Decomposition and Crystallization of a Sol–Gel-Derived PbTiO 3 Precursor Sverre M. Selbach, Guozhong Wang, Mari-Ann Einarsrud,* and Tor Grande* ,w Department of Materials Science and Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway A lead titanate (PbTiO 3 ) precursor, prepared by the Pechini method, has been heat treated to study the transformation from amorphous to crystalline PbTiO 3 . Nucleation of PbTiO 3 in the temperature interval 4001–4751C occurred before completion of the thermal decomposition of the polymeric precursor, resulting in nanocrystalline PbTiO 3 with an unexpectedly high tetragon- ality (c/a ratio). Annealing and crystallite growth at 6001C resulted in an increasing c/a ratio with annealing time in line with the expected finite size effect of PbTiO 3 . The unusually high c/a ratios observed in PbTiO 3 nucleated at 4001–4751C are discussed in relation to partial reduction and point defects in PbTiO 3 . I. Introduction O WING to the novel physical properties and potential appli- cations, nanostructures, and nanomaterials have been stud- ied intensively. 1 This interest results from the special properties of materials at the nanoscale, such as a large surface-to-volume ratio and increased surface activity, as compared with that of the bulk material. The properties of bulk materials usually depend on the size of the primary particles. Thus, the control of particle size and morphology plays a crucial role in the manufacturing process. Perovskite PbTiO 3 (PT) is a typical displacive ferroelectric material with a Curie temperature, T c , of 4901C. In its tetrag- onal form, the c/a 5 1.064. PT is widely applied in electronics such as multilayer capacitor, resonators, and ultrasonic trans- ducers, 2,3 due to a large pyroelectric coefficient and a relatively low permittivity. PT particles have been successfully synthesized using a variety of methods, such as coprecipitation, 4 the sol–gel process, 5–7 hydrothermal method, 8,9 solid-state mixing meth- od, 10,11 and molten salt method. 12 From all of these works, it has been assessed that the particle size and the properties of particles depend on the specific method of preparation and the experimental condition applied. It has been shown both exper- imentally 13–15 and theoretically 16,17 that the Curie temperature and latent heat decrease with decreasing particle size, which reflects a reduced tetragonality (c/a ratio). Here, we report on the crystallization of an amorphous PT precursor, prepared by the Pechini method, with the aim of finding optimized processing conditions for nanocrystalline PT. Particular emphasis is given to the correlation between the c/a ratio and particle size. An unexpected reduction of the c/a ratio with increasing crystallite size is reported. II. Experimental Procedure (1) Synthesis The PT polymeric precursor was prepared as described else- where. 5–7 The typical synthesis route was as follows: Titanium citrates were formed by dissolving 8.6 mL Ti(OC 3 H 7 ) 4 (98%, Acros Organic, Geel, Belgium) in an aqueous solution (3.4M, 601C, pH  5) of the complexing agent citric acid (CA) (BDH Laboratory Supplies, Dorset, U.K.). A stoichiometric amount (10.70 g) of Pb(CH 3 COO) 2  3H 2 O (99%, Merck Chemicals, Darmstadt, Germany) was dissolved in CO 2 -free de-ionized water and soon thereafter added to the clear Ti–citrate solution. To achieve polymerization, ethylene glycol (EG) (KEBO Lab, Prolab, EC, Stockhom, Sweden) (10 mL) was added to the Pb– Ti–citrate solution, which was kept under slow stirring until a clear solution was obtained. Ammonia solution (d 5 25%, Me- rck KGaA, Merck Chemicals, Darmstadt, Germany) was used to adjust the pH and to prevent precipitation of lead citrates. Upon continued heating at about 901C to promote the polyes- terification reaction, the solution became more viscous, without any visible phase separation. The polymeric precursor was ob- tained after 3 h. The molar ratio of lead/titanium in this work was 1:1, the CA/metal molar ratio was 3:1, and the CA/EG mass ratio was 3:2. The polymeric precursor was calcined at different tempera- tures (3501–6001C) for 0–62 h in an air atmosphere; the heating rate was 1201 or 4001C/h and the cooling rate was 2001C/h. (2) Characterization The amorphous PT precursor was characterized by thermo- gravimetric analysis (TGA) using a Netzsch STA 449 C (Jupiter, Selb, Germany) using 30 mL/min gas flow of air (H 2 O and CO 2 free). An alumina crucible with the crushed PT precursor was heated at 101 or 11C/min. to investigate the crystallization of the PT precursor. The tetragonal–cubic phase transition was inves- tigated by differential scanning calorimetry (DSC) using a Per- kin Elmer DSC 7 (Waltham, MA). The calcined powders were analyzed by powder X-ray diffraction (XRD) (Siemens D5005, Karlsruhe, Germany) us- ing CuKa1 radiation (l 5 1.54184 A ˚ ). Particle sizes were esti- mated from the diffraction patterns using the Scherrer equation corrected for instrumental broadening. 15 Si was used as an in- ternal standard in diffraction patterns collected at room tem- perature. Lattice parameters for PT were obtained by the Rietveld method using the TOPAS R (Bruker AXS v. 2.1) soft- ware. Qualitative high-temperature XRD (HTXRD) (Siemens D5005) equipped with a high-temperature camera (HTK 16, Anton Paar GMbh, Graz, Austria) was also utilized for the in situ study of the phase transformation of the polymeric PT G. Kowach—contributing editor This work was supported by the Research Council of Norway (NANOMAT-program, Grant no. 158518/431). Based in part on the thesis submitted by Selbach for the M.S. degree in Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim, Norway, 2005. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: tor.grande@material. ntnu.no Manuscript No. 22753. Received January 31, 2007; approved March 30, 2007. J ournal J. Am. Ceram. Soc., 90 [8] 2649–2652 (2007) DOI: 10.1111/j.1551-2916.2007.01789.x r 2007 The American Ceramic Society 2649