Shigeo Suzuki

Topographic Study on Ferroelectric NaNO_2 Crystals. I. : Structure of 180° Domain Walls

180° domain walls have been observed on X-ray diffraction topographs of sodium nitrite (NaNO 2 ) single crystals. Contrast of the walls depends on the kinds of the Bragg reflections used for the topographs. The reflection dependence of the contrast suggests that the contrast is produced neither by strain field around the domain walls nor misorientation of the lattice in the domain walls. A model of the crystal structure of the walls which is consistent with the observed contrast is presented. Thickness of the walls is estimated as the order of 1µm on X-ray topographs and 0.3∼1.0 µm on electron micrographs of replica films extracted from polar surfaces of the crystal.

Topographic Study of Ferroelectric NaNO 2 Crystals. II. Mechanism of Polarization Reversal

X-ray topographic study was made of a sodium nitrite (NaNO 2 ) single crystal which had been partially polarized with an external d. c. electric field at room temperature. Images with characteristic diffraction contrast found in topographs of the crystal suggest the following process of polarization reversal. By increasing thickness of pre-existing domain boundary walls the intermediate state regions having the same crystal structure as that of the domain boundary walls, are formed first, and on the next step the intermediate state regions once formed are eroded by the domains having the right direction of polarization. Rotation of NO 2 radicals by 180° around their O-O axes will be the molecular mechanism of the polarization reversal under electric field at room temperature.

X-Ray Topographic Study on Lattice Strain at 180° Domain Wall in Ferroelectric BaTiO3

Lattice strain at 180° domain walls in BaTiO 3 has been studied by X-ray topography for the case of a -plate in which the spontaneous polarization P s is parallel to the surface of the plate. At the 180° domain boundaries which were not parallel to P s , domain walls of 2 ∼3 µm thick with strained lattices were observed. Estimated lattice distortion was the shear strain of x 23 ≃1.6 ×10 -4 . The additional polarization normal to P s was estimated from the shear strain and the resultant polarization became nearly parallel to the boundary. The strained wall was not detected for the wall which was parallel to P s . Moire fringes observed at the wedge-shaped domain boundary were well explained by the interference between the diffracted X-rays from two antiparallel domains having a strained domain wall between them.

Domain Formation in Ferroelectric BaTiO3 Studied by X-Ray Topography. I

Three dimensional structure of antiparallel ferroelectric domains formed in c -plates of BaTiO 3 , partly covered with electrodes, has been studied by X-ray topography. Outside the electrodes, clusters of needle-like domains of about (10 µm) 2 in cross section due to the leakage field were observed. These clusters were nucleated from the surface with the negative electrode and almost all of the needle-like domains have terminated inside the crystal near the opposite surface with positive electrode. At the domain boundaries of large domains and at the walls of these needle-like domains, the crystal lattice is strained with the strain field parallel to the c -axis. Under an optical microscope domains are visible with very poor contrast due to the reflexion of light at the domain walls.

X-Ray Bragg Reflection and Infrared Absorption Topography of Ferroelectric NaNO2

X-ray diffraction topographic study of the partly electroded plates of ferroelectric NaNO 2 crystals (S. Suzuki and M. Takagi: J. Phys. Soc. Jpn 32 (1972) 1302) has revealed many small regions where the half way state of the rotational motion of NO 2 radicals at the time of the polarization reversal had been quenched. By making use of the sensitive dependence of infrared v 2 absorption on the orientation of the NO 2 radicals, rotation angle of NO 2 at the regions was estimated through the topographic (point by point) measurements of the infrared absorption. Topographic coincidence between X-ray topographs and infrared absorption was confirmed. Present study is the first work which had detected orientational change of molecules in a single crystal by using infrared absorption measurements in cooperation with X-ray topography.

Domain Formation in Ferroelectric BaTiO_3 Studied by X-ray Topography. II

The nucleation and growth of ferroelectric domains in a c -plate of BaTiO 3 , partly covered with electrodes, has been studied by X-ray topography. By the application of an electric field of 2.8 kV/cm for 10 seconds, small domains of several microns in size are formed outside the electrodes. On further applications of electric field, some of these nucleated domains does not grow and disappeares after the electric field was removed. By the detailed observation it is concluded that domains are not nucleated spontaneously but at the preferential points in the plate, and domains which were effective for further growth are that nucleated under the surface covered with the negative electrode. Once a nucleus has grown into a needle-like domain, new domains are easily formed around the pre-existing domain, and as the results a cluster of needle-like domain is formed.

Use of the Synchrotron X-Ray Topography for Ferroelectric Crystals

Synchrotron X-ray topography was applied to highly strained c-plates of NaNO2 in which new domains had been formed by application of an A. C.electric field. A precise determination on orientation of the lattice in the domain was made by analyzing the topographs. The study displays an example of the effective application of the topography to the ferroelectric crystals with complicated domain structure.