Size-driven domain reorientation in hydrothermally derived lead titanate nanoparticles
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High-resolution transmission electron microscopy studies of hydrothermally derived platelike lead titanate nanoparticles reveal that below a critical size of approximately 70 nm, the single ferroelectric domain polarization axis reorients from perpendicular to parallel to the plate. We suggest that during particle growth, ions in the hydrothermal processing medium compensate for the ferroelectric depolarization energy. When the processing medium is removed by washing and drying, single domain nanoparticles minimize their depolarization energy by c-axis flipping.
The effects of particle size on ferroelectricity in perovskites have been known since the 1950s.1,2 Due to the application of ferroelectric perovskites in nonvolatile memory,3,4 the size effects in ferroelectric materials have been extensively studied experimentally5–12 and theoretically.13–19 Research has focused on size effects on the Curie temperature, spontaneous polarization, dielectric response, and phase stability. As the grain size decreases, the Curie temperature and the spontaneous polarization typically decrease.5,7,8,10,17 In recent years, new synthesis technologies have made it possible to study size effects in isolated nanocrystalline particles. For example, as the particle size in ZrO2 decreases to the nanometer range, the high-temperature tetragonal phase becomes stable at room temperature.20 Similar phenomena have been reported for ferroelectric perovskites. Bulk BaTiO3 has a Curie temperature of about 130 °C. However, the paraelectric phase is stable at room temperature for particles less than a critical size ranging from 30 to 115 nm, depending on the study.7,9,11 The critical size of PbTiO3 ranges from 7 to 13.8 nm in experimental studies5,8,10,21 and 4.2 nm according to a theoretical model.17 In nonferroelectric systems, sizedriven phase transitions may be attributed to the large surface energy contribution to the total free energy. Determining the origin of phase stabilization in ferroelectric materials is complicated by spontaneous polarization, the presence of ferroelectric domains, and piezoelectricity. The mechanisms proposed for the size effects in ferroelectric materials, summarized by Frey and Payne,11 fall into four categories: depolarization effects, correlation a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0097 558
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 13 Mar 2015
length of ferroelectric interaction, influence of point defects, and elastic constraints. Also, the surface layers of ferroelectric particles may behave differently from the ferroelectric core.17 Recent work suggests that depolarization effects may dominate phase stabilization,22–25 as indicated by the observation that individual nanoscale BaTiO3 particles were observed in the cubic paraelectric phase, while clusters of the same particles were tetragonal and ferroelectric.23 Also, coating the same nanoparticles with Cu stabilized the tetragonal phase.24 Both clust
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