Mieko Takagi

Electron-Diffraction Study of Liquid-Solid Transition of Thin Metal Films

Structure changes of thin films of Pb, Sn and Bi at various temperatures has been studied by electron diffraction method. The films were prepared, in the electron diffraction camera by evaporating the metals, on cleavage surfaces of several kinds of crystals. The mean thickness of the films ranged from 10A to 1000A. The observed melting points are found to be lower than those of bulk metals. Anticipating this effect to be attributed to the small size of the crystal, the author has calculated the melting temperature as a function of crystal size. The observed values of the lowering of the melting point agree fairly well with the calculated values for the crystal sizes estimated from the mean thickness and the breadth of the Debye-Scherrer rings in diffraction patterns. On cooling, the films are supercooled down to considerablly lower temperatures which can be ascribed to the slowness of the rate of nucleation in a small crystal.

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.

Electron-Diffraction Study of the Structure of Supercooled Liquid Bismuth

Thin bismuth films were prepared on crystal surfaces by evaporation And the liquid state of the films at temperatures above and below the ordinary melting point (271°C) has been studied by means of the reflexion method of electron diffraction. The marked supercooling of the films (M. Takagi: J. Phys. Soc. Japan 9 (1954) 359) has been utilized for the study below the melting point. A method to eliminate difficulties inherent to the intensity measurement by reflexion method and a method to refine the observed intensity values were developed. The radial distribution curves at 400°C, 271°C (the melting point), 175°C, 130°C and 110°C (a little above the solidification temperature) were computed from experimental data. The curves showed a systematic change with temperature. It was revealed that the atomic arrangement at 110°C resembles closely that of solid bismuth. Above the melting point a tendency towards close-packed structure was noted.

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.

Topographic Study on Domain Boundaries in TGS. I

Intensity distribution on the section topographs for ferroelectric triglycine sulfate crystals containing an antiparallel domain boundary was calculated by using the dynamical theory of X-ray diffraction. The result shows an intensity peak corresponding to the domain boundary image. The intensity depends on the phase of the crystal structure factor. The calculated intensity distribution was compared with the results of the X-ray topographic observation reported by the same authors (Part I: J. Phys. Soc. Japan 44 (1978) 1266).

The Thickness Dependence of the Phase Transition Temperature in Thin Solid Films

Dependence of the phase transition temperatures on the thickness of films has been studied by a thermodynamic theory. The result shows that the depression of the transition temperature is inversely proportional to the thickness. Calculated values of the lowering of the melting points of gold are 6.5°C for 100 nm thickness and 65°C for 10 nm thickness.

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-Wall-Like Structures in Ferroelectric Crystals

X-ray diffraction topographic studies on the procedure of polarization reversal of ferroelectric thiourea and on the change in the domain configuration of NaNO 2 near the Curie temperature T c have been made. Polarization reversal of thiourea takes place via an intermediate state which is domain-wall-like in structure. This procedure is the same as the case of NaNO 2 . On the topographs of NaNO 2 taken at the temperatures near T c width of the domain walls become wider on approaching to T c , and at the ferro-antiferro phase boundaries all domains are eroded by the broadened domain walls. The results of the present study together with our previous X-ray topographic studies on NaNO 2 and on thiourea suggest that the domain walls, the intermediate states of polarization reversal and the modulated incommensurate phases above T c resemble in structure.