High Pressure Studies

Abstract

In principle, the methods described elsewhere in this volume are available at high pressures (HP). Of course, “in principle” implies the availability of techniques for maintaining samples of a sufficient volume at the HP to allow observation of a signal. Compared to studies on the same material at ambient conditions, neutron scattering at HP is a flux hungry technique. The HP device, irrespective of its design, will necessarily force compromise between signal and parasitic scattering and absorption from the cell. Breakthroughs in HP neutron science therefore depend on novel HP cell and beamline design that maximize pressure, sample volume, and signal to noise discrimination. ### Overview and historical context Successful high-pressure structural studies using neutron scattering techniques date from at least the 1960s (Bloch et al. 1966; Lechner and Quittner 1966; Litvin et al. 1966; Brugger et al. 1967). Although early HP cell designs were installed at reactor sources (Paureau and Vettier 1975) it became clear the geometric restrictions associated with collecting data in the angular dispersive geometry lead to contributions from parasitic scattering from the high pressure cell. Typical of these early designs was the so-called “McWhan cell” (McWhan et al. 1974), a piston cylinder apparatus (Fig. 1⇓) with an alumina cylinder radially supported by compressing two steel plates with a hydraulic ram. This left a neutron window between the plates ~6 mm high and allowed a number of very important studies, with pressures up to 3 GPa achieved routinely. Despite difficulties at the higher pressures with poor counting statistics, and severe peak overlap problems because of diffraction from alumina, several important studies were carried out including careful work on the high pressure phases of ice, D2O-VI, -VII and VIII (Kuhs et al. 1984). Apart from parasitic scattering from the HP cell, another disadvantage was the requirement that …