Tatsuo Sakashita

Polarized Raman scattering of domain structures in polycrystalline lead zirconate titanate

The intensity change upon crystal rotation of selected Raman modes of a lead-zirconate-titanate-based relaxor has been examined from both theoretical and experimental viewpoints. Periodic functions, representing intensity ratios between Raman bands as a function of crystal orientation, have been theoretically derived from the Raman tensor of both Ag and E phonon modes. Theoretical computations were compared with experimental data on a poled polycrystal and a discussion provided on reliability of different spectroscopic approaches for quantitative assessment of domain orientation.

Stress dependence of the polarized Raman spectrum of polycrystalline lead zirconate titanate

The stress dependence of the Raman spectrum of a relaxor-based polycrystalline ferroelectric lead zirconate titanate–lead nickel niobate–lead zinc niobate (PZT–PNN–PZN) has been investigated using polarized Raman microprobe spectroscopy. Emphasis has been placed on explicitly working out the second harmonic equations that relate Raman intensities to the angle between the laser polarization direction and selected crystalline axes. Based on these assessments, the effect under polarized light of crystal orientation on the intensity of selected Raman modes has been rationalized and the obtained experimental data interpreted. Raman spectra were collected with both parallel and cross polarization filters and their dependence on stress calibrated with loading the PZT–PNN–PZN samples in a four-point flexural jig. The use of polarized light allowed us to clarify the effect of domain orientation on the Raman spectrum of PZT–PNN–PZN and to perform precise calibrations of the stress dependence of the A1(TO4) Raman mo...

Stress dependence of the Raman spectrum of polycrystalline barium titanate in presence of localized domain texture

The stress dependence of the Raman spectrum of polycrystalline barium titanate (BaTiO3, BT) ceramics has been examined with microprobe polarized Raman spectroscopy. The angular dependence of the Raman spectrum of the tetragonal BT crystal has been theoretically established, enabling us to assess the stress dependence of selected spectral modes without the influence of crystallographic domain orientation. Upon considering the frequency shift of selected Raman modes as a function of orientation between the crystallographic axis and the polarization vector of incident and scattered light, a suitable instrumental configuration has been selected, which allowed a direct residual stress measurement according to a modified piezospectroscopic procedure. The analysis is based on the selection of mixed photostimulated spectral modes in two perpendicular angular orientations.

Quantitative analysis of domain texture in polycrystalline barium titanate by polarized Raman microprobe spectroscopy

A quantitative determination of domain distribution in polycrystalline barium titanate (BaTiO3, henceforth BT) ceramics has been pursued with the aid of a microprobe polarized Raman spectrometer. The crystallographic texture and domain orientation distribution of BT ceramics, which switched upon applying stress according to ferroelasticity principles, were determined from the relative intensity of selected phonon modes, taking into consideration a theoretical analysis of the angular dependence of phonon mode intensity for the tetragonal BT phase. Furthermore, the angular dependence of Raman intensity measured in polycrystalline BT depended on the statistical distribution of domain angles in the laser microprobe, which was explicitly taken into account in this work for obtaining a quantitative analysis of domain orientation for in-plane textured BT polycrystalline materials.

Self-Consistent Anharmonic Theory and Its Application to Ferroelectric Crystal

The dielectric properties of barium titanate near the Curie temperature are studied using the variational principle method on a self-consistent anharmonic model. A trial Hamiltonian was devised using the terms of the anharmonic oscillator and asymmetric potential energy, which was adopted as the inter-atomic potential of the ferroelectric crystal. The temperature dependence of the dielectric property of barium titanate that was derived from this theory is in good agreement with the experimental result of the inter-atomic potential determined from first-principles ultrasoft pseudo potential calculations.

Effects of Titania Surface Orientation on the Heterogeneous Formation of Barium Titanate by Hydrothermal Synthesis

We investigated the mechanism of hydrothermal synthesis of BaTiO3 in the system of Ba(OH)2 and TiO2 in the early growth stage, based on model experiments using various single-crystal rutile TiO2 substrates with (001), (110), (100), and (101) orientations as starting reactants and a practical case study of widely utilized anatase TiO2 powder. The BaTiO3 products represented heterogeneous island growth directly on rutile TiO2 substrates and morphological features depending on the surface orientations of the substrates. Well-defined orientation relationships between BaTiO3 and the TiO2 substrate depended on the surface crystal planes of rutile TiO2 and were classified into four independent crystallographic types, including the unique topotaxial relationship already reported in barium diffusion-induced solid-state reactions. The hydrothermally treated anatase powder exhibited a dumbbell-like precipitation of BaTiO3 on the anatase TiO2 particles, preserving a crystallographic relationship of (001)[100] BaTiO3 ∥ (001)[100] anatase. The experimental findings revealed the heterogeneous dissolution–precipitation mechanism via surface orientation-dependent epitaxial growth of BaTiO3 directly on rutile and anatase TiO2 in the initial stage of hydrothermal reactions, suggesting the importance of controlling surface morphology and crystallinity of TiO2 as hydrothermal synthesis sources.

Theoretical Consideration of Size Effect for Barium Titanate

The dielectric property of the ferroelectric crystal was studied in the variational method on the basis of the self-consistent model. The anharmonic oscillator was used as a trial Hamiltonian, and a double-well potential which was formed by superposing the back-to back Morse potentials was adopted as an inter atomic potential in the ferroelectric crystal. On the basis of the self-consistent model, a theoretical treatment for the temperature dependence of dielectric constant and the size dependence thereof is presented.

Raman Piezo-Spectroscopic Investigation of Microscopic Residual Stresses in Ni-MLCC Devices

Residual stresses in Ni-MLCC were investigated by Raman microprobe piezo-spectroscopic (PS) methods. The shape of the Raman spectrum of BaTiO3 in Ni-MLCC depended on the angle between the polarizing direction of the incident laser beam and the direction of the internal electrode. From a set of precise calibrations, we show that the orientation dependence arises from the interaction between internal stresses and the polycrystalline microstructure; by taking into account such an orientation dependence, we were able to establish a technique for the measurement of the distribution of residual stresses in Ni-MLCC.

First-Principles Calculations of Structural Changes Induced by Oxygen Vacancies in Tetragonal Phase BaTiO3

Structural changes induced by oxygen vacancies in tetragonal phase BaTiO3 are studied with first principles calculations within density functional theory. In our calculations, the incorporation of oxygen vacancies greatly decreases c/a ratio of the tetragonal phase BaTiO3, and a phase transition from tetragonal to cubic phase occurs when the incorporation of the oxygen vacancies reaches about 4%. Our results also shows that the generation of the oxygen vacancies slightly increases the volume of BaTiO3, and the increases are typically less than 0.5% even in heavily reduced conditions.

Self-Consistent Anharmonic Theory and Its Application to the Isotope Effect on Ferroelectric Phase Transition in BaTiO3 Crystal

The isotope effect on ferroelectric phase transition is studied by the variational principle method using a self-consistent anharmonic model. A trial Hamiltonian was devised using the terms of the anharmonic oscillator and arbitrary potential energy between atoms. The isotope effects on BaTiO3 ferroelectric phase transition, especially on the cubic-to-tetragonal transition, are given through a function consisting of the optical phonon. It is revealed that a heavy-isotope-enriched crystal shows a lower TC than a light-isotope crystal, which is in agreement with the experimental Ti-isotope effects in BaTiO3.