Hiroo Toyoda

Growth Ridges, Etched Hillocks, and Crystal Structure of Lithium Niobate

Growth feature of a LiNbO3 crystal obtained by the pulling method has been studied. The formation of growth ridges was interpreted as the intermittent growth of the form {10}, which was the most readily grown form from the melt. The point group of the high temperature, paraelectric phase was uniquely determined as 2/m from the symmetry elements of the configuration of ridges. On (00) planes, the triangular etched hillocks were observed. Orientations of the hillocks on the antiparallel domains were related by 2-fold rotation axis parallel to [100] consistent with the phase transition of the type 2/m→3m. The atomic arrangement in the form {10} was examined, and a three-dimensional framework of the corner-shared Nb-O octahedra, similar to the one in perovskite, was found.

Hall Effect and Thermoelectric Power in Semiconductive TiO2

The electrical properties of the conductive TiO2 (rutile) ceramics, as well as single crystals, doped with Nb2O5 or Ta2O5 were investigated by Hall effect and thermoelectric power measurements. These properties were shown to be strongly dependent on the doping concentration which covered the range up to 4 mole-percent. The experimental results indicate that the specimens are non-degenerated n-type semiconductor with a density-of-states effective mass of m*=20 m. At room temperature, where lattice scattering plays a dominant role, Hall mobility is about 0.4 cm2/Vs. for lower doping level and it falls down to 10-2 cm2/Vs. with the increasing doping concentration. The mobility increases in the slightly doped specimens at liquid nitrogen point, while it decreases in the specimens doped more heavily than 0.1 mole-percent. The activation energy of the donor centers due to the doping is 0.02 eV. The reduced specimen not intentionally doped, shows a shallower level nearly equal to 0.01 eV.

The Effects of Additives and of Ambient Atmosphere during Heating on the Electrical Resistivity of Semiconducting BaTiO3

The effects of doping and firing atmosphere on the electrical resistivity of polycrystalline barium titanate were studied. For the firing in nitrogen flow, the resistivity fell down to 10 ohm-cm with 0.05 mole percent Dy2O3 addition and did not show any appreciable change for larger additions. The firing in oxygen flow brought about a resistivity minimum near 0.1 mole percent Dy2O3 addition. The grain growth was suppressed with the increase of Dy content and in oxygen flow. The small grain was easily oxidized to have a high resistivity, while the large grain subjected to surface oxidation showed a pronounced PTC effect. Additions of Mn-, Cu-, Bi- or Tl-oxide together with Dy2O3 served to increase the electric resistivity. The resistivity rise also depended on the firing atmosphere in the respectively characteristic ways.

Switching of optical rotatory power in ferroelectric 5pbo 3Ge02 single crystal

An enantiomorphic transition was observed for the first time by observing Airys spirals of the oppositely polarized ferroelectric 5PbO 3GeO2 single crystals, which were grown by the Czochralski method. Switching time follows an exponential law at moderate field strengths and a power law at higher field strengths.

Electrical Resistivity of Arc-evaporated Carbon Film

The electrical resistivities of arc-evaporated carbon films were studied. The resistance has a large negative temperature coefficient. The annealing diminishes remarkably both the resistance and its temperature dependency. The specific resistivity depends on the film thickness and increases rapidly below 500A. It seems that the lattice imperfections play the most important role, and then from the annealing curve, their distributions are discussed. The aging effect on the resistance were also measured.

Optical and Structural Anomalies in Potassium Ferrocyanide Trihydrate Crystals

Potassium ferrocyanide trihydrate crystals grown from aqueous solution have been studied with polarizing microscope, X-ray diffraction methods, etching technique, and dielectric measurements at temperature region between 20° and -140°C. Various types of optical anomalies were observed, and the crystals were classified into five types by optical and X-ray investigations. Crystallographic data of monoclinic and tetragonal single crystals were measured. The various optic anomalies were clarified by twinning and parallel growth characteristic to the layer structure of the two modifications. Dislocation pits were observed by etching with a mixture of water and alcohol. Ferroelectric monoclinic phase, and the behaviors of tetragonal phase at low temperatures were studied by low temperature polarizing microscope, dielectric measurements and low temperature precession X-ray photographs. Teteragonal phase was shown to undergo a monotropic phase transition at about -55°C, and transform into a twinned monoclinic phase.

Electrical Properties of Cadmium Selenide Single Crystals –Effect of Heat-Treatment in Selenium Vapor–

The Hall effect and the effect of heat-treatment in selenium vapor were investigated for n-type conductive cadmium selenide single crystals. The temperature dependence of Hall coefficient showed the crystal to be nearly degenerate. The crystal had the electron concentration of 3.6×1017 cm-3 with the effective mobility of 580 cm2/V. sec. at room temperature. Heat-treatment in selenium vapor produced a rectifying layer on the crystal surface. Rectifying characteristics, barrier admittance, photoelectric effects and pulsed field effect were measured on the treated surfaces. These measurements showed that a p-type layer was formed on the n-type crystal and that the mobility of holes in the layer is about 50 cm2/V. sec. The band gap of the crystal was estimated to be 1.83 eV.

Dispersion of the Refractive Indices of LiNbO3 Crystal between 20? and 900?C

The refractive indices of lithium meta-niobate single crystal were measured with a prism up to 900°C over a wave length range from 420 to 670 mµ. The temperature dependence of the extraordinary refractive index ne is marked in contrasts with small change of the ordinary one n0. At 670 mµ, they take values ne=2.191 and n0=2.279 at room temperature and amount to 2.283 and 2.296 at 900°C, respectively. The results were discussed in connection with the effect of thermal expansion and the spontaneous polarization of ferroelectric LiNbO3.

The Surface Effect on the Electrical Properties of BaTiO3 Single Crystals

The effect of electrode materials and atmospheres on the polarization reversal and the electrical conduction of barium titanate single crystals were investigated. It was revealed From d. c. conduction measurements that evaporated indium and tellurium metals give an ohmic and a blocking contacts, respectively. It was found that the switching rate under the application of sinusoidal field is primarily determined by the anode substance and that the indium electrode markedly slows down the rate. The polarization reversal of the crystals with evaporated gold electrodes is affected considerably by atmosphere. The switching rate was decreased by evacuation and increased by the adsorption of polar molecules. The switching behavior was determined by the atmosphere in contact with anode. The condition to bring about a sharp switching required at least one blocking contact so biased that the carriers were easy to flow across it. The mechanism is discussed.

Structures of Ferroelectric Domains in Triglycine Sulfate

Domain structures in ferroelectric glycine sulfate are studied by etching in various alcohols. Methyl alcohol etches the head of polarization more readily than the tail end, and gives, in a relatively short time, clear domain patterns. Moreover, this etchant etches the (001) surface very slowly and after a long time delineates the arranging manner of domains along the polarization axis. Domain boundaries have cylindrical surfaces parallel to b axis, but the sections perpendicular to the axis are quite indefinite in dimensition and direction. Domain terminated with a rounded tip in the antiparallell domain was observed. Crystal imperfection seems to have a considsrable effect on the domain structure of this crystal. Some preliminary studies of domain wall motion under the application of electric field are also reported.