Koichi Toyoda

Electrical properties of SbSI crystals in the vicinity of the ferroelectric curie point

The electrical properties of SbSI crystals have been studied with particular reference to the ferroelectric phase transition. The dielectric constant along the c-axis obeys the Curie-Weiss law with C = 2.54 × 105 K and T0 = 15.2°C. The coefficients in the Gibbs free energy expanded by the electric polarization have been determined to be ξ = -3.02 × 108 Vm5C-3 and ζ = 6.99 × 109 Vm9C-5 by using the temperature dependence of the dielectric constant and the spontaneous polarization. An anomaly in the electric conductivity has been found at the Curie point. Effects of substitution of Bi for Sb in SbSI on the behavior of the phase transition have been also studied. The Curie point decreases with increasing Bi concentration x according to the relation Tc(x) = Tc(0) - 520x (°C). The phase transition, which is a first-order for pure SbSI, becomes a second-order between x = 0.10 and 0.17.

Refinement of crystal structure of SbSI

Abstract The crystal structure of SbSI has been refined at 27°C and 48°C, where the crystal is paraelectric. The anisotropies in the temperature factors were taken into account. The three-dimensional x-ray intensity data used were gathered by an automatic four-circle diffractometer, and the final R values of 0.048 were obtained in both cases. A large anisotropic temperature factor B33 of the Sb atom is obtained, and a trial was made to determine whether the large anisotropy could be explained as disorder of the Sb atom. The result shows that no choice could be made between a displacive or an order-disorder type phase transition.

Self-Trapped Exciton Luminescence in KBr1-xIx and RbBr1-xIx Solid Solutions

Emission spectra and decay characteristics of luminescence in KBr 1- x I x and RbBr 1- x I x solid solutions have been investigated in the whole range of composition x . The emission band due to localized excitons at I - dimer centers in bromides, which consists of singlet and triplet components, is found to connect continuously with intrinsic σ band in pure iodides. The triplet component of the band is transformed into another emission with larger Stokes-shift in the intermediate region of x . Time-resolved studies on this emission in KBr 1- x I x have clarified that it consists of two triplet emission bands differing by ∼ 0.2 eV in their peak energies. The lower energy one corresponds to π band in KI. We demonstrate that the relaxed exciton configurations of different types are selectively realized depending on the local environment surrounding the I 2 - molecular core of the exciton. Discussion is given on the dynamical relaxation process of free excitons in iodides.

Luminescence of Iodine Dimers in KCl:I

Properties of the 4.64eV(UV) emission in KCl:I have been investigated under the excitation with light in a region from 6.5 to 7.7eV. Measurements of optical spectra have been made at 80 K for KCl crystals doped with iodine ions of 8.1×10 -4 ∼1.9×10 -2 mole fraction. The UV emission is found to be excited strongly on the low energy side of each of three monomer absorption bands. It is confirmed that intensity ratio of the UV emission to the 2.64eV emission, which is due to monomer, increases linearly with iodine concentration. This fact indicates that at the low energy side of each monomer absorption, there exist absorption bands due to dimers of iodine ions, excitation of which leads to the UV emission.

Re-examination of the phase transitions in BaTiO3-based solid solutions

Abstract The successive phase transitions in BaTiO3-BaSnO3 and BaTiO3-BaZrO3 systems have been re-examined. The temperature dependencies of the complex dielectric constant and pyroelectric currents of these ceramic specimens were measured in order to determine the temperatures of the phase transitions. The behavior of the dielectric constants near the lower phase transitions become smeared with increasing BaSnO3. or BaZrO3 substitution, however, the imaginary part of the dielectric constant and the pyroelectric currents show clear anomalies. The phase diagrams for these systems, deduced from these measurements, have been analyzed by means of Matsubara-Yoshimitsu theory.

Luminescence from Self-Trapped Excitons in KBr: Na

Luminescence from KBr containing Na + ions (3×10 -5 -9×10 -2 mol fraction) is investigated at 10 K either by exciting crystals with the UV light or the F band light in X-rayed crystals. Emission bands related to Na + ions are identified at 2.72 and 2.90 eV; both of them polarize perpendicular to the V K type molecular axis. Their decay times obtained with an N 2 laser excitation are 35 and 24 µs, respectively. It is concluded that the Na emission is π-type emission from a self-trapped exciton, ( V K +e) * , perturbed by Na + ion. Relative intensity of the 2.72 eV band to the 2.90 eV band varies depending upon the excitation energies and the thermal history of X-rayed crystals. This means that at least two types of the V KA center with defferent thermal stabilities should be assumed.

Initial Stage of Growth Process of Lead Titanate Fine Particles

A study on the initial growth stage of lead titanate fine particles with nanometer size is described in this paper. A mixture of the constituent metal alkoxides was hydrolyzed to obtain amorphouslike particles of lead titanate around 5 nm in diameter. The hydrolysate was calcined to crystallize in atmosphere at various temperatures. The volume of the particle increased in proportion to the duration of the calcination. This time dependence is explained by assuming that coalescence occurs at limited areas of the particles such as corners or kinks. The size of as-prepared fine particles was uniform but their distribution became varied as the average size was increased by calcination.

Re-examination of the phase transitions in solid solutions of perovskite-type oxides

Abstract The successive phase transitions in BaTi1−xZr x O3 and BaTi1−xSn x O3 systems have been re-examined. Ceramic specimens of these solid solutions for various compositions were prepared by ordinary sintering techniques. The temperature dependencies of the complex dielectric constant and pyroelectric currents of these specimens were measured in order to determine the temperatures of the phase transitions. The behavior of the dielectric constants near the lower phase transitions become smeared with increasing Zr or Sn concentration, however, the imaginary part of the dielectric constant and the pyroelecric currents show clear anomalies. The phase diagrams for these systems, deduced from these measurements, have been analyzed by means of Matsubara-Yoshimitsu theory. The agreements between the experimental facts and theoretical predictions are quite satisfactory.

Optical Absorption and Luminescence of Nacl-NaBr Solid Solution

The optical absorption and luminescence of NaCl-NaBr solid solutions were studied in the whole concentration range at liquid nitrogen temperature. The change of the profile observed in the absorption spectra around 0.2 mole fraction of NaBr content is attributed to the percolation of minority composition in this system. With increasing the concentration of NaBr, the 5.15 eV emission band, which is known to arise from Br - -dimers, shifts gradually toward the low energy side to reach finally the location of the intrinsic emission band in pure NaBr, whereas the 4.0 eV emission band arising from Br - -monomers diminishes its intensity and fades out above 0.2 mole fraction of NaBr. This suggests that the relaxed excited state of the Br - -dimer, not of the Br - -monomer, in NaCl can be recognized as a proto-type of the self-trapped exciton in the pure NaBr.

Bibliography of early studies in perovskite ferroelectrics

Eighty years have past since the discovery of the first ferroelectric crystal, rochelle salt, by Valasek in 1921. In perovskite compounds, the ferroelectricity was discovered in BaTiO 3 at the early stage of the 1940s in USA, Russia and Japan. 10 years later, the antiferroelectricity was first discovered in PbZrO 3 at 1951 by Sawaguchi, Shirane and Takagi [51S2]. From the following bibliography of perovskite and its related compounds until 1955, we can easily imagine vivid activities and enthusiasm in this field all over the world. This reminds us recent every effort and passion to search and understand high-temperature superconductors. When we looked hack over this golden era of ferroelectricity, we realize so many interesting ideas were proposed.