Steven J. Duclos

Crystal Structures at Megabar Pressures Determined by Use of the Cornell Synchrotron Source

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10-9 cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.

The madelung constant for the tetragonal distortion of the cscl lattice

Abstract The Madelung constant is calculated for the tetragonal distortion of the CsCl lattice using Ewalds method over a large c a range. The results deviate considerably from the analytical expressions for the Madelung constant in the literature. As a checkpoint for our calculation, the Madelung constant for the ideal cubic AuCuI structure ( c a = √2) is also computed by direct lattice summation using Evjens method.

Polychromatic x-ray beam produced by a wiggler: A new dimension in x-ray diffraction at megabar pressures

The wiggler provides a substantially higher flux of photons through a given collimator at high energies than is available from the bending magnet at the Cornell High Energy Synchrotron Source. This opens a new dimension in ultrapressure x-ray diffraction studies. Examples are shown in which a good diffraction pattern of gold at 112 GPa (1.12 Mbar) was obtained in 13 s and in which the diffraction pattern for primitive hexagonal silicon at 35 GPa using the beam from the wiggler contains twice as many peaks as is the case for the beam from the bending magnet.