Hirofumi Kasatani

Epitaxial growth and dielectric properties of BaTiO3 films on Pt electrodes by reactive evaporation

Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high‐energy electron diffraction and cross‐sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two‐dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant er. Good agreement between the experimental results and the theoretical calculations leads to ...

Structure Analysis and Hydrogen Bond Character of K3H(SO4)2

Structure analysis of K 3 H(SO 4 ) 2 was performed by X-ray diffraction experiments in order to study the hydrogen bond character in connection with the geometrical isotope effect. Crystal data at room temperature were obtained as a=9.777(1)A, b=5.674(1)A, c=14.667(4)A, β=102.97(2)° and the space group A 2/ a , Z =4. The hydrogen bond length R OO determined from the positional parameters was 2.493(1)A which was close to but still longer than the so called critical bond length r c . The electron density of split hydrogen atoms was observed on a differential Fourier map. It is concluded that the hydrogen atoms take the disordered state at room temperature and the distance between hydrogen atoms R HH is approximately 0.65 A. The possibilities for the existence both of the low temperature phase and the high temperature prototype phase are discussed.

Temperature dependence of hydrogen bond nature in K3H(SO4)2

Structure analysis of K 3 H(SO 4 ) 2 was performed by X-ray diffraction experiments at various temperatures down to 27 K in order to investigate the hydrogen bond character in connection with the geometrical isotope effect. The hydrogen bond length R OO monotonically decreases with decreasing temperature, accompanied by the contraction of lattice parameters, and it crosses the critical bond length r c ( H) at about 100 K. Simultaneously, the double site feature of hydrogen atoms changes into the single site state at the center of the hydrogen bond. This behavior explains why K 3 H(SO 4 ) 2 has no phase transition in spite of the existence of the disordered character at room temperature.

Structural Study of Epitaxial BaTiO3 Crystals

Epitaxial crystals of BaTiO 3 with the thicknesses of 67 A, 230 A, 600 A and 1500 A were grown on SrTiO 3 substrates by activated reactive evaporation. The tetragonality of the crystal decreases with increasing thickness and remains in the temperature range of 15 K to 670 K. The epitaxial effect is discussed in terms of a critical thickness, as well as of a surface layer of fine particles.

Possible origin of the isotope effect on the phase transition of K3(D, H)(SO4)2

Structural data of K 3 H(SO 4 ) 2 and K 3 D(SO 4 ) 2 reported by Noda et al . [J. Phys. Soc. Jpn. 59 (1990) 2809 and ibid. 3249] are precisely examined in order to investigate the possible origin of the isotope effect in a hydrogen bond material. The SO 4 molecule is slightly distorted from a regular tetrahedron. The distortion is explained by the displacement of the sulfur atom as well as that of the oxygen atom which is in charge of forming the hydrogen bond. The size of the SO 4 molecule in the D-compound is slightly larger than that in the H-compound, but the difference seems to be marginally small. The major geometrical difference is only seen in the hydrogen bond length R OO . In addition, electron transfer between hydrogen and oxygen atoms with opposite directions was found. The difference of the ionicity of H and D atoms is considered to be the origin of the difference of the bond length R OO .

New Organic Material of Bis(benzylidene)cycloalkanone Derivatives for Efficient Optical Second-Harmonic Generation

We have synthesized a series of bis(benzylidene)cycloalkanone derivatives with up to 30 members, and examined their conversion efficiencies of second-harmonic generation (SHG) and related characteristics. As a result, we find that a few derivatives exhibit prominent features in SHG. That is, 2,5-bis(4-methylbenzylidene)cyclopentanone, 2,5-bis(4-bromobenzylidene)cyclopentanone and 2,7-bis(4-methylbenzylidene)cycloheptanone are 30, 20 and 16 times more efficient in SHG, respectively than urea, and they presumably satisfy the criteria required for practical application, concerning phase matching, cutoff wavelength, stability and laser-power tolerance. The crystallographic structures of a few typical compounds are also presented. Furthermore, the difference in effectiveness of the present design principle compared with conventional ones is discussed.

X-Ray Study of the Phase Transitions in K3D(SO4)2 and K3H(SO4)2

Structural phase transition of K 3 D(SO 4 ) 2 was studied by X-ray scattering experiments, and new reflections were observed below T c =85 K. The space groups are A 2/ a ( Z =4) above T c and P 2 1 / a ( Z =4) below T c . The postulated structure of the low temperature phase has an antipolar character. Temperature dependence of lattice parameters was also investigated, whose anomaly below T c was marginally small. The deviation from the curve calculated by the Debye approximation was utilized to estimate the nature of the order parameter. Similar experiments in K 3 H(SO 4 ) 2 have not shown any evidence of the phase transition.

Ferroelectric phase transition in BaTiO3 films

Abstract Epitaxial BaTiO 3 single crystals were grown on Pt/MgO (Pt for electrodes, MgO for substrates) by means of the activated reactive evaporation technique. X-ray diffraction patterns were measured to clarify the properties such as the temperature dependence of lattice parameters. It was found that the temperature dependence of lattice parameters and integrated intensities of Bragg reflections showed different behavior depending on the film thickness. The relative dielectric constant ϵ r was measured in order to investigate the dielectric properties of the films. Film crystals of BaTiO 3 showed the phase transitions which are different from those in bulk state.

Incommensurate Phase Transition in 4,4'-Dichlorodiphenyl Sulfone

The structural phase transition in a 4,4 ′ -dichlorodiphenyl sulfone single crystal was investigated by means of X-ray diffraction. It has been found that the crystal shows the normal-incommensurate phase transition at 150 K with the modulation wavevector q = a * ±(1/5+δ) b * , where a * and b * represent the reciprocal lattice vectors in the normal phase and δ represents the small misfit parameter in the incommensurate phase. No lock-in phase transition was confirmed within the temperature range 20-150 K.

Synchrotron X-Ray Diffraction Study of Third-Order Fibonacci Lattices

Third-order Fibonacci lattices have been grown by means of molecular beam epitaxy. Many satellite peaks are observed by using synchrotron X-rays. A broad peak observed by a conventional diffraction method is found, at high resolution, to be a gathering of many sharp peaks. The satellite peaks are assigned by a power law, implying that the third-order Fibonacci lattices have a self-similar feature.