Kiyoshi Sakaue

STRUCTURAL CHARACTERIZATION OF BATIO3 THIN FILMS GROWN BY MOLECULAR BEAM EPITAXY

Epitaxial ultrathin films of BaTiO3 were prepared using molecular beam epitaxy. For the substrate, (001)-oriented SrTiO3 single crystals were used. Controlling the growth conditions of these films as well as the semiconductor thin films, led to the successful growth of the BaTiO3 films as single crystals, characterized by x-ray diffraction even in the ultrathin range. The ultrathin BaTiO3 films are highly c-axis-oriented tetragonal phaselike bulk BaTiO3 crystals. The tetragonality of the thin film crystals is much larger than bulk crystal’s. We also measured the saturated polarization (Ps) of the BaTiO3 films at temperatures ranging from room temperature to 600 °C. The results confirmed again that the films are ferroelectric tetragonal phase crystals. Moreover, they showed that the transition temperature for the ferroelectric-paraelectric phase transition of the films is higher than bulk crystal’s.

The local structure of mixed-ion perovskites

The temperature-dependent Nb K-edge absorption spectra of several mixed-ion Pb-containing perovskite compounds were analysed to determine the Nb displacement and to trace its changes through the phase transitions. Both extended x-ray absorption fine structure (EXAFS) and the pre-edge region of the spectra were involved in the analysis. The results show that, in the compounds studied, Nb occupies an off-centre position with symmetry lower than that implied by macroscopic symmetry. The magnitude and direction of the Nb off-centre displacement do not display any noticeable temperature change and are not affected by the change in macroscopic symmetry. The Nb–O distribution and its temperature evolution do not show any distinct dependence on the degree of compositional ordering and properties of the samples.

Local structure and nature of phase transitions in KNbO3

Abstract The direction and magnitude of Nb displacements from the center of oxygen octahedron in orthorhombic, tetragonal and cubic phases of KNbO 3 were determined by means of polarized X-ray absorption fine structure (polarized XAFS). The XAFS spectra for Nb K -edge were obtained from plate-like single crystals in a transmission mode. It is found that in the orthorhombic phase the direction of the Nb displacement is very close to the polar axis. However, the disorder of Nb positions grows gradually with increasing temperature. All phase transitions are governed by both displacive and order-disorder mechanisms. The rhombohedral-to-orthorhombic phase transition is considered as essentially a displacive type, while the order-disorder component is dominant in the tetragonal-to-cubic phase transition.

Study of the ω-Phase in Ti–Mo Alloys by Diffuse X-Ray Scattering

The X-ray diffraction profiles of the ω-phase in Ti 1- x Mo x , 0.07 ≤ x ≤0.20, are carefully examined for the first time. The intensity distribution near the wave vector q 0 =(2/3, 2/3, 2/3) is asymmetric. The ω-phase appears in rod shaped clusters along the direction in the body-centered-cubic lattice. The incommensurate diffuse scattering can be detected above the critical composition x c =0.10. The scattering profiles are almost independent of temperature over a temperature range -120°C to 50°C. The observed diffuse scattering is discussed in terms of the planar displacements described by a stacking soliton and the coherency of the ω-phase domains.

Epitaxial Growth of Single-Crystal C60 Films on GaAs(111) Substrates.

Single-crystal C 60 films were successfully grown on GaAs (111) A surface by MBE. The C 60 film was investigated by in situ observation of RHEED. C 60 molecules grew into a monolayer film with the close-packing fcc configuration. The film was confirmed by X-ray diffraction study to be of high-quality (111) oriented crystal. The results may be put single crystal C 60 on semiconductor into use for many devices.

Phase transition of BaTiO3 thin films

The phase transition of ferroelectrics is one of their most important bulk-nature attributes. In well-epitaxial ferroelectric thin films, the crystal structures are highly strained and the ferroelectric phase transitions tend to dissappear because of an epitaxial effect. We investigated the epitaxial effect by structural analysis and clarified the growth conditions of BaTiO3 films that show their bulk nature. As a result, epitaxial crystals of BaTiO3 with a thickness of 67 A were grown on a MgO substrate with a Pt electrode by the activated reactive evaporation method. A clear thermal anomaly was observed at 140 °C that suggests the phase transition of BaTiO3.

Dielectric Investigation of BaTiO3 Thin-Film Capacitor

The dielectric properties of BaTiO3 thin film were investigated on a highly c-axis-oriented epitaxial thin crystal. It was confirmed by X-ray diffraction that the film thickness was 120 A and that it was high-quality single-crystal film. The spread of crystal orientation of the BaTiO3 thin film was the same as that of SrTiO3 substrate. However, the dielectric properties of this thin film were different from those of the BaTiO3 bulk crystals and were explained by the space-charge effect. The space-charge polarization of this film strongly depends on temperature and this behavior was observed after the incubation time had passed. The experimental results are discussed in terms of the interaction between the polarities of domain and space charges.

The local structure of PbIn1/2Nb1/2O3

The local structure of ordered and disordered PbIn1/2Nb1/2O3 has been investigated as a function of temperature using X-ray absorption fine structure (XAFS) technique. Both EXAFS (extended XAFS) and near edge structure of the spectra were analysed. Radial distribution of oxygen atoms around Nb appeared to be similar for the both modifications and independent of the temperature. Presence of the pre-edge peak in the spectra indicated, that the value of the Nb displacement is quite large both below and above the phase transition point. The local structure of PbIn1/2O3 is discussed in comparison with the other Nb-containing perovskite compounds.

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.

Structural phase transition of benzil

The temperature dependence of order parameters such as spontaneous strains and atomic displacements in benzil (C6H5COCOC6H5) has been obtained by the x‐ray diffraction method. The diffuse x‐ray scattering has also been examined just above the transition temperature (Tc = 83.5 K). It has been found that the primary order parameter is a spontaneous strain ezx and the benzil crystal is ferroelastic below Tc.