Synthesis and structural evolution of barium titanate nanoparticles

This thesis describes our work towards understanding the behavior of nanoparticles of the ferroelectric, barium titanate (BaTiO3). Bulk barium titanate undergoes a ferroelectric to paraelectric (tetragonal to cubic) phase transition at 130°C. Landau theory is commonly applied to ferroelectric phase transitions on the nanoscale, predicting that the phase transition temperature is size-dependent. In spite of the fact that the ferroelectric properties of barium titanate have been known since the 1940's, there are still questions that remain about the nature of ferroelectricity in this material. One such question is whether the phase transition is a displacive transition or an order-disorder transition (or both). A second question is whether there is in fact an intrinsic size effect in this material as described by Landau theory. The main mechanism for an intrinsic size effect, if one exists, is also an outstanding issue. In Chapter 1, we give a brief introduction to ferroelectrics and to Landau Theory.;In order to study the size dependent properties of BaTiO3, it is necessary to have a high-quality sample, as the phase transition is sensitive to defects and strain. In Chapter 2 we describe two methods for the synthesis of high quality barium titanate nanocrystals. The first method is a sol-gel type synthesis from a bimetallic alkoxide precursor in conjunction with a solvothermal technique, which produces crystalline particles with controllable size. The second method involves the organic-metallic reaction of the bimetallic alkoxide precursor with hydrogen peroxide at high temperature. This procedure forms monodisperse BaTiO3 particles that are soluble in non-polar solvents.;In Chapter 3 we report structural and phase transition studies that we have made on BaTiO3 nanoparticles using X-ray diffraction (XRD), atomic pair distribution function (PDF), and Raman spectroscopy. Through temperature-dependent XRD studies, we find that all sizes of particles undergo a structural evolution with change in temperature, and are tetragonal to some degree until close to the bulk TC. Using Raman spectroscopy, we find that larger particles undergo a clear phase transition although at a less sharply-defined temperature than in the bulk; the structure evolves more gradually as particle size decreases. We attribute this behavior to the larger range of latent heats that participate in phase transitions of small particles, due to the increased influence of the surface. Our PDF analysis provides evidence for an order-disorder aspect to the material. We find that as particle size decreases, there is a clear trend of increasing distortion of the Ti-O octahedra. Although there is an increased tetragonal-like distortion of the bonds in small particles, the c/a value is less than that of the bulk material. We believe that this is due to a reduced potential for long-range coherence in small particles, causing the orientation of the distortions to become more random.;In Chapter 4 we report the study of barium titanate nanocrystals by electrostatic force microscopy (EFM). EFM is a scanning probe technique that is able to measure charges and dipoles on nanoscale features. Our goal was to observe a signal from the structural dipole of BaTiO3 nanoparticles. However, we found that electrostatic force from the particles is most likely due to surface effects and not to the measurement of a permanent dipole on the sample.