Effect of Er doping on structural and dielectric properties of sol-gel prepared PZT ceramics

Materials Research Bulletin, Vol. 34, Nos. 12/13, pp. 1875–1884, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

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EFFECT OF Er DOPING ON STRUCTURAL AND DIELECTRIC PROPERTIES OF SOL-GEL PREPARED PZT CERAMICS

S.R. Shannigrahi, R.N.P. Choudhary*, and H.N. Acharya Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India (Refereed) (Received September 18, 1998; Accepted January 11, 1999)

ABSTRACT The polycrystalline complex compounds of (PbzEr1⫺z)(Zr0.60Ti0.40)1⫺z/4O3 (z ⫽ 0.07, 0.08, 0.10) (PEZT) were synthesized using the metal–alkoxide sol-gel technique. Powder X-ray diffraction (XRD) studies of these compounds suggest that they can be formed in single-phase perovskite structure by a one-step process at a relatively low temperature. Scanning electron microscopy (SEM) analysis shows that the grains are nearly spherical and distributed uniformly in the compounds. Detailed studies of dielectric properties (⑀, tan␦, and ␴) of the compounds as a function of temperature show a broadening of the permittivity peak and a change in transition temperature with the increase of Er3⫹ concentration. Analysis of the diffusivity (␥) of the peaks in these compounds provides a value between 1 and 2. The higher value of ␥ indicates the greater disordering of the system. The hysteresis loop study confirms the phase transition of the system is of second order. © 2000 Elsevier Science Ltd KEYWORDS: A. ceramics, C. X-ray diffraction, D. dielectrics, D. ferroelectricity, D. phase transitions INTRODUCTION Among all of the lead-based ferroelectric perovskite compounds of ABO3 types (A ⫽ mono or divalent, B ⫽ tri-hexavalent ion) studied so far, Pb(ZrTi)O3, popularly known as PZT [1–2], either in bulk or thin film form, has been found to have many applications, in, for

*To whom correspondence should be addressed. 1875

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example, computer memory and display, transducers, electro-optic modulators, and sensors [3– 6]. The device parameters of the systems can be modified with a wide variety of substitutions at the A- and/or B-sites [7–10]. For example, lanthanum-modified PZT, popularly known as PLZT, has been found useful in various nonlinear optical applications. The use of different dopants allows the enhancement of specific properties and therefore results in even wider application. Not much has been reported on Er-modified PZT. Here we report our work on the synthesis and characterization (structural, microstructural, dielectric, and hysteresis properties) of a (Pb1⫺zErz)(Zr0.60Ti0.40)1⫺z/4O3 (z ⫽ 0.07, 0.08, 0.10) (denoted as PEZT) complex system. EXPERIMENTAL Sample Preparation. The precursors used to prepare the polycrystalline samples of PEZT were lead acetate trihydrate Pb(CH3COO)2䡠3H2O (99.9%, E. Merck, Germany), erbium acetate tetrahydrate (CH3COO)3Er䡠4H2O (99.9%, Aldrich, USA), zirconium isopropoxide Zr(C3H7O)4 (70 wt% in I-propanol, Fluka, Switzerland), and titanium isopropoxide Ti[(CH3)2CHO]4 (⬎97% Ti, E. Merck, Germany). Glacial acetic acid and distilled water were used as solvents, while ethylene glycol was used as an additive in order to obtain a monolithic gel. First, lead acetate and erbium acetate were dissolved separately in acetic acid at a ratio of 2 gm of salt to 1 ml acid and were heated at 110°C for half an hour to remove the water content before cooling them down to 80°C. These two solutions were mixed in a vessel and stirred. During stirring, first zirconium isopropoxide and then titanium isopropoxide were added to the mixture. Ethylene glycol was then added in the proportion of 1 ml to 10 gm of lead acetate in the solution. The initial reaction had to be completed before the glycol was added, since residual titanium isopropoxide and zirconium isopropoxide alcolyzed with ethylene glycol to form a condensed solid. A small amount of distilled water was added to obtain the final sol. The sol was then kept at 60°C for 24 h to form a clear transparent gel. The gel was dried at 100°C for 72 h, and a light brown powder was obtained. The oven-dried powdered gels were calcined at 550°C for 15 h. The powders were cold-pressed into discs (pellets) at a pressure of 6 ⫻ 107 kg䡠m⫺2 using a uniaxial hydraulic press. The pellets were then sintered at 1300°C for 7 h. In order to prevent PbO loss due to vaporization during sintering, an equilibrium PbO vapor pressure was established, with PbZrO3 as the setter and everything placed in the covered platinum crucible to maintain the stoichiometry of the compounds. The density of the sintered pale yellow pellets was measured by Archimedes’ method and found to be 97–98% of the theoretical density. X-ray Diffraction and Scanning Electron Microscopy. The formation and quality of the desired compounds were checked by X-ray diffraction (XRD) technique with a powder diffractometer (Philips PW 1877) using Cu K␣ radiation (␭ ⫽ 1.5418 Å) in a wide range of Bragg angles (20° ⱕ 2␪ ⱕ 60°) at room temperature with the scanning rate 3°min⫺1 on powders as well as on sintered pellets. The microstructures of the sintered pellet samples were analyzed by SEM using a CAMSCAN 180. Dielectric Study. To create electrodes, high-purity silver paste was painted on the flat polished surfaces of the sintered pellets. The paste was dried at 100°C before taking any electrical measurements. The dielectric constant (ε) and tangent loss (tan␦) of the samples

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FIG. 1 Comparison of XRD patterns of PEZT. were obtained using a GR 1620 AP capacitance measuring assembly with a three-terminal sample holder as a function of frequency (400 –104 Hz) at room temperature and as a function of temperature (30 – 400°C) at frequency 104 Hz. Polarization Study. The temperature variation of saturation polarization (Ps) and coercive field (Ec) of the samples were recorded at 50 Hz with an ac field of 20 kV䡠cm⫺1 using a dual-trace oscilloscope attached to a modified Sawyer-Tower circuit.

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TABLE 1 Comparison of Some Physical Parameters of PEZT z

a ⫽ b (Å) c (Å) d (gm䡠cc⫺1) Avg. grain size (SEM) (␮m) Tc (°C) Activation energy Ea (eV)

0.07

0.08

0.10

4.016 4.164 8.034 4.50 360 1.64

4.075 4.134 7.972 4.16 354 1.43

4.111 4.152 7.646 — — —

RESULTS AND DISCUSSION The room temperature XRD patterns (Fig. 1) of the calcined powders and sintered pellets of the PEZT samples show the formation of single-phase compounds. All the reflection peaks were indexed and lattice parameters of PEZT were determined using the least-squares refinement method with the help of the computer program POWD [11]. From this, we conclude that the basic crystal structure of PZT is not affected by the substitution of Er3⫹. However, a minor shift can be observed in the peak positions, indicating a small change in the lattice parameters. The lattice parameters of the compounds are given in Table 1. At room

FIG. 2 SEM micrographs of PEZT.

FIG. 3 Variation of dielectric constant (ε) and loss (tan␦) of PEZT with frequency, at room temperature.

FIG. 4 Variation of dielectric constant (ε) of PEZT with temperature, at 104 Hz.

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FIG. 5 Variation of ln␴ vs. 103/T of PEZT at 104 Hz.

temperature, all the Er3⫹-modified PZT compounds belong to the tetragonal crystal system. Good agreement has been found between the calculated and observed interplanar spacing d of all diffraction lines in the tetragonal phase of PEZT. Figure 2 shows SEM micrographs of the sintered PEZT compounds. The grains are almost spherical. The average sizes of the grains are given in Table 1. Figure 3 shows the variation of dielectric constant (ε) and tangent loss (tan␦) as a function of frequency (400 –104 Hz) at room temperature. The values of tan␦ have been found to decrease with an increase in frequency. Figure 4 shows the variation of ε with temperature at 104 Hz for all three different compositions. It has been observed that the compounds for z ⫽ 0.07 and 0.08 undergo a diffuse ferroelectric–paraelectric phase transition at 360 and 354°C, respectively, whereas there is no such anomaly for z ⫽ 0.10 in the temperature range of 30 to 400°C. For grain sizes in the range of 1–5 ␮m, it has been established that, with decreasing grain size εmax, the dielectric peak broadens and the ferroelectric transition shifts to a higher or lower temperature [12]. From SEM measurements, we have seen that the grain size decreases as the concentration of the doped ions increases in all the PEZT. In order to determine and analyze the degree of diffusivity in the materials, the following expression was used [13]:

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FIG. 6 Room temperature P-E hysteresis loop of PEZT at 50 Hz.

冉

冊

1 1 ⫺ ⬀ (T ⫺ T c) ␥ ε ε max

where ε is the dielectric constant at temperature T, and εmax is its maximum value at Tc. The values of ␥ (diffusivity), calculated from the graphs, were found to be between 1 (for ferroelectrics) and 2 (completely disordered materials), confirming the diffuse phase transition in the materials. Electrical conductivity ␴ is related to the dielectric constant and loss by the expressions ␴ ⫽ ␼ ε ε0 tan␦ and ␴ ⫽ ␴0 exp(⫺Ea/KBT), where ε0 is the dielectric constant in free space, ␼ is the angular frequency, KB is the Boltzmann constant, and Ea is the activation energy.

FIG. 8 Temperature dependence of coercive field (Ec) of PEZT at 50 Hz.

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FIG. 7 Temperature dependence of spontaneous polarization (Ps) of PEZT at 50 Hz.

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TABLE 2 Comparison of Dielectric and Hysteresis Parameters of (Pb0.93R0.07)(Zr0.60Ti0.40)0.9825O3 (R ⫽ Rare-Earth Ions) R

εmax at 10 kHz

Tan␦Tc at 10 kHz

Tc (°C) at 10 kHz

Ea (eV) at 10 kHz

Ps (␮C/cm2) at 50 Hz

Ec (kV/cm) at 50 Hz

La Nd Sm Dy Gd Eu Er Pr Yb

20384 5878 7613 13838 14143 8530 14811 1620 6562

0.024 0.030 0.037 0.039 0.053 0.210 0.250 0.009 0.320

191 196 315 355 343 364 359 77 378

0.66 0.32 0.52 0.20 0.62 0.64 0.10 0.30 0.29

18.7 8.64 8.27 10.46 9.33 5.15 11.26 2.73 6.87

12.58 6.00 4.54 7.11 8.67 5.29 6.53 2.12 3.79

Activation energy was calculated from the plot of ln␴ vs. 103/T (Fig. 5) in the paraelectric region. The values of activation energy of the materials are given in Table 1. The typical hysteresis loop at room temperature of PEZT is shown in Figure 6. A high electric field (⬃ 20 –25 kV䡠cm⫺1) was required to obtain saturation polarization. Saturation polarization (Ps) and coercive field (Ec) were determined from the hysteresis loop. It is observed that, at room temperature, the value of polarization decreases with increases in doped ion concentration. The temperature dependence of Ps and Ec of the materials are shown in Figures 7 and 8, respectively. The slow and continuous changes in both parameters indicate a second-order phase transition in these compounds [14]. In Tables 2 and 3, the dielectric and electrical properties of PRZT (R ⫽ rare earth) are compared. From these tables, it is clear that the PLZT compounds have higher values than other rare-earth doped compounds. Out of several rare-earth doped PZTs, PEZT has the highest dielectric and spontaneous polarization values, next to PLZT. There is no systematic trend in the other physical

TABLE 3 Comparison of Dielectric and Hysteresis Parameters of Pb0.92R0.08(Zr0.60Ti0.40)0.98O3 (R ⫽ Rare-Earth Ions) R

εmax at 10 kHz

Tan␦Tc at 10 kHz

Tc (°C) at 10 kHz

Ea (eV) at 10 kHz

Ps (␮C/cm2) at 50 Hz

Ec (kV/cm) at 50 Hz

La Nd Sm Dy Gd Eu Er Pr Yb

18924 6577 7613 12673 12864 7588 11123 2367 —

0.016 0.007 0.085 0.140 0.060 0.070 0.036 0.086 —

156 182 315 368 337 349 356 56 —

0.89 0.23 0.25 0.56 0.83 0.34 1.43 0.19 —

21.90 8.84 7.58 6.75 8.31 5.41 10.78 — —

6.65 6.00 5.51 3.97 3.11 5.28 7.22 — —

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properties of the compounds with regard to the concentration or atomic number of the dopants. CONCLUSION PEZT ceramics of the present composition were synthesized from acetate–alkoxide sols and found to be very fine and homogeneous. There is very good agreement between the observed and calculated d-values from the XRD patterns. Variation in grain size with doping concentration of Er3⫹ was observed from the SEM images. The transition temperature, dielectric constant, Ps, and Ec decrease with increasing Er3⫹ concentration. Thus, PEZT leads to a stable ferroelectric state over a wide range. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

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