K Matsuda

Wide angle X-ray scattering measurements of supercritical water using synchrotron radiation

Wide angle X-ray scattering measurements of sub- and supercritical water have been carried out using synchrotron radiation at SPring-8 in Japan, to study density dependence of the local structure. X-ray diffraction spectra were measured from 300 K to 1023 K at 40 MPa, and up to 1100 K at 100 MPa, corresponding to the density ? from 1 g cm?3 to 0.10 g cm?3. The nearest neighbour distance and the nearest neibour coordination number depending on the density indcate that water is uniformly expanded from 1 g cm?3 to 0.6 g cm?3 with destruction of hydrogen bonding. With further volume expansion, the coordination number becomes approximately one at 0.15 g cm?3 while the nearest neighbour distance is approximately 3.3 ?. This experimental fact may be a first direct observation of dimeric molecules in the supercritical water.

X-ray Compton scattering experiments for fluid alkali metals at high temperatures and pressures

We have developed a high-pressure vessel and a cell for x-ray Compton scattering measurements of fluid alkali metals. Measurements have been successfully carried out for alkali metal rubidium at elevated temperatures and pressures using synchrotron radiation at SPring-8. The width of Compton profiles (CPs) of fluid rubidium becomes narrow with decreasing fluid density, which indicates that the CPs sensitively detect the effect of reduction in the valence electron density. At the request of all authors of the paper, and with the agreement of the Proceedings Editor, an updated version of this article was published on 10 September 2015. The original article supplied to AIP Publishing was not the final version and contained PDF conversion errors in Formulas (1) and (2). The errors have been corrected in the updated and re-published article.

Structural fluctuations in expanded fluid Se accompanying the semiconductor-metal transition

We carried out small angle X-ray scattering (SAXS) measurements for expanded fluid Se up to the semiconductor-metal (SC-M) transition region and found that there appears a broad peak in the SAXS profiles in the temperature range from 1273 K to 1873 K at 60 MPa in addition to the critical scattering with the Ornstein-Zernike behaviour. The spectra were fitted using a model function and two characteristic lengths, the domain size of the dense and rare regions, and the correlation length concerning with the damping behaviour, were obtained. The present results suggest peculiar density fluctuations accompanying the SC-M transition in fluid Se.

Collective dynamics of liquid Fe

The dynamic structure factor S(Q,W) of liquid Fe was measured near the melting point of 1570°C using high-resolution inelastic x-ray scattering spectrometer at BL35XU/SPring-8. Although liquid Fe has large values of the ratio of heat capacities γ and the viscosity η, the S(Q, W) spectra show well-defined collective excitations. The excitation energy WQ of the inelastic excitation modes was obtained within the generalized Langevin formalism. A positive dispersion of the sound velocity, as compared to the hydrodynamic value by about 20 %, was found as in other liquid metals. The η(Q) values decrease rapidly with Q, which may result in the well-defined collective excitations.

Small-angle x-ray scattering and density measurements of liquid Se50−Te50 mixture at high temperatures and high pressures using synchrotron radiation

We have carried out small-angle x-ray scattering and x-ray transmission measurements of liquid Se50−Te50 mixture at SPring-8 in Japan and obtained the structure factor S(Q) at small-Q region (0.6 < Q < 3.5 nm−1) and the density at high temperatures and high pressures up to 1000 °C and 180 MPa. We report preliminary results in this paper. With increasing temperature, the density shows a minimum at around 500 °C and a maximum at around 700 °C. On the other hand, S(0) becomes maximum and S(Q) strongly depends on Q at around 600 °C, which is about the middle temperature where the density shows the minimum and maximum. The temperatures shift to lower side when the pressure increases. These results prove that, with increasing temperature, the sample exhibits gradual transition from low-density structure to high-density structure, which causes mesoscopic density fluctuations in the intermediate temperature region.

Small-angle x-ray scattering of supercritical fluid Hg: Bi-impurity effect

We have carried out small-angle x-ray scattering and x-ray transmission measurements of supercritical fluid Hg-0.2%Bi and pure Hg systems at SPirng-8 in Japan. We have obtained the static structure factors S(Q) in the low-momentum transfer region (0.5< Q <3 nm−1) for both samples at temperatures and pressures up to 1600 °C and 200 MPa. A Bi-impurity effect can be seen in both S(Q) and density deduced from x-ray transmission, only in the high temperature and high pressure region; the sign of the phase separation is seen near the critical density for Hg-0.2%Bi sample. We have analyzed S(Q) by Ornstein-Zernike formula and deduced the structure factors in the long-wavelength limit S(0), correlation lengths ξ, and corresponding short-range correlation lengths R = ξ≡/√S(0). Temperature variation of R is very similar to that of the density in each sample. This experimental fact hints that the variation of R is highly dependent on its metallic nature because the density is the most relevant parameter for the electronic features in fluid Hg and its impurity systems.

X-ray diffraction measurements of expanded fluid sulfur up to the supercritical region

The structure factor, S(Q), of fluid sulfur was measured up to 1573 K and 30 MPa using synchrotron radiation at SPring-8 in Japan. The pair distribution function, g(r), deduced from S(Q) exhibits the first peak well separated from higher coordination shells in the entire density region from liquid to dense vapour. At 0.6 g cm−3 near the critical density the coordination number of the first peak is one but the bond length remains approximately 2 A, longer than that of an isolated S2 molecule. These results hint a strong intermolecular interaction between S2 molecules in the dense vapour phase near the critical density in the supercritical region.