Search

We investigated effects of a BaTiO3 overlayer deposited epitaxially on an SrTiO3(001) surface on the 105 K phase transition of SrTiO3. With the use of x-ray reflectivity and Bragg reflection we observed the critical behaviour of the 105 K phase transition in the interfacial region and in the bulk. For T < Tc(Tc = 105 K), rocking-curve-widths of x-ray reflectivity and of Bragg reflections are found to exhibit power law behaviour (Tc − T)2β, where β obtained by x-ray reflectivity (βINTERFACE) is almost the same as that obtained by Bragg reflection (βBULK = 0.34 ± 0.02). The present result shows a sharp contrast to the critical behaviour of SrTiO3 having free SrTiO3(001) surfaces, where 2βSURFACE ≈ βBULK is reported. In this study, we find that the critical behaviour of the BaTiO3/SrTiO3 interface is influenced significantly by the surface condition generated by the epitaxial cap layer. It is likely that a change of dimensionality and/or tensile stress at the interface could affect the critical behaviour of this interfacial region. Copyright © 2005 John Wiley & Sons, Ltd.

The local structure of ordered and disordered PbIn 1/2 Nb 1/2 O 3 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 PbIn 1/2 O 3 is discussed in comparison with the other Nb-containing perovskite compounds.

Ferroelectric BaTiO 3 thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO 3 (0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO 3 films were highly c -axis oriented single crystals with good film quality.

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.

Chemical Li ion extraction processes have been carried out for pristine LiCoO 2 , LiCo 0.95 Ga 0.05 O 2 , and LiCo 0.9 Ga 0.1 O 2 compounds by swirling them in 0.35 M H 2 SO 4 solution. It is confirmed from XRD patterns that the compounds maintain the two-dimensional framework with pristine-type structure even after the acid treatment up to 12 h. The Ga-substituted compounds keep Li ions for longer time on the acid treatment rather than the LiCoO 2 . The average oxidation state of Co ions increases with the Li + ion extraction time up to 3.45+. The local structure refinements for the chemically Li + ion extracted compounds have been investigated by Co K-edge X-ray absorption spectroscopy. The extraction causes the increase of Debye–Waller (DW) factor or static disorder around the Co ion. The DW factor of the Co–Co bond pair less increases with the extraction time for the Li y Co 0.95 Ga 0.05 O 2 , and Li y Co 0.9 Ga 0.1 O 2 compounds than that for the LiCoO 2 . The Ga-substituted compounds are more stable against acid treatments than the LiCoO 2 , since more basic Ga 3+ ion retards the structural distortion of the CoO 6 octahedra against the Li ion extraction.

The surface structure of Au(111) electrodes in hydrophobic ionic liquids, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]TFSA) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM]TFSA) was investigated using surface X-ray scattering. In both [BMP]TFSA and [BMIM]TFSA, the reconstruction of the surface structure was observed and the phase transition between the (1×1) and the reconstructed structure was found to be reversible. On the other hand, the area of the reconstructed structure in [BMP]TFSA was larger than that in [BMIM]TFSA. These differences suggest that the [BMIM]+–Au interaction is stronger than the [BMP]+–Au interaction. Keywords: Ionic liquid, Bis(trifluoromethylsulfonyl)amide, Au(111) electrode, Surface structure, Surface X-ray scattering

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.

Epitaxial crystals of BaTiO3 with thickness of 67 A were grown on MgO substrate with a Pt electrode by the activated reactive evaporation method. The lattice parameter was characterized by x-ray diffraction. The tetragonality of the crystal was much higher than that of the bulk crystal and the growth layer was strained by the epitaxial effect. Thermal measurements were performed to clarify the phase transition of the thin crystal. A clear thermal anomaly was observed at 140 °C that suggests a phase transition of BaTiO3. This is the first observation of a phase transition of such a thin crystal of BaTiO3. We discuss the requirements for a phase transition in terms of the epitaxial effect.

The average and local structures of a dipole glass, KBr1-x(NO2)x, were clarified using X-ray diffraction and extended X-ray absorption fine structure (EXAFS). The average structure exhibits the distorted or fluctuated lattice below the glass transition temperature Tg . The mean square displacement of potassium and bromine ions largely deviates from the Debye approximation and the deviation is characterized by the Edwards—Anderson order parameter. On the other hand, the local structure shows no anomalous behavior and the mean square displacement of the pair distribution of potassium—bromine ions obeys the Debye approximation even below Tg . The results indicate that a cluster with only short-range correlation induced by the short-range ferroelectric order of nitrite ions exists within the distorted or fluctuated lattice caused by the reorientational freezing of nitrite ions.

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.

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.

Nb positions in KNbO 3 crystal were determined at room temperature via polarized Nb K-XAFS measurements. It has been revealed that the direction of Nb displacement in orthorhombic phase is very close to polar axis direction. This refute eight-site model which assumes that Nb atoms are disordered among several equivalent positions and phase transition are of order-disorder type. Analysis of polarized XAFS data showed that only slight disorder among close positions is possible. The distance between these positions is of an order of temperature atomic displacements, so the disorder, if any, is likely of dynamic nature. Results obtained are in perfect agreement with X-ray diffraction data.

We investigated the elastic behavior of an In 0.2 Ga 0.8 As GaAs strained superlattice grown on a (001)-oriented GaAs substrate by molecular beam epitaxy. Using high-resolution X-ray diffraction, we found that the strained superlattice has monoclinic, and no longer tetragonal crystal symmetry. The measurements of the peak shift of X-ray Bragg reflections were performed using a diamondanvil cell. The behavior of the system under hydrostatic pressure gave the magnitude of the stress from the monoclinic distortion. The experimental value of the stress was about 5 kbar. We propose the microscopic aspects of the In 0.2 Ga 0.8 As GaAs superlattice including elastic and structural aspects.

[(C 60 ) 5 (Cu) 5 ] 52 multilayer film was grown on a GaAs(111) substrate with a C 60 buffer. The [(C 60 ) 5 (Cu) 5 ] 52 multilayer film was investigated by in situ observation of RHEED. In the growth plane, the [(C 60 ) 5 (Cu) 5 ] 52 multilayer film grew into a textured film within the coherence length of the probed electron beam. The multilayer film was confirmed by X-ray diffraction study to be of (111)-oriented crystal in the growth direction. The transition temperature of the C 60 layers of the multilayer film was 260 K. We also observed the glassy behavior of the C 60 layers of the [(C 60 ) 5 (Cu) 5 ] 52 multilayer film. The results may put multilayer film, which consists of metal and fullerene, into use for many applications.

X-ray diffraction measurements of single-crystal C 60 film with the thickness of 800 A (about 60 ML) were carried out at temperatures from 300 K to 20 K. An anomaly of the lattice parameter was observed at the transition temperature T C =240 K. The transition temperature of C 60 film is 20 K lower than that of bulk crystal. The phase transition of C 60 film was somewhat different from that in the bulk state.

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.

We observed the relaxation behavior of the X-ray diffraction intensity arising from the glassy phase transition in C 60 single crystals for the first time. The (7 10 0) reflection which appeared below about 160 K showed an anomalous cusp-type feature near 90 K. It was found that this behavior strongly depended on the cooling rate of the specimen. Intensity relaxation of the reflection at 80 K at a cooling rate of 66 mK/s followed the single exponential law, I ( t )∝ I 0 (1-exp (- t /τ)) with a relaxation time constant τ of 1.8×10 4 seconds. These results strongly support those previously reported.

Epitaxial crystals of BaTiO3 have been grown by activated reactive evaporation. The ultra-thin crystal less than 100A thickness shows an elongation along the c-direction. The ferroelectric phase transition of the thin films is discussed in terms of the epitaxial effect and the surface layer.

Temperature dependence of the Nb displacement relative to the center of oxygen octahedron in NaNbO3 has been studied by polarized Nb K XAFS. Spectra were measured at two orientations of a single crystalline sample. Room temperature EXAFS data are in a good agreement with earlier X-ray diffraction data: Nb antiferroelectric displacements were found to be orthogonal to the b axis. Analysis of the temperature dependent EXAFS data didn't reveal any abrupt changes of Nb-O distances in the phase transition points. In all high-temperature paraelectric phases Nb appeared to be displaced to the off-center positions. Displacements, orthogonal to b axis, remained almost constant, while displacement along b axis gradually increased with temperature, so that in the cubic phase the displacements along all axes became equal. This shows, that disorder plays an important role in the high temperature phases. The above results are supported also by the analysis of the pre-edge structure, - the integral intensity of the pre-edge peak was temperature-independent when the polarization vector of the X-rays was orthogonal to b axis and gradually increased with temperature when the polarization was parallel to b.

Synthetic multilayered films with alternating MoSi layers were fabricated by electron beam evaporation in a high vacuum of 10 −8 Pa. They are designed according to a calculation based on a dynamical theory including absorption. A molybdenum buffer layer with a thickness of 50 nm was epitaxially grown on the substrate in order to obtain a smooth surface. The reflectivity of the first Bragg peak with Cu Kα radiation was determined as 28% while the calculated value was 54%. Silicon layers were grown as the amorphous and molybdenum layers as the polycrystalline state. The discrepancy between the observed and calculated reflectivity is due to both the interfacial roughness and the variation in the period.

EXAFS studies of KBr1-x(NO2)x mixed crystals (x

Rayleigh waves of single crystal Cu/Ni superlattices have been studied by Brillouin scattering. The Rayleigh wave propagating along the [ 1 1 ¯ 0 ] direction on the (1 1 1 ) surface shows an anomalous increase of the velocity around the modulation period of 40A. On the other hand, no modulation period dependence has been observed for the Rayleigh wave propagating along the [1 0 0] direction on the (0 0 1) surface.

The Co/Cu superlattices have been prepared by molecular‐beam epitaxy on Si(001) substrates with a Cr buffer layer. X‐ray‐diffraction experiments showed that highly (111)‐oriented superlattices were obtained. The magnetoresistance was investigated as a function of Cu spacer thickness tCu from 20 to 45 A. The giant magnetoresistance (GMR) is oscillatory and the period of oscillation is about 8–9 A. The maximum GMR was found as high as 10% with 300 Oe low field at room temperature. In addition, the resistance versus field curves were found to be clearly hysteretic, and the fields corresponding to the maximum resistence Rmax are oscillatory with Cu layer increasing, as long as the saturated field has oscillatory changes with tCu.

X-ray study of single-crystal C 60 was carried out at low temperatures to investigate the relaxational properties of the glassy phase. We observed the time-dependent X-ray integrated intensity of the reflection characteristic of the low-temperature stable phase. It is found that the relaxation times of molecular reorientation follow the Arrhenius law. The relaxation process is discussed in terms of a cluster model, in which the small crystal with the space group P2 1 3 is considered to appear in a matrix with the space group Pa3.

Single-crystal (001) Cu/Ni superlattices were successfully grown on the (001) GaAs substrates and the (111) Cu/Ni superlattices on the (111) yttrium-stabilized zirconium substrates by molecular beam epitaxy. The velocity of the surface acoustic phonon increases with decreasing modulation wavelength. This result is quite different from those previously reported.

The iodine K-edge EXAFS study of K1−x (NH4) x I mixed crystals was performed to clarify the local structure of the dipole glass associated with the random reorientational freezing of ammonium ions. The pair-distribution function of the nearest iodine-iodine ions is strongly reduced with increasing ammonium concentration. The reduction originates in the local distortion of lattice induced by the short-range antiferroelectric order of ammonium ions. We discuss the relationship between the reorientational freezing of ammonium ions and the local distortion of lattice in comparison with the results of X-ray diffraction measurements.

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.

EXAFS study was perforated on K 1-x (NH 4 ) x I mixed crystals with x=0, 0.48 and 0.68 in order to clarify a local structure of the dipole-glassy state. From the analysis with k X (k) spectra, broadening of the peaks indicating the distance between the nearest iodine ions was observed in the glassy state. The back-Fourier transforms of the iodine ions was well reproduced by the one-shell model in the whole concentration and temperature range. The Debye-Waller factor increases largely with increasing concentration, which is caused by the static disorder. The result indicates existence of the local distortion of lattice formed by iodine ions which is induced by reorientational freezing of ammonium ions with only short-range antiferroelectric correlation.

Single crystal (001)-Cu/Ni superlattices were successfully grown on (001) GaAs substrates and (111)- Cu/Ni superlattices on the (111)-yttrium-stabilized zirconium substrates by molecular beam epitaxy. The structure of the superlattice was characterized using two ways of X-ray diffraction geometry. Structural parameters, such as lattice distortion, indistinct layers and fluctuation of the period, were extracted by model fitting to the X-ray diffraction spectra. The elastic features of the Cu/Ni superlattice was characterized by the Brillouin scattering measurement. Clear modulation wavelength dependence of the velocity of surface acoustic wave were seen in (111)-Cu/Ni superlattices. These results were quite different from those previously reported.

The local structure in oxide superconductor La2−xSrxCuO4 was determined by the polarized XAFS study. The structure was found to be different from the averaged structure determined by diffraction studies. Especially, the average distance between copper and oxygen atoms, which increases by 0.02 A at low temperatures, is discussed in terms of the broken inversion symmetry.

Ferroelectric properties depend on the thickness of films. To observe the dielectric properties of ultrathin ferroelectric thin films, a [(SrTiO3)6(BaTiO3)6]2 multilayer thin film with a total thickness of 96 Å was investigated. X-ray diffraction confirmed that it was a well-epitaxial and highly strained film with a perovskite structure. The multilayer film exhibited ferroelectric behavior at room temperature and dielectric behavior above 250°C. However, the dielectric properties of this multilayer film were different from those of the (Ba,Sr)TiO3 bulk mixture crystals and seemed to be influenced by the size effect and ferroelectric coupling across BaTiO3 layers.

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 Å 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.

Sychrotron diffraction study of barium titanate films grown by molecular beam epitaxy method were performed in grazing incidence geometry. Thin barium titanate film with 8 nm thick was psudomorphic nature. Upper part of the film with 40 nm showed that c-axis lay parallel to the substrate surface and in the middle of the film a-axis lay along the surface. The region, where c-axis lies parallel to the substrate, is quite possible to be 90° domain of BT film.

XAFS studies as well as dielectric and X-ray diffraction studies of II-VI semiconductive ternary alloys Cd1-xZnxTe with x=0.0, 0.05, 0.1, 0.2, 0.3, 0.7 and 1.0 were carried out in the temperature range of 10K to 600K. Ferroelectric phase transitions occurred in the mixed crystals with 0.0

Polarized XAFS studies of high-Tc superconductive La2-xSrxCuO4 with x=0.0, 0.09,0.1 and 0.14 were carried out over the temperature range of 15K to room temperature by the transmission method. The c-plate single crystal was used and the polarization vector of the SOR beam was perpendicular to the c-axis and the a-axis in the tetragonal system. At room temperature, polarized XANES spectrum was slightly different from the non-doped La2CuO4-y reported by Oyanagi et al.. A bond length between the copper atom and the oxygen atom was drastically changed below 60K.

The acoustic instability has been studied in several molecular crystals, where the coupling between translational and rotational degrees of freedom plays an important role.

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.

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.

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.

A pulsed X-ray source and a diffractometer used with this source have been constructed. The pulsed X-ray tube can be driven at a peak voltage of 300 kV and a peak anode current of 5 kA to give X-ray pulses with a duration of about 40 ns at a repetition rate of up to 10 Hz. The high voltage is generated by a Marx generator coupled to a pulse forming line. One X-ray pulse contains about 3×106 Cu Kα photons. Some preliminary observations of the diffraction patterns from an LiF crystal have been made using a multichannel photodiode array as a detector. The use of this apparatus as a time-resolved X-ray diffractometer are discussed in this report.