NEW DEVELOPMENTS IN NEUTRON-SCATTERING METHODS UNDER HIGH-PRESSURE WITH THE PARIS-EDINBURGH CELLS

Neutron diffraction on powder samples can now be performed by time-of-flight methods above 25 GPa. The patterns are obtained in a few hours and yield interatomic distances with a precision of a few thousandths of an angstrom. For this, the Paris-Edinburgh cells are used, at the Test and Polaris stations of the ISIS Facility (UK). The procedures for the generation and characterization of pressure are described, as well as the methods for loading fluid samples, and the performance of the cell at low temperature. The quality of the data depends critically on adequate collimation and shielding of the beams, as well as on the efficiency of the detectors. Precise attenuation corrections must be applied to the raw data, prior to profile refinement procedures, in order to obtain significant structural information and thermal parameters. Results obtained by time-of-flight methods under different experimental conditions are reviewed. The use of the cell for inelastic scattering measurements at a reactor source in angular dispersion is described. The shift of a phonon mode of a single crystal under pressure has thus been measured up to 7 GPa. Prospects are discussed for extending these measurements to a wider domain of pressures and temperatures.