R. Poloni

Optimization of Paris–Edinburgh press cell assemblies for in situ monochromatic X-ray diffraction and X-ray absorption

We describe some important improvements allowed by the development of new cell assemblies coupled to opposed conical sintered diamond anvils in the Paris–Edinburgh press. We provide X-ray absorption and diffraction experiments carried out at pressures up to 16.5 GPa. The maximum temperature reached was 1800 K for P<10 GPa and 1300 K for higher pressures. The sintered diamond anvils are X-ray transparent and give access to a much larger X-ray window than the tungsten carbide anvils, even at the highest pressure. Therefore, X-ray measurements are performed using in situ cross-calibration simultaneously. We also describe a new heating setup used to reach high temperatures, despite the low conductivity of the sintered diamond core by deviating the electrical current using copper strips. These improvements are illustrated by recent data collected using angle dispersive in situ X-ray diffraction on liquid Fe-18%wt S and using EXAFS at the barium K-edge on Ba8Si46 silicon clathrates and at the iodine K-edge on iodine-intercalated nanotubes.

A diamond anvil cell with resistive heating for high pressure and high temperature x-ray diffraction and absorption studies

In this paper we describe a prototype of a diamond anvil cell (DAC) for high pressure/high temperature studies. This DAC combines the use of a resistive oven of 250 W power in a very small volume, associated with special conical seats for Boehler-type diamond anvils in order to have a large angular acceptance. To protect the diamond anvils from burning and to avoid the oven oxidation, the heated DAC is enclosed in a vacuum chamber. The assemblage was used to study the melting curve of germanium at high pressure (up to 20 GPa) and high temperature (up to 1200 K) using x-ray diffraction and x-ray absorption spectroscopy.

Amorphization of Rb6C60 and Cs6C60 under high-temperature and high-pressure conditions

X-ray diffraction measurements have been performed on Rb6C60 and Cs6C60 compounds at a constant pressure around 5 GPa as a function of temperature. The data show the occurrence of an irreversible transition starting at 900 K for Cs6C60 and 1300 K for Rb6C60 leading to a progressive amorphization of both compounds. Raman data collected at ambient conditions on the recovered samples show that such transition is accompanied in both cases by the loss of the molecular character of the system.

Metastable Bi under Extreme Conditions Investigated by Combined XAS and XRD

We report the results of a detailed experimental study carried out at the European Synchrotron Radiation Facility (ESRF) providing an unprecedented insight into the metastable phase diagram of Bi. In particular we focus the attention on a novel metastable crystalline structure of Bi compatible with the β‐tin structure. It is reasonable to identify the new structure with the unknown stable Bi‐II’ structure. These findings have been achieved using the single energy x‐ray absorption detection (SEXAD) technique and angular dispersive x‐ray diffraction (ADXD) both combined with a “Paris‐Edinburgh” V5 large volume press at the BM29 beamline. The results obtained show that the combination of SEXAD and ADXD is a unique and reliable tool for detecting subtle structural modifications in condensed matter.