Characterization of Dense Lead Lanthanum Titanate Ceramics Prepared from Powders Synthesized by the Oxidant Peroxo Method

Materials Chemistry and Physics 124 (2010) 1051–1056

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Materials Chemistry and Physics

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Characterization of dense lead lanthanum titanate ceramics prepared from powders synthesized by the oxidant peroxo method

Alexandre H. Pinto a, Flavio L. Souza b, Adenilson J. Chiquito c, Elson Longo d, Edson R. Leite a, Emerson R. Camargo a,∗ a LIEC-Laboratório Interdisciplinar de Eletroquímica e Cerâmica, Departamento de Química, UFSCar-Universidade Federal de São Carlos, Rod.Washington Luis km 235, CP 676 São Carlos, SP 13565-905, Brazil b Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia 166, Bangu, Santo André, SP 09210-170, Brazil c Departamento de Física, UFSCar-Federal University of São Carlos, Rod.Washington Luis km 235, CP 676 São Carlos, SP 13565-905, Brazil d Instituto de Química de Araraquara, UNESP-Universidade Estadual Paulista, Rua Francisco Degni, CP 355 Araraquara, SP 14801-907, Brazil article info abstract

Article history: Nanosized powders of lead lanthanum titanate (Pb1−xLaxTiO3) were synthesized by means of the oxidant- Received 10 December 2009 peroxo method (OPM). Lanthanum was added from 5 to 30% in mol through the dissolution of lanthanum Received in revised form 14 July 2010 oxide in nitric acid, followed by the addition of lead nitrate to prepare a solution of lead and lanthanum Accepted 8 August 2010 nitrates, which was dripped into an aqueous solution of titanium peroxo complexes, forming a reactive amorphous precipitate that could be crystallized by heat treatment. Crystallized powders were char- Keywords: acterized by FT-Raman spectroscopy and X-ray powder diffraction, showing that tetragonal perovskite Oxides structure is obtained for samples up to 25% of lanthanum and cubic perovskite for samples with 30% Chemical synthesis ◦ Crystal structures of lanthanum. Powders containing 25 and 30% in mol of lanthanum were calcined at 700 C for 2 h, Dielectric properties and in order to determine the relative dielectric permittivity and the phase transition behaviour from ferroelectric-to-paraelectric, ceramic pellets were prepared and sintered at 1100 or 1150 ◦C for 2 h and subjected to electrical characterization. It was possible to observe that sample containing 25% in mol of La presented a normal behaviour for the phase transition, whereas the sample containing 30% in mol of La presented a diffuse phase transition and relaxor behaviour. © 2010 Elsevier B.V. All rights reserved.

1. Introduction called the “oxidant peroxo method”, sometimes referred by the abbreviation OPM [11–17]. This wet-chemical technique of syn- Isomorphic substitution of lead by lanthanum induces some thesis is characterized by the fundamental oxy-reduction reaction interesting changes in the physical properties of lead titanate. between lead (II) ions and some water soluble peroxo complexes One of the most studied effects is the dependence between struc- that results in the formation of an amorphous and highly reac- tural characteristics of the lanthanum-modified lead titanate and tive precipitate. This precipitate is free of common contaminants, the lanthanum concentration [1]. In fact, Pb1−xLaxTiO3 solid solu- such as halides or graphitic carbon, usually found in materials tions (thereafter referred to as PLT) have been used as model to synthesized by others chemical routes. Since the OPM precipitate understand various phenomena related to the cubic-to-tetragonal is formed at molecular-level, its composition can be efficiently transition [2]. However, it is well known that nanosized particles controlled, and because of its reactivity, it was observed that the generally display properties that differ from those observed in bulk crystallization occurs at temperatures below than those reported material, and for this reason alternative synthetic routes to the tra- for PbTiO3 and PZT systems synthesized by solid-state reaction. ditional solid-state reaction have been developed to obtain a wider Moreover, the OPM technique uses water as solvent and a rela- number of compounds at nanometric scale [3–7]. Particularly, PLT tively simple experimental apparatus, without the necessity of dry and PbTiO3 have been synthesized by several wet-chemical routes atmosphere or toxic compounds. [8–10], and recently one of us developed a new synthetic route Previously, OPM route was used with success to prepare several lead-based compositions, for instance lead titanate, lead hafnate, lead zirconate and specially several compositions of lead zirconate ∗ titanate [11–16]. In a recent paper, we prepared and characterized Corresponding author. Tel.: +55 16 3351 8090; fax: +55 16 3351 8350. dense ceramics bodies of PZT, with a Zr:Ti molar ratio of 50 to 50, E-mail addresses: fl[email protected] (F.L. Souza), [email protected] (A.J. Chiquito), [email protected] (E. Longo), [email protected] (E.R. Leite), from powders prepared by the OPM route, demonstrating that OPM [email protected] (E.R. Camargo). powders can be efficiently used to prepare high quality ferroelectric

Fig. 2. Raman spectra of PLT powders calcined at 900 ◦C for 1 h with different com- Fig. 1. XRD patterns collected at room temperature of PLT powders calcined at positions collected at room temperature. 900 ◦C for 1 h with different compositions. The L0 pattern refers to pure lead titanate tetragonal phase and the L30 pattern to cubic phase. ceramic samples [17]. Although lead lanthanum titanate materials have been extensively studied because of its interesting structural and electrical properties, only recently we developed the OPM technique to be used with rare earth elements. In this paper, we are reporting the synthesis of lead lanthanum titanate (PLT) powders by the oxidant-peroxo method (OPM) with up to 30% in mol of lanthanum, the sinterization of dense ceramics bodies at different temperatures and their electrical characterization.

2. Experimental

2.1. Synthesis

All of the chemical reagents were used as received without any puriﬁcation. Lead lanthanum titanate with nominal composition of Pb1−xLaxTiO3, with x up to 0.30, was prepared by the OPM route [11–17] through the addition of 1 g (0.02 mol) of titanium metal powder (99.7%, Aldrich, Germany) into a mixture of 90 mL of hydrogen peroxide (analytical grade, Synth, Brazil) and 60 mL of ammonia aqueous solution (analytical grade, Synth-Brazil). This solution was kept at rest in an ice-water bath Fig. 3. Transmission electron microscopy image of the PLT powder with 25% of for approximately 5 h, resulting in a yellow transparent aqueous solution of solu- ◦ − −1 lanthanum (L25) calcined at 900 C for 1 h. ble peroxytitanate [Ti(OH)3O2] ion with concentration of 0.14 mol L . Lanthanum oxide (99.5%, Merck, Germany) previously heat treated at 400 ◦C for 1 h to eliminate carbonate was weighted and added into 30 mL of diluted nitric acid solution at pH = 2 to as “precursor”, were calcined at 700 ◦C for 2 h under a heating rate of 10 ◦Cmin−1 (analytical grade, Synth, Brazil). The volume was corrected to 50 mL with distilled using closed alumina boats. water, and lead nitrate (99.5%, Riedel-de Haën, Germany) was added to complete the stoichiometric ratio of each sample composition. Finally, the lanthanum and 2.2. Sinterization lead nitrates solution was slowly dripped into the peroxytitanate solution under stirring and cooled with ice-water bath, resulting in a vigorous evolution of gas. An Pellets in form of disks with 6.8 mm of diameter and 1.9 mm of thickness were orange precipitate was immediately formed and the solution lost its yellow colour. prepared with the calcined powders and pressed using an isostatic pressing with This precipitate was ﬁltered and washed with acetone to eliminate the adsorbed load of 32 tons for 1 min. After pressing, the pallets were sintered at two differ- water and nitrate ions. This washed amorphous precipitate was dried at 70 ◦C for ent temperatures (1100 and 1150 ◦C) in a tubular oven for 2 h with heating rate of 5 h and ground using a mortar. Amounts of 0.30 g of this powder, thereafter referred 10 ◦Cmin−1. In order to avoid lead volatilisation, samples were sintered using closed

Table 1 Lattice parameters of PLT samples calcined at 900 ◦C for 1 h with different compositions.

Sample Lattice parameters (Å) Angles Cell volume (Å3) Symmetry

abcc/a ˛ = ˇ =

L5 900 3.9018 3.9018 4.0676 1.0425 90.00◦ 61.924 Tetragonal L10 900 3.9099 3.9099 4.0395 1.0331 90.00◦ 61.752 Tetragonal L20 900 3.9367 3.9367 3.9599 1.0059 90.00◦ 61.369 Tetragonal L25 900 3.9261 3.9261 3.9284 1.0006 90.00◦ 60.554 Tetragonal L30 900 3.9256 3.9256 3.9256 1.0000 90.00◦ 60.496 Cubic A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056 1053

Fig. 4. FESEM images (A) of the fractured surface of the SL30B ceramic sample sintered at 1150 ◦C for 2 h, and (B) of the L30 powder calcined at 700 ◦C for 2 h. (C) FESEM image of the fractures surface of the SL30B sample showing a larger area.

zirconia boats and immersed in PLT powders previously prepared by solid-state it means that less energy is necessary to assembly nanosized crys- reaction. talline particles and second, ﬁnal materials are more homogeneous and uniform than those obtained by traditional milling and ﬁring 2.3. Characterization process. In the OPM route, the oxy-reduction reaction between lead

The precursor and all of the calcined powders were characterized by Raman ions and the peroxo titanium complexes results in a precise and spectroscopy with Fourier transform (FT-Raman) and powder X-ray diffraction stoichiometric precipitate with the desired Pb:La:Ti mole ratio. This (XRD). Raman spectra were collected at room temperature between 100 and precipitate can be described, at least, as a complex mixture of amor- 950 cm−1 using a FT-Raman Bruker RFS 100/S spectrometer, using the 1064 nm line phous and hydrated oxides PbO2, TiO2 and La2O3 that will result in of an yttrium aluminium garnet (YAG) laser. The X-ray powder patterns were also ◦ ◦ crystallized PLT after the firing step, as described by Eq. (1). collected at room temperature in the 2 range from 10 to 80 with step scan of 0.02◦ using a Rigaku D/MAX 2500 diffractometer with a rotary anode (Cu K␣ radia- Pb(OH) 2− + La(H O) 3+ + [Ti(OH) O ]− tion) operating at 150 kV and 40 mA. Crystallographic coherence lengths (crystallite 4 2 6 3 2 size) were calculated according to Scherer’s equation [18], adjusting each peak with →{PbO ·TiO ·La O }nH O → Pb −xLaxTiO (1) a Lorentzian function to determine the full width at half maximum (FWHM). The 2 2 2 3 2 1 3 (1 0 1) reflexion of monocrystalline silicon was used as FWHM reference, and the crystallite size was estimate from the average of three peaks. The morphology of powder and the surface of fractured sintered PLT ceramic samples were charac- 3.1. Formation of the PLT terized by high-resolution field emission scanning electron microscopy (FESEM, Zeiss SUPRA 35). For electrical measurements, gold contacts were deposited on 3+ 2+ 4+ In La modified PbTiO3,La replaces Pb rather than Ti and the samples surfaces and the dielectric constant (ε ) measured at different frequencies (10 and 100 kHz) and different temperature ranges during the heating to keep the electrical neutrality, site vacancies should be created in and cooling modes (Pb0.75La0.25TiO3 samples were measured from 300 to 700 K and the crystalline structure [2]. Another possibility is to reduce some 4+ 3+ Pb0.70La0.30TiO3 from 50 to 300 K) using a Keithley 3330 (LCZ) meter and a Agilent Ti to Ti to compensate the excess of positive charge introduced 4263B (LCR) Meter coupled with Helio Cryo CCS model. by the lanthanum ions. Therefore, the properties of PLT can be influenced by the presence of vacancies and, in some extension, 3. Results and discussion also by the Ti3+ concentration. However, isomorphic substitution of lead by lanthanum atoms in the A site of the perovskite induces Usually, samples of PLT are prepared by the conventional solid- some changes in the structural properties of PbTiO3 host mate- state reaction. In this traditional method, PbO, La2O3 and TiO2 are rial. For instance, when the lanthanum content is higher than weighed according to the planned stoichiometry and mixed by ball 25% in mol, a diffuse character of the ferroelectric-to-paraelectric milling. On the other hand, the OPM approach to the synthesis of phase transition (DPT) is observed, which can be described by the PLT is based on a molecular level reaction between soluble the tetragonal-to-cubic transition. From this point of view, some compounds, following a “bottom-up” strategy of synthesis. First, researchers reported that the modification in PbTiO3 induced by 1054 A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056

Fig. 5. (A) Relative dielectric constant in function of the temperature for SL25B and SL30B samples. (B) Temperature dependence of the relative dielectric constant of the SL25B sample collected at different frequencies. (C) Temperature behaviour of the inverse of the dielectric constant at 100 kHz of the SL25B sample. (D) Fitting of the (1/ε − 1/εm) vs (T − Tm) at 100 kHz of the SL25 sample. the introduction of lanthanum ions results in structural changes early with the increases of lanthanum amount until the unit for directly related to the nature of the phase transition. In the OPM L30 sample. Neves et al. [1] found similar results for PLT sam- procedure to synthesize PLT, La and Pb ions are added together at ples prepared by traditional solid state reaction and after Rietveld molecular level, which should drive to a homogeneous distribu- refinements, they pointed out that the defects occur mainly in A tion of La in substitution of Pb in the A-site. However, considering site shared by lanthanum and lead ions, discarding defects in B the oxidizing environment that results by the presence of peroxo site and oxygen vacancies, which is in agreement with our discus- group, the electrical neutrality should be obtained mainly because sion based in chemical considerations regarding the OPM route. of vacancies in the cation site than because of the presence of Ti3+ The XRD patterns of Fig. 1 show that samples L5 and L10 are indu- or oxygen vacancies. bitably tetragonal and sample L30 is cubic. Pattern L20 can also be Fig. 1 shows the XRD patterns of PLT samples with different assigned as tetragonal when (0 0 1) and (2 0 0) reflection peaks are amounts of lanthanum, ranging from 5% of lanthanum up to 30% observed in detail. It is important to note that the diffraction peaks of lead substituted, all of them calcined at 900 ◦Cfor1h.Itis (0 0 2) and (2 0 0) are clearly separated in the L20 pattern, indicat- possible to observe the structural modification from tetragonal to ing the tetragonal structure of the sample with 20% of lanthanum. cubic phase as the amount of lanthanum is increased, with the However, if XRD is the most popular and widely accepted tech- sample containing 5% of lanthanum (L5) showing tetragonal and nique to characterize crystalline structures, sometimes its relative the sample containing 30% of lanthanum (L30) the cubic structure low sensibility makes impossible to assert the correct structure. In [19–23]. Table 1 shows the lattice parameters and cell volumes this way, Raman scattering spectroscopy has been employed with calculated using the Unit Cell software [24], where is possible to success to check the presence of secondary phases at levels of 1% observe that the tetragonality parameter (c/a ratio) decreases lin-

Table 3

Table 2 Experimental parameters extracted by Fig. 6,(Tc) transition temperature, inverse of Experimental parameters extracted by Fig. 5,(Tc) transition temperature, Curie- dielectric constant (1/εm), diffuseness parameter () and Curie-Weiss constant (C*). Weiss temperature (T ) inverse of dielectric constant (1/ε ) and Curie-Weiss cw m ε constant (C). Frequency (Hz) Tc (K) 1/ m C* PLT 30 sintered at 1100 ◦C for 2 h Frequency (Hz) T (K) T 1/ε C c cw m 10 K 256 1.4E−4 1.23 1.54E−6 120 378 442 1.12E−4 3.2E−6 100 K 263 1.3E−4 1.26 1.48E−6 1 K 378 442 1.13E−4 3.4E−6 PLT 30 sintered at 1150 ◦C for 2 h 10 K 378 442 1.13E−4 3.5E−6 10 K 258 1.16E−4 1.51 1.41E−6 100 K 378 442 1.15E−4 3.5E−6 100 K 265 1.22E−4 1.55 1.39E−6 A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056 1055

Fig. 6. (A) Temperature dependence of the relative dielectric constant of SL30A (1100 ◦C) and SL30B (1150 ◦C) samples measured at different frequencies. (B) Critical parameter

() fitted by ln(1/ε − 1/εm) as function of ln(T − Tm) at 100 kHz for the SL30A, and (C) the critical parameter () for SL30B sample. in mass or even less, or to assign correctly some crystalline phases phase, pressed and sintered at different temperatures for 2 h that are not easily distinguishable by XRD [25]. (1100 ◦C for L30, thereafter referred to as SL30A, and 1150 ◦C for To check the correct structural characteristic of the L25 sample, L30 and L25, referred to as SL30B and SL25B, respectively). It is all of powders were subjected to FT-Raman analysis and their spec- well known that pure lead titanate ceramics spontaneously crack tra in the range from 100 to 800 cm−1 are shown in Fig. 2, using the on cooling after sintering, which is attributed to the large lattice spectrum of pure lead titanate (L0) as reference for tetragonal and anisotropy of PbTiO3. On the other hand, the substitution of Pb by the L30 for cubic structure. The first impression of Fig. 2 is the grad- La in PLT makes possible to obtain dense ceramics without cracking ual changing of the phonon modes between these two extremes. with the sinterability enhanced due to the formation of vacancies The general shape of L0 spectrum is easily identified in the L5 and in the crystalline lattice, which facilitates the interatomic diffusion L10 spectra, confirming the tetragonal symmetry of these two sam- during the sintering process [27–29]. The apparent densities of the ples. The broadening of the Raman peaks observed in these spectra sintered ceramics were measured as 94–99% of theoretical density, indicates that the incorporation of La in the lattice results in struc- calculated by ratio between the density of the single crystal and tural disorder together to the change in the crystalline structure the geometrical density of the sintered bodies. These values are [26]. The tetragonal-to-cubic transition could be evaluated through relatively high when compared to those reported previously for the merging of the E(3TO) and A1(3TO) modes and the vanishing similar compositions. Fig. 4 shows the FESEM images of the frac- of the A1(2TO) mode. tured surface of SL30A sintered sample (A) and (C), and the image Fig. 3 shows a representative transmission electronic (TEM) of the precursor powder (B). A high degree of densification with image of the L25 sample calcined at 900 ◦C for 1 h. One of the most fairly uniform grains sizes could be observed, despite the fact that outstanding features of the OPM powders is its reactivity and it the precursor powder used to prepare the pressed pellets seems is interesting to note the presence of necks between particles of agglomerated. approximately 40 nm with rounded sides, confirming the partial sintering of these nanometric powders. 3.3. Electric and dielectric properties

3.2. Sintered bodies Fig. 5A shows the dependence of the relative dielectric permittivity with temperature of the SL25B and of the SL30A samples Considering that L25 and L30 samples are in the limits of the measured at 10 kHz, where is possible to observe that the Curie tetragonal-to-cubic transition, powders with these two composi- point (Tc) is shifted to lower temperatures when the amount of tions were calcined at 700 ◦C for 2 h to crystallize the perovskite lanthanum is increased from 25% (SL25B) to 30% (SL30B), which 1056 A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056 is related to the decrease in tetragonality with the amount of indicating a diffuse transition (Table 3), with diffuseness parameter lanthanum [27–31]. The dielectric constant of the SL25B sample 1< ≤ 2, in good agreement with the literature [34]. measured in function of the temperature at four different frequencies is shown in Fig. 5B, and as expected for a ferroelectric material 4. Conclusions that follows the Curie-Weiss law in the vicinity of the transition temperature, a sharp transition between tetragonal (ferroelectric) It was demonstrated that it is possible to prepare nanosized and cubic (paraelectric) phases was observed [17–19,32]. When the powders of lead lanthanum titanate (Pb1−xLaxTiO3) by means of Currie-Weiss temperature (To) is smaller than the phase transi- the oxidant peroxo method (OPM). Tetragonal phase was observed tion temperature (Tc), a first-order transition is observed, whilst by X-ray diffraction in the samples with lanthanum up to 25% in mol a second-order transition is observed when To = Tc [33]. In order and cubic for samples with 30% of lanthanum, which was confirmed to determine the ferroelectric-to-paraelectric phase transition, the by Raman spectroscopy. The dielectric measurements indicated dependence of the temperature with the inverse of the dielec- a normal behaviour for the phase transition from ferroelectric to tric constant (1/ε − 1/εm vs T − Tm) was presented in Fig. 5C. The paraelectric for the sample with 25% in mol of lanthanum, whereas, dielectric stiffness was fitted at a narrow temperature range near a diffuse phase transition and relaxor behaviour were observed to to the ferroelectric-to-paraelectric phase transition temperature the sample containing 30% in mol of lanthanum. (Tc) and the Currie-Weiss temperature (To = 368 K) calculated was lower than the transition temperature (Tc = 378 K), indicating a Acknowledgements first-order phase transition. The Curie-Weiss constant (C) value was also estimated fitting the experimental data (Fig. 5D), and the The financial support from FAPESP (Projects 07/50904-2 and values obtained for SL25B at different frequencies presented simi- 07/58891-7), FINEP, CNPq and CAPES is gratefully acknowledged. − lar values around 3.5 × 10 6. There is an empirical relationship for The INCTMN/CMDMC also provided support for this work. the Curie-Weiss law (ε = C*/(T/To) ) that describes the diffuseness parameter () of the phase transition. The value calculated for the References SL25A sample measured at different frequencies and temperatures was of 0.9987, with close to the unit, indicating a sharp tran- [1] P.P. Neves, A.C. Doriguetto, V.R. Mastelaro, L.P. Lopes, Y.P. Mascarenhas, A. sition [34]. It is possible to note a non-diffuse character and the Michalowicz, J.A. Eiras, J. Phys. Chem. B 108 (2004) 14840. [2] T.Y. Kim, H.M. Jang, Appl. Phys. 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