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Ferroelectric phase transition in Bi-doped PLZT ceramics
Materials Science and Engineering B98 (2003) 74 /80 www.elsevier.com/locate/mseb
Ferroelectric phase transition in Bi-doped PLZT ceramics Soma Dutta a, R.N.P. Choudhary a,*, P.K. Sinha b a
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur 721302, India
b
Received 22 May 2002; received in revised form 20 November 2002; accepted 2 December 2002
Abstract Polycrystalline samples of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 (z /0.0, 0.3, 0.6, 0.9, 1) (PLBZT) were synthesized using a high-temperature solid-state reaction technique. The formation of the PLBZT compounds in a particular crystallographic phase as a function of Bi concentration was checked through X-ray diffraction (XRD). S.E.M. study of PLBZT with higher magnification and resolution shows the uniform distribution of particles/grains on the surface of the samples. Dielectric constant and tangent loss of the samples in wide temperature and frequency range provided the nature of phase transition. The diffuse phase transition was observed for higher z . An increase in permittivity maxima temperature (i.e. the transition temperature) was also observed with increasing Bi concentration. The values of resistivity of all the samples are very high (109 V-cm). # 2003 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Diffuse phase transition; Transition temperature
1. Introduction Discovery of ferroelectric properties in BaTiO3 in 1945 [1] led to study a large number of oxides of different structural families. Amongst all the oxides studied so far, some lead based compounds of the perovskite family ABO3 (A /mono or divalent, B /tri / hexa valent ions) were found to exhibit many interesting and important ferroelectrics properties useful for devices. Lead zirconate titanate Pb(ZrTi)O3 (PZT); a solid solution of ferroelectric PbTiO3 (Tc /490 8C) and antiferroelectric PbZrO3 (Tc /230 8C), is one of the important members of the perovskite family which has wide industrial applications such as computer memory and display, transducer, actuator, detector, microphone, sensor etc. [2 /8]. It has been found in many studies that the variation of Zr/Ti ratio and with a suitable substitution at Pb-sites have a significant influence on physical properties and device parameters of PZT [9,10]. The La-modified PZT (i.e. PLZT) is one such system
* Corresponding author. Tel.: /91-322-28-3814; fax: /91-322-2755303. E-mail address: [email protected] (R.N.P. Choudhary).
which has wide spread applications in electronics and electrooptoics [11,12]. PZT and/or modified PZT have extensively been studied in the past using various advanced experimental techniques. Though a large amount of work has been reported by us on PLZT [13 /15], the effect of some double doping (i.e. trivalent pair ions) at the Pb-site has not been reported much [16]. Therefore, we have carried out synthesis and some characterization (structural, micro-structural, dielectric and electrical properties) of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3(z /0.0, 0.3, 0.6, 0.9, 1) (PLBZT) complex system.
2. Experimental The polycrystalline samples of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 with different z values (z / 0.0, 0.3, 0.6, 0.9, 1) were prepared by a high-temperature solid-state reaction technique from oxides: PbO (99.99%, M/S Aldrich Chemical Company Inc. USA), La2O3 (99.9%, M/S Indian rare earth ltd.), Bi2O3, ZrO2 (97%, M/S Loba Chemie, Inc. Bombay, India), TiO2 (99.5%, M/S Loba Chemie, Inc.) in a suitable stoichio-
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00612-8
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
75
Fig. 1. Room temperature XRD pattern of PLBZT samples.
metry. The thoroughly mixed anhydrous oxides were fired (calcined) at 1050 8C for 7 h. The mixing and calcination were repeated at the same temperature for 6 h in an air atmosphere. The homogeneous fine powders of PLBZT were then used to make pellets of diameter 10 mm and thickness 1/2 mm under an isostatic pressure of about 6 /107 kg m 2 using a hydraulic press. The pellets were finally sintered at 1250 8C for 24 h in a platinum crucible in PbZrO3 atmosphere. The density of the samples was measured by Archimedes’ method and found to be /97% of the theoretical density of PLZT. The formation of the desired compound was checked by preliminary structural studies with the help of X-ray diffractogram taken on calcined powder in a wide range of Bragg angles, 2u (2085/2u 5/808) at room temperature using an X-ray diffractometer (Philips PW1710, ˚ ). The Holland) with CoKa radiation (l /1.790 A average crystallite size (P ) of the samples was calculated from the broadening of X-ray diffraction (XRD) peaks of different Bragg’s angles using Scherrer’s equation:
Phkl /(0.89 l)/(b1/2 cos uhkl) where b1/2 is half peak width and u is Bragg angle. The peak broadening due to mechanical strain, instrumental and other means have been ignored because powder samples were used. The surface morphology/grain distribution on the pellet surface was studied with JEOL JSM-5800 scanning electron microscope (SEM). A high-purity silver paste was then painted on the flat polished surfaces of the sintered pellets, which acts as an electrode, and then fired-on at 150 8C before taking any electrical measurements. The dielectric constant (o ) and loss tangent (tan d ) of the samples were obtained as a function of frequency (103 /104 Hz) at different temperatures (room temperature/450 8C), using a HIOKI 3532 LCR Hitester and a laboratory-made 3-terminal sample holder; which compensate stray capacitance. The dc electrical conductivity/resistivity was measured as a function of temperature (room temperature /450 8C) at constant electric field (60 V cm 1) with the help of a KEITH-
Table 1 ˚ ), unit cell volume V , average crystallites size P in nm and density d in g cc 1 of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 for Lattice parameters (in A different Bi-concentrations (z ) Compositions z
Tetragonal phase a
0.0 0.3 0.6 0.9 1.0
4.0087 4.0125 4.0406 4.0711 4.0746
c (7) (8) (9) (14) (8)
Standard deviations are given in parenthesis.
4.0125 4.0294 4.0762 4.0889 4.0636
(7) (8) (8) (14) (8)
c /a
V
1.0009 1.0042 1.0088 1.0044 0.9973
64.48 64.87 66.55 67.77 67.47
P
d
11.7 17.69 16.12 19.3 24.78
7.33 7.82 8.06 7.88 7.20
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
76
Fig. 2. SEM micrographs of PLBZT samples for z /0.0 (a), /0.3 (b), /0.6 (c), /0.9 (d) and 1.0 (e).
LEY-617 programmable electrometer and a laboratorymade heating arrangement.
3. Results and discussion 3.1. Structural study Fig. 1 gives comparison of XRD pattern of calcined powder of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 with different z (0.0, 0.3, 0.6 and 0.9) values. The XRD pattern corresponding to z /l is also included in inset.
The single and sharp XRD peaks, which are different (i.e. in position and intensity) from that of ingredients, confirm the formation of single-phase new compounds. All the reflection peaks were indexed and lattice parameters of the compounds were determined and refined by the least-squares refinement method using a computer program package (POWD MULT). Compared with PLZT, a small shift in diffraction peak position of XRD pattern of all PLBZT was observed (Fig. 1), suggesting no change in the basic crystal structure of PZT (Zr/Ti / 60/40) on substitution of La3 and Bi3 ions at Pbsites. However, a small shift in the peak position gives a
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
77
3.2. Dielectric study
Fig. 3. Variation of, (a) dielectric constant (o ) and; (b) dielectric loss (tan d ) of PLBZT with frequency.
small change in the lattice parameters (Table 1). A careful analysis of XRD patterns showed that PLBZT compounds for z /0.0, 0.3, 0.6, 0.9, 1.0 belong to the tetragonal phase at room temperature. However, a significant increase in lattice parameters and unit cell volume was observed with increase of z (up to z /0.9). This is quite consistent because atomic radius of Bi is larger than that of La. But complete replacement of Bi for La showed lower value of c /a and volume as compared with that of z /0.9 The scanning electron micrographs of the PLBZT samples are given in Fig. 2. The grains were found to be nearly spherical throughout the surface. The tiny, small and uniformly distributed grains confirm high density of the ceramic samples.
Fig. 3 shows the variation of dielectric constant (o ) and tangent loss (tan d ) as a function of frequency at room temperature. The o of all the Bi-doped PLZT first decreases in low frequency range and then becomes almost constant with increasing frequency. The loss tangent (tan d ) of PLBZT slightly increases with increasing frequency. Fig. 4(a /e) show the temperature dependence of dielectric constant (o ) of PLBZT (for z /0.0 /1.0) compounds at five different frequencies. Transition temperature (Tc) shifts towards higher temperature side with increasing frequency (i.e. relaxor behavior) was observed for z /0.0 /0.6, but interestingly the Tc shift towards lower temperature side for higher values of z . Fig. 5 (a, b) show the variation of o and tan d of PLBZT with temperature at 10 kHz. It is found that dielectric constant of the compounds first increases (upto z /0.6) then it starts decreasing with increasing z. The dielectric peaks for all the Bi-doped PLZT samples (z /0.0 /1.0) were found to broaden and diffused over a certain temperature interval. The broadening of the dielectric peaks increases with increasing z (0.0 /0.9) (Fig. 5a) and frequency. From the various experimental techniques it has been established that the diffuse phase transition occurs mainly due to, (a) grain size; (b) ferroelectric nano-region in higher temperature region; (c) compositional fluctuations; and/or (d) substitutional disordering in the arrangement of cation in one or more crystallographic sites of the structure. This leads to microscopic or nanoscopic heterogeneity in the compounds, which causes a distribution of the different local Curie points [17]. The variation of tangent loss with temperature shows an anomaly near the transition temperature. For all the Bi-doped PLZT tan d increases with the rise in temperature and its peak value reaches at a temperature close to Tc and after that it starts decreasing. The degree of diffuseness of dielectric peaks of the compounds was estimated using an empirical relation (1/ o /1/o max) (T /Tmax)g , where o max is the maximum value of dielectric constant at a temperature called Tmax (close to Tc). The value of exponent g (diffusivity) calculated for all the samples of PLBZT (Fig. 6) was found to be between 1 and 2 (1 5/g 5/2) where g /1 follows the Curie /Weiss law in the para-electric phase, and g /2 for completely disordered ferroelectrics. The decrease in the value of g with increase in z suggests that the PLZT compounds become more ordered with increase of Bi concentration at the La-site. The nature and/or amount of doping play an important role in tailoring the material properties such as o , tan d and Tc. The broadening of the dielectric peak also depends on the grain size of the samples. For smaller grain size the o max decreases and the dielectric peak broadened with
78
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
Fig. 4. Variation of dielectric constant (o ) as a function of temperature of [Pb0.92(La1 z Biz ]0.08][Zr0.60Ti0.40]0.98O3 at five different frequencies for z /0 (a), /0.3 (b), /0.6 (c), /0.9(d), /1.0(e).
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
79
Fig. 5. Variation of, (a) dielectric constant (o ) and; (b) tan d as a function of temperature of PLBZT at a frequency of 10 kHz.
Table 2 Comparison of dielectric parameters of PLBZT at 10 kHz Compositions
z /0.0
z /0.3
z /0.6
z /0.9
z /1.0
Tc (8C) o max at Tc Tan d at Tc Diffusivity constant g
176 4858 0.046 1.77
225 9547 0.028 1.75
262 18 040 0.033 1.72
297 17 044 0.024 1.68
413 2746 0.114 1.79
80
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
Fig. 6. Plot of ln (l/o /l/o max) vs. ln(T /Tc) of PLBZT at 10 kHz.
shifting of transition temperature to higher temperature [18]. Values for all z are given in Table 2.
Fig. 7. Resistivity (ln r ) vs. temperature characteristics of PLBZT ceramics.
suggest that PLBZT can be a good candidate for some devices.
3.3. Electrical resistivity The temperature dependence of dc resistivity (r ) of PLBZT at constant biasing field (60 V cm 1) is shown in Fig. 7. The r of all the pair doped (La, Bi) samples decreases with increasing temperature within the ferroelectric region. The nature of variation of resistivity with temperature of PZT compound or its solid solutions, with varying (La) or pair (La, alkali) ions, was found to be very small [19] in the high-temperature (para electric) region. In the low temperature (ferroelectric) region, the value of resistivity increases with increase of La and Bi ions content because the valency of these ions are /3 where as that for Pb is /2 [20]. Therefore, La, Bi ions may be accepted to function as a donor resulting in an increase in the resistivity due to electron-hole compensation.
4. Conclusion The PLBZT ceramics prepared by a solid-state reaction technique exhibit formation of single-phase compounds. Detailed studies of dielectric constant show a ferroelectric diffuse phase transition of these compounds. It is also observed that the transition temperature increases continuously with the increase of Bi concentration. The dielectric constant initially increases and then decreases with the increase of z. The dielectric and electrical resistive properties strongly
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20]
B. Wul, L.M. Goldman, C.R. Acad. Sci. USSR 46 (1945) 123. K.K. Deb, Ferroelectrics 82 (1988) 45. Y. Masuda, Ferroelectrics 63 (1985) 261. R.P. Tandon, R. Singh, R.P.P. Singh, S. Chandra, Ferroelectrics 120 (1991) 293. S.T. Liu, J.D. Heaps, O.N. Tufte, Ferroelectrics 3 (1971) 281. Y.L. Wang, Z.M. Cheng, Y.R. Sun, X.H. Dai, Physica B 150 (1988) 168. G.H. Haertling, Am. Ceram. Soc. Bull. 46 (1964) 374. V.A. Isupov, Ferroelectrics 143 (1993) 109. T.K. Kundu, M. Mukherjee, D. Chakraborty, T.P. Sinha, J. Mater. Sci. 33 (1998) 1758. G.H. Haertling, Ferroelectrics 75 (1987) 25. G.H. Haertling, in: R.C. Buchanan (Ed.), Ceramic Materials for Electronics (Chapter 3), Marcel Dekker, New York, 1991. B.S. Chiou, J.N. Kuo, H.T. Dai, J. Electron. Mater. 19 (1990) 393. R.N.P. Choudhary, J. Mal, Mater. Sci. Eng. B90 (2002) 1. S.R. Shannigrahi, R.N.P. Choudhary, H.N. Acharya, Mater. Res. Bull., 34 (1999) 1875. H.R. Rukmini, R.N.P. Choudhary, V.V. Rao, J. Mater. Sci. 34 (1999) 4815. K.L. Yadav, R.N.P. Choudhary, Indian J. Pure Appl. Phys. 31 (1993) 585. M.E. Lines, A.M. Glass, Principles and Application of Ferroelectric and Related Materials, Oxford University Press, London, 1977. L.E. Cross, Ferroelectrics 76 (1987) 107. H.R. Rukmini, R.N.P. Choudhary, D. Prabhakara, Mater. Chem. Phys. 64 (2000) 171. R. Gerson, H. Jaffe, J. Phys. Chem. Solids 24 (1963) 979.
Ferroelectric phase transition in Bi-doped PLZT ceramics Soma Dutta a, R.N.P. Choudhary a,*, P.K. Sinha b a
Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Department of Aerospace Engineering, Indian Institute of Technology, Kharagpur 721302, India
b
Received 22 May 2002; received in revised form 20 November 2002; accepted 2 December 2002
Abstract Polycrystalline samples of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 (z /0.0, 0.3, 0.6, 0.9, 1) (PLBZT) were synthesized using a high-temperature solid-state reaction technique. The formation of the PLBZT compounds in a particular crystallographic phase as a function of Bi concentration was checked through X-ray diffraction (XRD). S.E.M. study of PLBZT with higher magnification and resolution shows the uniform distribution of particles/grains on the surface of the samples. Dielectric constant and tangent loss of the samples in wide temperature and frequency range provided the nature of phase transition. The diffuse phase transition was observed for higher z . An increase in permittivity maxima temperature (i.e. the transition temperature) was also observed with increasing Bi concentration. The values of resistivity of all the samples are very high (109 V-cm). # 2003 Elsevier Science B.V. All rights reserved. Keywords: Ceramics; Diffuse phase transition; Transition temperature
1. Introduction Discovery of ferroelectric properties in BaTiO3 in 1945 [1] led to study a large number of oxides of different structural families. Amongst all the oxides studied so far, some lead based compounds of the perovskite family ABO3 (A /mono or divalent, B /tri / hexa valent ions) were found to exhibit many interesting and important ferroelectrics properties useful for devices. Lead zirconate titanate Pb(ZrTi)O3 (PZT); a solid solution of ferroelectric PbTiO3 (Tc /490 8C) and antiferroelectric PbZrO3 (Tc /230 8C), is one of the important members of the perovskite family which has wide industrial applications such as computer memory and display, transducer, actuator, detector, microphone, sensor etc. [2 /8]. It has been found in many studies that the variation of Zr/Ti ratio and with a suitable substitution at Pb-sites have a significant influence on physical properties and device parameters of PZT [9,10]. The La-modified PZT (i.e. PLZT) is one such system
* Corresponding author. Tel.: /91-322-28-3814; fax: /91-322-2755303. E-mail address: [email protected] (R.N.P. Choudhary).
which has wide spread applications in electronics and electrooptoics [11,12]. PZT and/or modified PZT have extensively been studied in the past using various advanced experimental techniques. Though a large amount of work has been reported by us on PLZT [13 /15], the effect of some double doping (i.e. trivalent pair ions) at the Pb-site has not been reported much [16]. Therefore, we have carried out synthesis and some characterization (structural, micro-structural, dielectric and electrical properties) of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3(z /0.0, 0.3, 0.6, 0.9, 1) (PLBZT) complex system.
2. Experimental The polycrystalline samples of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 with different z values (z / 0.0, 0.3, 0.6, 0.9, 1) were prepared by a high-temperature solid-state reaction technique from oxides: PbO (99.99%, M/S Aldrich Chemical Company Inc. USA), La2O3 (99.9%, M/S Indian rare earth ltd.), Bi2O3, ZrO2 (97%, M/S Loba Chemie, Inc. Bombay, India), TiO2 (99.5%, M/S Loba Chemie, Inc.) in a suitable stoichio-
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00612-8
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
75
Fig. 1. Room temperature XRD pattern of PLBZT samples.
metry. The thoroughly mixed anhydrous oxides were fired (calcined) at 1050 8C for 7 h. The mixing and calcination were repeated at the same temperature for 6 h in an air atmosphere. The homogeneous fine powders of PLBZT were then used to make pellets of diameter 10 mm and thickness 1/2 mm under an isostatic pressure of about 6 /107 kg m 2 using a hydraulic press. The pellets were finally sintered at 1250 8C for 24 h in a platinum crucible in PbZrO3 atmosphere. The density of the samples was measured by Archimedes’ method and found to be /97% of the theoretical density of PLZT. The formation of the desired compound was checked by preliminary structural studies with the help of X-ray diffractogram taken on calcined powder in a wide range of Bragg angles, 2u (2085/2u 5/808) at room temperature using an X-ray diffractometer (Philips PW1710, ˚ ). The Holland) with CoKa radiation (l /1.790 A average crystallite size (P ) of the samples was calculated from the broadening of X-ray diffraction (XRD) peaks of different Bragg’s angles using Scherrer’s equation:
Phkl /(0.89 l)/(b1/2 cos uhkl) where b1/2 is half peak width and u is Bragg angle. The peak broadening due to mechanical strain, instrumental and other means have been ignored because powder samples were used. The surface morphology/grain distribution on the pellet surface was studied with JEOL JSM-5800 scanning electron microscope (SEM). A high-purity silver paste was then painted on the flat polished surfaces of the sintered pellets, which acts as an electrode, and then fired-on at 150 8C before taking any electrical measurements. The dielectric constant (o ) and loss tangent (tan d ) of the samples were obtained as a function of frequency (103 /104 Hz) at different temperatures (room temperature/450 8C), using a HIOKI 3532 LCR Hitester and a laboratory-made 3-terminal sample holder; which compensate stray capacitance. The dc electrical conductivity/resistivity was measured as a function of temperature (room temperature /450 8C) at constant electric field (60 V cm 1) with the help of a KEITH-
Table 1 ˚ ), unit cell volume V , average crystallites size P in nm and density d in g cc 1 of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 for Lattice parameters (in A different Bi-concentrations (z ) Compositions z
Tetragonal phase a
0.0 0.3 0.6 0.9 1.0
4.0087 4.0125 4.0406 4.0711 4.0746
c (7) (8) (9) (14) (8)
Standard deviations are given in parenthesis.
4.0125 4.0294 4.0762 4.0889 4.0636
(7) (8) (8) (14) (8)
c /a
V
1.0009 1.0042 1.0088 1.0044 0.9973
64.48 64.87 66.55 67.77 67.47
P
d
11.7 17.69 16.12 19.3 24.78
7.33 7.82 8.06 7.88 7.20
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
76
Fig. 2. SEM micrographs of PLBZT samples for z /0.0 (a), /0.3 (b), /0.6 (c), /0.9 (d) and 1.0 (e).
LEY-617 programmable electrometer and a laboratorymade heating arrangement.
3. Results and discussion 3.1. Structural study Fig. 1 gives comparison of XRD pattern of calcined powder of [Pb0.92(La1 z Biz )0.08][Zr0.60Ti0.40]0.98O3 with different z (0.0, 0.3, 0.6 and 0.9) values. The XRD pattern corresponding to z /l is also included in inset.
The single and sharp XRD peaks, which are different (i.e. in position and intensity) from that of ingredients, confirm the formation of single-phase new compounds. All the reflection peaks were indexed and lattice parameters of the compounds were determined and refined by the least-squares refinement method using a computer program package (POWD MULT). Compared with PLZT, a small shift in diffraction peak position of XRD pattern of all PLBZT was observed (Fig. 1), suggesting no change in the basic crystal structure of PZT (Zr/Ti / 60/40) on substitution of La3 and Bi3 ions at Pbsites. However, a small shift in the peak position gives a
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
77
3.2. Dielectric study
Fig. 3. Variation of, (a) dielectric constant (o ) and; (b) dielectric loss (tan d ) of PLBZT with frequency.
small change in the lattice parameters (Table 1). A careful analysis of XRD patterns showed that PLBZT compounds for z /0.0, 0.3, 0.6, 0.9, 1.0 belong to the tetragonal phase at room temperature. However, a significant increase in lattice parameters and unit cell volume was observed with increase of z (up to z /0.9). This is quite consistent because atomic radius of Bi is larger than that of La. But complete replacement of Bi for La showed lower value of c /a and volume as compared with that of z /0.9 The scanning electron micrographs of the PLBZT samples are given in Fig. 2. The grains were found to be nearly spherical throughout the surface. The tiny, small and uniformly distributed grains confirm high density of the ceramic samples.
Fig. 3 shows the variation of dielectric constant (o ) and tangent loss (tan d ) as a function of frequency at room temperature. The o of all the Bi-doped PLZT first decreases in low frequency range and then becomes almost constant with increasing frequency. The loss tangent (tan d ) of PLBZT slightly increases with increasing frequency. Fig. 4(a /e) show the temperature dependence of dielectric constant (o ) of PLBZT (for z /0.0 /1.0) compounds at five different frequencies. Transition temperature (Tc) shifts towards higher temperature side with increasing frequency (i.e. relaxor behavior) was observed for z /0.0 /0.6, but interestingly the Tc shift towards lower temperature side for higher values of z . Fig. 5 (a, b) show the variation of o and tan d of PLBZT with temperature at 10 kHz. It is found that dielectric constant of the compounds first increases (upto z /0.6) then it starts decreasing with increasing z. The dielectric peaks for all the Bi-doped PLZT samples (z /0.0 /1.0) were found to broaden and diffused over a certain temperature interval. The broadening of the dielectric peaks increases with increasing z (0.0 /0.9) (Fig. 5a) and frequency. From the various experimental techniques it has been established that the diffuse phase transition occurs mainly due to, (a) grain size; (b) ferroelectric nano-region in higher temperature region; (c) compositional fluctuations; and/or (d) substitutional disordering in the arrangement of cation in one or more crystallographic sites of the structure. This leads to microscopic or nanoscopic heterogeneity in the compounds, which causes a distribution of the different local Curie points [17]. The variation of tangent loss with temperature shows an anomaly near the transition temperature. For all the Bi-doped PLZT tan d increases with the rise in temperature and its peak value reaches at a temperature close to Tc and after that it starts decreasing. The degree of diffuseness of dielectric peaks of the compounds was estimated using an empirical relation (1/ o /1/o max) (T /Tmax)g , where o max is the maximum value of dielectric constant at a temperature called Tmax (close to Tc). The value of exponent g (diffusivity) calculated for all the samples of PLBZT (Fig. 6) was found to be between 1 and 2 (1 5/g 5/2) where g /1 follows the Curie /Weiss law in the para-electric phase, and g /2 for completely disordered ferroelectrics. The decrease in the value of g with increase in z suggests that the PLZT compounds become more ordered with increase of Bi concentration at the La-site. The nature and/or amount of doping play an important role in tailoring the material properties such as o , tan d and Tc. The broadening of the dielectric peak also depends on the grain size of the samples. For smaller grain size the o max decreases and the dielectric peak broadened with
78
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
Fig. 4. Variation of dielectric constant (o ) as a function of temperature of [Pb0.92(La1 z Biz ]0.08][Zr0.60Ti0.40]0.98O3 at five different frequencies for z /0 (a), /0.3 (b), /0.6 (c), /0.9(d), /1.0(e).
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
79
Fig. 5. Variation of, (a) dielectric constant (o ) and; (b) tan d as a function of temperature of PLBZT at a frequency of 10 kHz.
Table 2 Comparison of dielectric parameters of PLBZT at 10 kHz Compositions
z /0.0
z /0.3
z /0.6
z /0.9
z /1.0
Tc (8C) o max at Tc Tan d at Tc Diffusivity constant g
176 4858 0.046 1.77
225 9547 0.028 1.75
262 18 040 0.033 1.72
297 17 044 0.024 1.68
413 2746 0.114 1.79
80
S. Dutta et al. / Materials Science and Engineering B98 (2003) 74 /80
Fig. 6. Plot of ln (l/o /l/o max) vs. ln(T /Tc) of PLBZT at 10 kHz.
shifting of transition temperature to higher temperature [18]. Values for all z are given in Table 2.
Fig. 7. Resistivity (ln r ) vs. temperature characteristics of PLBZT ceramics.
suggest that PLBZT can be a good candidate for some devices.
3.3. Electrical resistivity The temperature dependence of dc resistivity (r ) of PLBZT at constant biasing field (60 V cm 1) is shown in Fig. 7. The r of all the pair doped (La, Bi) samples decreases with increasing temperature within the ferroelectric region. The nature of variation of resistivity with temperature of PZT compound or its solid solutions, with varying (La) or pair (La, alkali) ions, was found to be very small [19] in the high-temperature (para electric) region. In the low temperature (ferroelectric) region, the value of resistivity increases with increase of La and Bi ions content because the valency of these ions are /3 where as that for Pb is /2 [20]. Therefore, La, Bi ions may be accepted to function as a donor resulting in an increase in the resistivity due to electron-hole compensation.
4. Conclusion The PLBZT ceramics prepared by a solid-state reaction technique exhibit formation of single-phase compounds. Detailed studies of dielectric constant show a ferroelectric diffuse phase transition of these compounds. It is also observed that the transition temperature increases continuously with the increase of Bi concentration. The dielectric constant initially increases and then decreases with the increase of z. The dielectric and electrical resistive properties strongly
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