Hydrothermal syntheis and characterization of BaTiO3 nanoparticles
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This thesis investigates the size dependent dipole disorder in ferroelectric materials, which leads to a loss of macroscopic ferroelectricity, and the role of the depolarization field on this disorder. Structural characterization was done on particles dispersed in mediums with different dielectric constant, providing different electrostatic boundary conditions on the particles. BaTiO3 particles was prepared using hydrothermal synthesis, with varying process parameters to achieve particles with crystallites between 11 and 31 nm. An additional annealing step gave particles with crystallites between 40 and 94 nm. For particle characterization, the different dispersion mediums was either oil or water, with dry particles as a reference. The average structure was characterized using Raman spectroscopy, X-ray diffraction and subsequent Pawley refinement. The local structure was measured using synchotron total scattering X-ray diffraction and pair distribution function analysis. The three structural characterization methods reveal a complicated dipole ordering, as the influence from the dispersion medium is different over the different measurement length scales. The particle structures can not be explained on both local and average scale by symmetry models of either tetragonal P4mm or Pm-3m space group symmetry, suggesting a more complicated dipole ordering like domain structures or vortex states. The Raman spectroscopy data suggest an abrupt change in the dipole ordering between the particles with 20 and 24 nm crystallites, which could be interpreted as a transition in the dipole ordering from a coexisting polarization and vortex state to a polar domain structure. Further investigation is necessary before any final conclusions can be reached.