Structural studies of expanded fluid rubidium up to the supercritical regions

X-ray diffraction measurements have been carried out for fluid rubidium up to enormous high temperatures and pressures beyond the liquid-vapor critical point (Tc=2017K, Pc=12.45MPa) using synchrotron radiation at SPring-8. We have developed a new sample cell made of molybdenum which is resistant to highly corrosive nature of hot alkali metals. Single crystalline molybdenum electrolytically thinned to 30 μm was used as X-ray windows of the cell. The crystal orientation of the disks was controlled to avoid the appearance of Bragg peaks from the cell in the diffraction spectra. All parts of the cell were assembled and brazed together using a high-temperature Ru-Mo alloy. The use of single crystal contributes to reduce the background noise from the cell and made it possible to achieve stable and precise measurements at the supercritical region of fluid rubidium.