Materials Chemistry and Physics 124 (2010) 1051–1056 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 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 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.08.030 1052 A.H. Pinto et al. / Materials Chemistry and Physics 124 (2010) 1051–1056 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 tech- nique 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 tita- nium 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.