Metastable face-centered cubic lead zirconate titanate (PZT) and lead lanthanum zirconate titanate (PLZT) nanocrystals s
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Nanocrystalline lead zirconate titanate (PZT) and lead lanthanum zirconate titanate (PLZT) have been synthesized in powder form by a single-step auto-ignition of metal–polymer gel precursor. The nanocrystalline powders were characterized using analytical transmission electron microscopy (TEM) equipped with an energy-dispersive x-ray spectrometer (EDXS) for composition analysis. For both PZT and PLZT, nanoparticles of sizes as low as 1–5 nm along with larger nanoparticles of sizes up to 30 nm are observed in the TEM. The selected-area diffraction (SAD) patterns from the nanoparticles revealed a face-centered cubic (fcc) crystal structure for both PZT and PLZT with a lattice parameter of ∼0.51 nm. The formation of PZT and PLZT nanoparticles of sizes below 5 nm with metastable fcc crystal structure has been observed for the first time. It is concluded that, as the crystal size decreases, the system assumes crystal structures of higher symmetry initially through small changes in lattice parameters and, in extreme cases, through chemical disorder for ultrafine nanoparticles.
I. INTRODUCTION
Lead zirconate titanate (PZT) is a perovskitestructured ferroelectric ceramic that contributes in a wide range of Zr/Ti stoichiometry to a group of electroceramic materials with exceptional dielectric, ferroelectric, and piezoelectric properties.1–3 It is known that Ti-rich tetragonal (P/4mmm) compounds exhibit a higher pyroelectric coefficient and that Zr-rich, rhombohedral compounds have lower coercivity.1,2 An enormous amount of literature has been published on tetragonal, rhombohedral phases and mixed phases in the morphotropic phase boundary region.4–7 However, to the best of the present author’s knowledge, reports on other metastable phases in the Pb-based perovskite system in nanocrystalline or massive state are absent. Ayyub et al.8 have studied the effects of particle size on the lattice parameters of 45- to 300-nm-sized PbTiO3 and PbZrO3. The authors found that the tetragonal and pseudotetragonal distortions in PbTiO3 and PbZrO3, respectively, decrease with decreasing particle size i.e., size-induced structural modification towards the cubic phase. A complete elimination of tetragonal distortion resulting in the cubic paraelectric a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0099 J. Mater. Res., Vol. 23, No. 3, Mar 2008
phase was observed at the lower end of the particle size range in case of PbZrO3. In some other studies it was revealed that, in Al2O38 and in Fe2O3,8,9 the rhombohedral (␣) phase was found to be unstable when the crystal size became sufficiently small (艋30 nm) and the cubic (␥) phase was preferentially nucleated in the respective oxides. Studies have also shown that nanoparticles may exhibit phases unknown in the bulk state. For instance, nanoparticles of Be and Bi have been observed to have simple cubic lattice, the same metals in the bulk state being hexagonal-close-packed (hcp).10 In the present communication, we report for the first time the formation o
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