
c
c/a
Theoretical density (gm/cm3 )
3.9452 3.8552 4.2545 4.2215
4.2917 4.1917 4.6488 4.5430
1.08 1.08 1.09 1.07
6.21 7.20 6.76 6.52
Fig. 1. (a) X-ray diffraction patterns of Fe and Mn doped PLZT ceramics, (b) variation of lattice parameter with concentration (mol%) of different compositions and (c) plots of tetragonality and relative density vs concentration (mol%) of different compositions.
496
R. Rai et al. / Journal of Alloys and Compounds 487 (2009) 494–498
Fig. 1. (Continued).
and 0.60 or y = 0.40, 0.30, 0.25 and 0.20, respectively. The effect of the Fe and Mn on different Zr/Ti ratio on the structural and electrical properties of the samples have been studied for better understanding of the nature of phase transition present in them. 2. Experimental procedures Polycrystalline samples of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1−(x+y)}/2] Mn[{1−(x+y)}/2] O3 were synthesized from high purity oxides. PbO, Fe2 O3 , and ZrO2 (99.9% M/S Aldrich chemicals USA), TiO2 (99.9% M/S Sd. fine-chem Ltd.), La2 O3, MnO (99.99% M/S Indian Rare-Earth Ltd.) using high temperature solid-state reaction technique in air atmosphere. The constituent compounds in suitable stoichiometry were thoroughly mixed in an agate mortar for 4 h. The mixed materials were then calcined in alumina crucible at 800 ◦ C for 2 h. The calcined fine powder was cold pressed into cylindrical pellets of size 10 mm of diameter and 1–2 mm of thickness using a hydraulic press with a pressure of 6 × 107 kg m−2 . The pellets were sintered in an air atmosphere at 900 ◦ C for 2 h in presence of PbZrO3 powder in order to compensate for PbO loss. The formation and quality of compounds were checked by XRD technique. The X-ray diffraction pattern of the compounds was recorded at room temperature using X-ray powder diffractometer (Rigaku Minifiex Japan) with CuK␣ radiation ( = 1.5418 Å) in a wide range of Bragg angles 2 (20◦ ≤ 2 ≤ 60◦ ) at a scanning rate of 2◦ min−1 . The dielectric constant (ε) and loss tangent (tan ı) of the compounds were measured using a HP4623B LCR meter as a function of frequency at 298 K (room temperature, [RT]) and temperature (RT to 600 K) at 10 kHz. The dc resistivity of the compounds was measured using Keithley-617 programmable electrometer with laboratory fabricated experimental setup. The flat polished surface of sintered pellets were electroded with air drying silver paste and fired at 150 ◦ C for 2 h before taking any electrical measurement. For the dielectrics, electrical measurements, the samples were placed in a laboratory made three-terminal sample holder and heated at the rate of 2 ◦ C min−1 .
3. Results and discussion XRD patterns of the perovskite ceramics are shown in Fig. 1(a). All the reflection peaks were indexed using observed inter-planar spacing d, and lattice parameters of samples were determined by using least-squares refinement method with a computer program package POWD [26]. The calculated and observed d values of all diffraction lines (reflections) of Fe and Mn modified PLZT compounds suggest that there is no change in the basic crystal structure (tetragonal) as y changes from 0.20 to 0.40. The tetragonality of samples also varies with the composition (Table 1). The broadening and splitting of XRD (2 0 0) and (2 1 0) peaks clearly demonstrate the formation of tetragonal phase. Fig. 1(b) shows that the variation of lattice parameter of the samples. These ceramics showed high sintered densities of the samples (Fig. 1(c)). Relative density increases with decreases of y mol%. But tetragonality of samples was decreases with decreases of y upto 0.35 and after that it increases upto 0.25 and again decreases for 0.20. Fig. 2(a) and (b) shows the variation of ε and tan ı with frequency at room temperature for all compositions. It was found that, with
Fig. 2. (a) Variation of dielectric constant of Mn and Fe doped PLZT samples as a function of frequency. (b) Variation of tan ı of Mn and Fe doped PLZT samples as a function of frequency at room temperature.
the increase of frequency, ε slowly decreases following the logarithmic law [27–29] This type of relaxation could be related to the domain wall relaxation in a random media similar to that found in other ABO3 systems [30,31]. Damjanovic [32] observed frequencydependent piezoelectric effect in doped PZT ceramics and related it to the Rayleigh type dependence. The decrease of the dielectric constant simply stems from the fact that the polarization does not occur instantaneously with the application of the electric field as charges possess inertia. The delay in response as a function of driving electric field leads to loss and dielectric constant frequency relaxation. The loss tangent (tan ı) increases with increasing frequency. This is due to effect of Fe and Mn ions on lattice site. Fig. 3(a) shows the temperature dependence of ε for different concentration of Zr and Ti at the B-site of PLZT at 10 kHz. Like any normal ferroelectric the dielectric constant increases gradually with rise in temperature up to its maximum value εmax and then decreases, indicating the phase transition in samples between room temperature and ∼600 K. The broadened peaks indicate that the transition in all the cases is of diffused type, an important characteristic of a disordered perovskite structure. The broadening is attributed to the disorder [27–28] in the arrangement of cations at
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497
Fig. 4. Variation of ln(1/ε − 1/εmax ) vs ln(T − Tmax ) in the paraelectric region at 10 kHz.
a ‘complete’ DPT, the following empirical power relation expression was proposed to parameterize the phase transition diffuseness [34] ε=
Fig. 3. (a) Variation of dielectric constant of Mn and La doped PLZT samples as a function of temperature at 10 kHz and (b) dielectric constant maximum and transition temperatures vs doping concentration (mol%) at 10 kHz.
A-site which is occupied here by Pb2+ with La3+ additives, and or B-site occupied by Fe2+ , Mn2+ , Zr4+ and/or Ti4+ with lattice vacancies, leading to a microscopic heterogeneity in the composition and thus result in the distribution of different local curie point. Fig. 3(b) shows that the variation of Tmax and εmax with concentration of y mol%. It is clear that with decreases of concentration the Tmax and εmax increase. In Kirilov and Isupov [33] model, the material compositional fluctuations give rise to the diffuse phase transition, considering micro-regions that present a slightly different phase transition temperature. A semi-quantitative treatment based on a Gaussian distribution of local polar micro-regions was evolved to parameterize the broadening of the phase transition. In this way, a phase transition diffuseness parameter can be determined from the fitting of the dielectric constant curve as a function of temperature with the equation ε=
εmax 1 + 1/2((T − Tmax )/ı)
2
εmax
(2)
1 + (T − Tmax ) /2ı2
In Eq. (2) the same parameters as for Eq. (1) are considered; however the exponent 2 is substituted by another adjustable parameter that is the diffuseness exponent of the phase transition, ı being only an adjustable parameter. In Eq. (2), = 1 should represent ‘normal’ Curie–Weiss behavior, while = 2 corresponds to a ‘complete’ diffuse phase transition. The degree of disorderness of the samples was evaluated using the expression (1/ε − 1/εmax ) ∝ (T − Tmax ), [35] where is a measure of diffuseness of the ferroelectric to paraelectric phase transition. The plots between the ln(1/ε − 1/εmax ) vs ln(T − Tmax ) is shown in Fig. 4. The values of (Table 2) are found to lie between 1.50 and 1.56, which confirms the diffuse phase transition in PLZT compounds. The conductivity of the samples was estimated from the dielectric parameters. As long as the pure charge transport mechanism is the major contributor to the loss mechanisms, the ac conductivity may be calculated using the relation = ωεε0 tan ı [36] in which ε0 is the vacuum dielectric constant and ω is the angular frequency. In reality, loss tangent is due to the culmination of a variety of loss mechanisms and separation of these is a formidable task. However, at higher temperature the loss tangents are relatively low and hence using the above expression in the evaluation of conductivity is justified. The conductivity for all the samples was estimated over certain temperature range. The plot of ln ac vs inverse absolute temperature is shown in Fig. 5(a). The activation energy (Ea ) of the samples was calculated at a temperature in the ferroelectric region near the Tmax , where the loss tangents are relatively low, using the relation = 0 exp(−Ea /kB T) [37–38] where kB is the Boltzmann constant. The values of activation energy Ea are also tabulated in Table 2. Fig. 5(b) shows the variation of activation energy and diffusivity of the sample with different concentration. With increase of
(1)
where εmax is the maximum of the dielectric constant, Tmax the temperature of maximum dielectric constant and ı the diffuseness parameter related to the peak broadening of the phase transition. In other words, ı indicates the degree of the DPT. However, the quadratic law proposed in Eq. (1) deals only with ferroelectric materials that are considered as ferroelectrics with a so-called ‘complete’ DPT. Where ferroelectric materials with diffuse phase transition and/or relaxor characteristics do not necessarily present
Table 2 Physical parameters of Pb0.92 La0.08 (Zrx Tiy )0.975 (Fe[{1–(x+y)}/2] Mn[{1–(x+y)}/2] O3 samples. Physical parameters
εRT Tmax (K) at 10 kHz εmax at 10 kHz Ea (eV)
Concentration (y%) 0.40
0.30
0.25
0.20
915 481 3000 0.33 1.56
925 490 3500 0.32 1.55
935 507 4750 0.25 1.50
940 515 6000 0.29 1.51
498
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of single-phase compounds with tetragonal structure. Mn and Fe pair doping in PLZT provide many interesting features, such as shift in transition temperature, diffuse phase transition and modification of dielectric constant. From the present study we can say that the doping of softener (La), with hardener Fe and Mn enhanced the properties. It also shows the dielectric constant initials increase and then decrease with increase of temperature. It is clear that with decreases of concentration (y mol%) the Tmax and εmax increase. References
Fig. 5. (a) Variation of ac conductivity (ln ac ) with the inverse of the absolute temperature and (b) variation of activation energy and diffusivity of Mn and La doped PLZT samples at 10 kHz, respectively.
concentration first the value of activation energy and diffusivity decreases and than again increases. 4. Conclusions Fe and Mn doped PLZT ceramics, using high temperatures solidstate reaction technique exhibit good homogeneity, and formation
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