2)O3 ferroelectric ceramics

Journal of Physics and Chemistry of Solids 60 (1999) 767–772

Diffuse phase transition in Ca-modified Pb(Li1/4La1/4Mo1/2)O3 ferroelectric ceramics S. Bera, R.N.P. Choudhary* Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India Received 30 January 1998; accepted 1 December 1998

Abstract Polycrystalline samples of (Pb1⫺xCax)(Li1/4La1/4Mo1/2)O3, x ˆ 0.00, 0.03, 0.07, 0.10, were prepared by solid-state reaction technique. The formation and quality of the compounds were checked by preliminary X-ray studies. Detailed studies of dielectric properties (constant and loss) and hysteresis loop as a function of temperature suggest that these compounds undergo ferroelectric diffuse phase transition. The substitution of Ca 2⫹ ion at Pb 2⫹ sites causes a major effect on their transition temperature and electrical properties. Measurements of dc resistivity as a function of temperature and biasing dc electric field suggest that the compounds have negative temperature coefficients of resistance (NTCR) above room temperature. 䉷 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Recently there is much interest in the complex and mixed systems in order to enhance the physical properties of the parent compounds for the possible industrial applications. Ever since the discovery of ferroelectricity in BaTiO3 [1], a large number of ferroelectrics with perovskite structure of general formula ABO3 (A is a mono or a divalent cation, B is a tri, tetra, penta or a hexavalent cation) have been discovered and studied for solid-state devices such as transducer, computer memory and display, pyroelectric detector etc. [2–5]. Smolenskii et al. (1959) proposed that a wide range of oxygenous complex perovskite compounds of a general formula (A1,…,Ak)(B1,…,B1)O3 can be prepared by keeping in mind the condition of electrical charge neutrality, the properties of a given structure, the affinity of ions to a given co-ordination number and Goldsmidth tolerance factor (t) [6]. For more than two decades lead-based complex perovskite e.g., PbTiO3, Pb(ZrTi)O3, and (PbLa)(ZrTi)O3 etc. plays a very important role in ferro-, piezo-, and pyroelectric devices [7–9]. Extensive literature survey on Pb-based complex perovskite shows that, except a * Corresponding author. Tel.:⫹91-3222-2221-2224; fax: ⫹913222-2303. E-mail address: [email protected] (R.N.P. Choudhary)

few studies, not much work has been reported on complex PbWO3 and PbMoO3, modified with alkali and rare-earth ions at Mo- and W-sites. Therefore, we carried out extensive and systematic studies on structural and ferroelectric properties of lead–alkali–rare-earth tungstates and molybdates [10–14], which provided many interesting properties (i.e., distorted perovskite structure and diffuse phase transition). This attracted us to make more complex modification in the compound by substituting group. IIA elements such as Ca, Sr, and Ba, to examine their effect on the aforementioned properties. In this article we report phase transitions in (Pb1⫺xCax)(Li1/4La1/4Mo1/2)O3 (hereafter PCLLM). 2. Experimental The polycrystalline samples of (Pb1⫺xCax)(Li1/4La1/4Mo1/ (x ˆ 0.00, 0.03, 0.07, 0.10) were prepared by conventional solid-state reaction technique comparatively at low temperature ( ⬍ 1100 K) using ingredient oxides and carbonates: PbO (99.99%, Aldrich Chemical Co., USA), CaCO3 (AR grade, Loba Chemie Industrial Co., Bombay, India), Li2CO3 (99%, s.d. Fine Chemical Pvt. Ltd, India), La2O3 (99.99%, Indian rare-earth Ltd) and MoO3 (99.9%, M/S John Baker Inc., USA) in a desired stoichiometry. The precursor powders were thoroughly mixed in agate mortar for 4 h in a wet medium (methanol) and calcined at 900 K

2)O3,

0022-3697/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00341-2

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S. Bera, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 60 (1999) 767–772

Fig. 1. Comparison of XRD profiles of PCLLM.

for 20 h. The mixing and grinding were repeated and finally the reactive powder was recalcined at 930 K for 10 h in an air atmosphere. The fine powders of PCLLM were then used to make cylindrical pellets of diameter 10 mm and thickness 1 mm under an isostatic pressure of about 6 × 10 7 N/m 2. Polyvinyl alcohol (PVA) was used as a binder to reduce the brittleness of the green pellets. The pellets were then sintered at 960 K for 5 h in a high purity alumina crucible. The organic binder was burnt out during sintering of the pellets and has no effect on the physical properties of the material under investigation. The formation of the desired compound was checked by preliminary structural studies with the help of X-ray diffractograms (XRD) of the calcined powder over a wide range of Bragg angle, 2u…15⬚ ⱕ 2u ⱕ 65⬚† at room temperature (RT) using X-ray powder diffractometer (Philips PW 1710 Holland) with CuKa radiation (l ˆ 0.15418 nm). The dielectric constant (e ) and loss tangent (tan d ) of PCLLM were measured both as a function of frequency (200 Hz– 10 kHz) and temperature (300–453 K) using GR 1620 AP capacitance measuring assembly with a laboratory-made three-terminal sample holder which compensates all sorts of stray capacitances. The measurement of polarization was carried out using a modified Sawyer–Tower [15] circuit with a dual trace oscilloscope at an ac field 4.5 kV/cm with frequency 50 Hz. The dc electrical resistivity (r ) was measured as a function of biasing electric field (1.5–9.0 kV/m) at RT (300 K) as well as a function of temperature (300– 550 K) at a constant electric field (8.5 kV/m). The

measurement was carried out with the help of a programmable electrometer (Keithley-617) and laboratory-made sample holder.

3. Results and discussion The prominent reflection peaks of the X-ray profiles were indexed and lattice parameters were determined and refined using least-squares method with the help of a standard computer program (POWD). Fig. 1 shows the XRD patterns of (Pb1⫺xCax)(Li1/4La1/4Mo1/2)O3 (x ˆ 0.00, 0.03, 0.07, 0.10). All the peaks remain at the same position up to 10 mol% of Ca 2⫹ doped at Pb-site. Good agreement between the observed and calculated interplaner spacing (d-values) suggests (Table 1) that doping of Ca 2⫹ ions at Pb-site does not effect the basic crystal structure of the mother compound, but a slight change in lattice parameter is because of the occupancy of Pb-site vacancies by Ca 2⫹ ions. The compounds are in orthorhombic phase at RT. Moreover, the sharp single diffraction peaks of the compounds suggest the formation of a single-phase desired compound. The average particle size (Lhkl) of all the four compounds was calculated from some strong and medium reflection peaks using Scherrer’s equation: Lhkl ˆ 0:89l=‰b1=2 cos uŠ and are given in Table 2. Fig. 2 shows the variation of dielectric constant (e ) and loss tangent (tan d ) in the frequency range (200 Hz– 10 kHz) at RT. The values of e and tan d decrease with increasing frequency as expected [16]. Fig. 3 shows the

S. Bera, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 60 (1999) 767–772

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Table 1 ˚ ) of some reflections of PCLLM at RT with relative intensity in parenthesis Comparison of dobs [o] and dcal [c] (in A hkl

x ˆ 0.00

x ˆ 0.03

x ˆ 0.07

x ˆ 0.10

200

[o] 3.4630 (100) [c] 3.4630 [o] 3.3438 (22) [c] 3.3644 [o] 3.2530 (50) [c] 3.2668 [o] 3.0652 (60) [c] 3.0650 [o] 3.0263 (26) [c] 3.0570 [o] 2.9321 (32) [c] 2.9238 [o] 2.8923 (36) [c] 2.9076 [o] 2.8171 (19) [c] 2.8028 [o] 2.5725 (18) [c] 2.5744 [o] 2.5503 (15) [c] 2.5477 [o] 2.2923 (16) [c] 2.2952 [o] 2.1609 (13) [c] 2.1616 [o] 2.0904 (16) [c] 2.0885 [o] 1.9917 (18) [c] 1.9920 [o] 1.9224 (16) [c] 1.9234 [o] 1.7000 (19) [c] 1.6984

3.4662 (100) 3.4662 3.3696 (15) 3.3969 3.2666 (52) 3.2733 3.0893 (68) 3.0723 3.0481 (31) 3.0618 2.9545 (32) 2.9278 2.9122 (40) 2.9114 2.8313 (15) 2.8088 2.5923 (18) 2.5775 2.5565 (11) 2.5512 2.3009 (13) 2.2991 2.1709 (10) 2.1644 2.0987 (16) 2.0919 1.9997 (18) 1.9969 1.9294 (16) 1.9268 1.7068 (23) 1.6999

3.4662 (100) 3.4662 3.3510 (16) 3.3772 3.2549 (78) 3.2757 3.0789 (68) 3.0744 3.0379 (30) 3.0643 2.9402 (30) 2.9284 2.8983 (35) 2.9150 2.8225 (19) 2.8104 2.5851 (22) 2.5788 2.5494 (10) 2.5538 2.2952 (14) 2.3000 2.1682 (11) 2.1671 2.0941 (16) 2.0931 1.9976 (19) 1.9681 1.9255 (17) 1.9278 1.7008 (21) 1.7000

3.4662 (100) 3.4662 3.3634 (17) 3.3732 3.2666 (81) 3.2768 3.0789 (69) 3.0764 3.0429 (30) 3.0656 2.9497 (28) 2.9291 2.9029 (33) 2.9029 2.8269 (20) 2.8120 2.5857 (26) 2.5794

122 114 015 123 203 030 115 222 132 215 041 225 018 226 410

2.3037 (15) 2.3009 2.1708 (11) 2.1679 2.0987 (17) 2.0939 1.9976 (21) 1.9995 1.9294 (19) 1.9287 1.7038 (22) 1.7002

Table 2 Comparison of orthorhombic cell parameters and average particle size (Lhkl) of PCLLM x

˚) a (A

˚) b (A

˚) c (A

˚) Lhkl (A

0.00 0.03 0.07 0.10

6.9260 6.9324 6.9324 6.9330

8.7228 8.7341 8.7450 8.7481

16.7228 16.4102 16.4205 16.4317

250 275 270 265

Table 3 Comparison of some dielectric and polarization (P) data of PCLLM at 10 kHz X

e at 300 K

tan d at 300 K

e max

Tc (K) from e max

tan d at Tc

g

P (mC/cm 2)

0.00 0.03 0.07 0.10

24 16 17 17

0.300 0.067 0.033 0.667

54 1700 2100 2400

372 393 386 383

0.81 0.25 1.23 0.833

1.37 1.75 1.78 1.80

0.11 0.07 0.06 0.06

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S. Bera, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 60 (1999) 767–772

Fig. 2. Variation of dielectric constant (e ) and loss (tan d ) as a function of frequency at RT of PCLLM.

Fig. 4. Variation of dielectric loss (tan d ) as a function of temperature at 10 kHz of PCLLM.

Fig. 3. Variation of dielectric constant (e ) as a function of temperature at 10 kHz of PCLLM.

S. Bera, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 60 (1999) 767–772

Fig. 5. Variation of dc resistivity of PCLLM as a function of biasing field at RT.

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variation of e as a function of temperature (300–453 K) at 10 kHz. The diffuse dielectric anomaly was observed for these compounds. The transition temperature was found to be shifted towards the RT as the concentration of Ca 2⫹ ion was increased. The diffusivity (g ) was calculated from the well known relation …1=e ⫺ 1=emax † / …T ⫺ Tc †g [17] and are given in Table 3. It was also observed that the diffuseness of phase transition increases with Ca-ion concentration. Fig. 4 shows the variation of tan d as a function of temperature (300–453 K) at 10 kHz. The value of tan d is quite high compared to its isomorphous compound [18]. The value of polarization of all these four compounds is small enough and its value decreases with increasing Ca 2⫹ ion concentration. The value of polarization at 300 K of all the compounds is given in Table 3 along with some important dielectric data. The variation of dc resistivity as a function of biasing field is shown in Fig. 5. The resistivity of (Pb1⫺xCax) (Li1/4La1/4Mo1/2)O3 (x ˆ 0.00, 0.03, 0.07, 0.10) decreases with increasing biasing field. This may be because of the following reasons: ceramic samples have some pores and cracks because the measured density of any ceramic sample is always less than its theoretical density. At RT the pores and cracks are filled with gas and moisture. Owing to the increase in electric field, ionization of gas and moisture

Fig. 6. Variation of dc resistivity of PCLLM as a function of temperature at constant biasing field 8.5 kV/m.

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S. Bera, R.N.P. Choudhary / Journal of Physics and Chemistry of Solids 60 (1999) 767–772

occurs which results in the generation of thermal stress and more cracks in the sample. This is why dc resistivity decreases with increasing electric field up to a certain extent [19]. From our experimental observation the dc resistivity increases with concentration of Ca 2⫹ ion at a fixed biasing field. This is because of the decrease in ionic conduction through the solid which is controlled by the concentration of Ca 2⫹ ion. The same conclusion can be drawn from our tan d measurement. The temperature dependence of dc resistivity of (Pb1⫺xCax)(Li1/4La1/4Mo1/2)O3 (x ˆ 0.00, 0.03, 0.07, 0.10) at a constant biasing electric field (8.5 kV/m) is shown in Fig. 6. The resistivity decreases with increasing temperature for the following reasons: owing to the addition of thermal energy, electrons could be set free from O 2⫺ ion, which results in unstable valance states and decrease in resistivity [20]. The nature of variation is the same for all the four compounds. This type of resistive behavior was found in many semiconductors and is usually called negative temperature coefficient (NTC) resistor. 4. Conclusion (Pb1⫺xCax)(Li1/4La1/4Mo1/2)O3 (x ˆ 0.00, 0.03, 0.07, 0.10) is a perovskite ferroelectric compound. Ca 2⫹ doped at Pbsite makes a large effect on their electrical properties and phase transition in them. These compounds show diffuse phase transition. Above 373 K the compounds behave as NTC thermistors.

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