Kozaburo Tamura

Energy-dispersive x-ray diffraction equipment for fluids at extreme conditions of high temperatures and high pressures

An energy-dispersive x-ray diffraction technique for fluid materials under high pressure, up to 2000 bar, and high temperature, up to 1650u2009°C, has been developed using an x-ray source with high energy up to 70 keV, a high-pressure vessel with Be windows pressurized with He gas, and a new sample cell made of single crystal sapphire. Using this technique one can overcome several difficulties intrinsic with x-ray diffraction measurements of fluids, such as high vapor pressure and high x-ray absorption constant of samples, x-ray scattering and absorption by sample cells, and x-ray absorption by high-pressure vessels. In order to show an excellent potential of the equipment, examples of the experiments for the liquid-vapor subcritical fluid Hg and Se are reported.

X-ray diffraction measurements for expanded liquid mercury in the metallic region

X-ray diffraction measurements for expanded liquid Hg have been carried out for the first time in the density range from 13.55 to 10.26 g cm-3, covering almost all of the metallic region of expanded liquid Hg. The authors have developed a new type of high pressure vessel and a new sample cell made of single-crystal sapphire for the X-ray diffraction measurements using the energy-dispersive method. They have obtained the structure factor and the pair distribution function as functions of density. It was found that the nearest-neighbour distance remains almost constant increasing slightly, less than 1%, with the reduction of density by about 24%. On the contrary, the coordination number decreases substantially and almost linearly with decreasing density. The volume expansion of liquid Hg in the metallic region is not accompanied by a uniform increase of interatomic distance but is mainly caused by a decrease of coordination number.

EXAFS measurements of liquid Se-Te mixtures

The authors have measured EXAFS spectra of liquid(1-)Se, Te, Se-Te mixtures and vapour (v-)Se for the first time. It is clearly observed that the two-fold coordinated chain structure is largely preserved in l-Se and that the intrachain covalent bond contracts on melting. The EXAFS spectra of l-Te can be reproduced well by assuming a two-fold coordinated chain structure with shorter and longer bonds within the chain. In order to obtain partial structure-parameters in liquid Se-Te mixtures, spectra are taken on both Se and Te K edges. The Se-Se bond elongates with increasing Te concentration. The number of the longer Te-Te bonds increases with increasing temperature or increasing Te concentration; this may be associated with the semiconductor-to-metal transition.

Structural changes and the metal-non-metal transition in supercritical fluids

Energy-dispersive x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and x-ray absorption fine-structure (XAFS) measurements for metallic fluids up to the supercritical region were carried out using synchrotron radiation. We obtained the structure factor S(k) and the pair distribution functions g(r) for expanded fluid Hg from the liquid to the dense vapour region including the metal-non-metal (M-NM) transition region in the density range from 13.6 to 1.9xa0gxa0cm-3 by means of XD measurements at SPring-8 in Japan. The density variations of the interatomic distance (r1) and coordination number (N1) obtained are discussed in relation to the M-NM transition in fluid Hg. To investigate structural change in the semiconductor-metal (SC-M) transition in expanded fluid Se, XD measurements, at SPring-8, and XAFS measurements, at the European Synchrotron Radiation Facility (ESRF) in France, were carried out at high temperatures and high pressures. It was found that the twofold-coordinated chain structure is preserved and contraction of the covalent bond occurs on the SC-M transition. XAFS measurements for dense Se vapour near the critical point were also carried out to study how dimers in the rarefied vapour condense to the metallic fluid. SAXS measurements were carried out to obtain information on the density fluctuation of fluid Se near the critical point. On the basis of the structural data obtained by XD, XAFS and SAXS measurements for fluid Se, we discuss how the density fluctuation affects the local structure and electronic properties of fluid Se near the critical point.

X-ray diffraction measurements for liquid As2Se3 up to the semiconductor-metal transition region

Abstract X-ray diffraction measurements were performed for liquid (1-) As2Se3 at temperatures and pressures up to 1500°C and 70 bar. A high-pressure vessel and a sapphire sample cell were developed, according to the authors own design, for measurements using the energy-dispersive method. The structure factor becomes broadened with increasing temperature and changes significantly at about 1000°C accompanied by the semiconductor-to-metal (SC-M) transition; the first sharp diffraction peak around 1.2A−1 disappears and the second and third maxima merge. The nearest-neighbor distance and the average coordination number start to increase when the SC-M transition occurs. The density of 1-As2Se3 in the temperature and pressure range up to 1600°C and 700 bar, respectively, were also measured by the X-ray absorption method. A volume contraction occurs in the SC-M transition region.

Semiconductor-metal transition in fluid selenium near the critical point

Abstract In the present paper we discuss the aspects and mechanism of the nonmetal-metal transition in fluid Se which occurs near the liquid-gas critical point. A brief review is presented on the unique and striking behavior in the electronic, structural and thermodynamic properties around the transition. New results of the optical absorption and X-ray diffraction measurements using techniques developed in our laboratory are also presented. The optical absorption coefficient of fluid Se was measured at temperatures up to 1700 °C and pressures up to 500 bar. Using the cell with the sample thickness of 3000 A we obtained the absorption coefficient in the wide range extending to the value of 2×10 5 cm −1 . The optical gap, derived from the analysis of the spectra, substantially decreases with temperature and pressure eventually to vanish near the critical point. X-ray diffraction measurements were performed at temperatures up to 1500 °C and pressures up to 510 bar by the energy dispersion method. From the detailed analysis of the pair distribution function we conclude that the twofold coordinated structure is largely preserved in the region where the optical gap disappears. On the basis of the findings we speculate the existence of the small chain molecule with the planar zig-zag conformation which may cause the transition to the metallic state.

Structural changes and the metal-non-metal transition insupercritical fluids

Abstract Experimental results on the structural properties of expanded fluid Se are described. Energy-dispersive X-ray diffraction measurements have been carried out up to the supercritical region including the semiconductor-metal transition region. It has been established that the twofold coordinated structure is largely preserved even in the metallic region near the critical point. The results of XAFS, XSAS and density measurements up to the supercritical region of fluid Se are also presented. On the basis of these results, the existence of small chain molecules with planar zig-zag conformation which may cause the transition to the metallic state in fluid Se is postulated.

XAFS measurements at high temperatures and pressures

The method of XAFS measurements at high temperatures and pressures for supercritical fluids with high critical constants have been developed. An internally heated high‐pressure vessel made of super‐high‐tension steel and a polycrystalline sapphire cell of our own design permit XAFS measurement up to 800 bar. XAFS experiments were carried out at high temperatures up to 1600u2009°C, and high pressures up to 150 bar. XAFS spectra of liquid Se at 1300u2009°C and 150 bar, and dense Se vapor at 1600u2009°C and 150 bar have been obtained and clear oscillations are observed at these temperatures.The method of XAFS measurements at high temperatures and pressures for supercritical fluids with high critical constants have been developed. An internally heated high‐pressure vessel made of super‐high‐tension steel and a polycrystalline sapphire cell of our own design permit XAFS measurement up to 800 bar. XAFS experiments were carried out at high temperatures up to 1600u2009°C, and high pressures up to 150 bar. XAFS spectra of liquid Se at 1300u2009°C and 150 bar, and dense Se vapor at 1600u2009°C and 150 bar have been obtained and clear oscillations are observed at these temperatures.

Extended x‐ray absorption fine structure studies of dense selenium vapor

We have measured the extended x‐ray absorption fine structure spectra for dense selenium vapor at high temperatures and pressures up to 1600u2009°C and 150 bar for the first time. For the experiment, we have developed a high‐pressure vessel and a polycrystalline sapphire cell of our own design. We have found that dense selenium vapor under the present experimental condition consists mainly of Se2 and the bond length is much shorter than that of liquid Se. With increasing pressure the bond length slightly increases. When the saturated vapor‐pressure curve is approached, the bond is largely elongated.

Optical absorption spectra of liquid As2S3 and As2Se3 over a wide temperature range

The authors have measured the optical absorption coefficient of liquid As2S3 and As2Se3 up to 900 degrees C in a wide absorption range up to about 3*105 cm-1. They have developed an optical cell of their own design with a sample thickness of about 2000 AA. The optical gap obtained from the spectra decreases rapidly with increasing temperature, eventually becoming zero at about 950 degrees C for liquid As2Se3, where the semiconductor-to-metal transition starts to occur.