X-RAY DIFFRACTION OF LIQUIDS AND GLASSES AT HIGH PRESSURE. Willem L. Vos¹ and Marco J. P. Brugmans². ¹van der Waals-Zeeman Instituut, Universiteit van Amsterdam, 1018 XE Amsterdam, The Netherlands; ²FOM Instituut voor Atoom- en Molecuulfysica, 1098 SJ Amsterdam, The Netherlands

In contrast to crystals, the usual subject study of crystallography, the constituent atoms of liquids and glasses have random spatial positions. Nevertheless, there is still some degree of (local) order that can be described with statistical distribution functions, e.g. the radial distribution function g(r). This function is proportional to the probability of finding an atom at a distance r from a central atom. The Fourier transform of this function is the structure factor S(Q), that can experimentally be measured.

Structural studies of liquids and glasses at high pressure are very scarce, which is probably due to the experimental difficulties: the scattered signal from amorphous samples is much broader than crystalline diffraction peaks. Thus, the signal is weak and hard to distinguish from the background caused by the relatively large cell windows.

The use of high-brightnes synchrotron x-ray radiation is an obvious choice, because samples in high pressure environments are usually very small, and we are interested in systems consisting of light elements. In this talk, we will discuss several sources of systematic errors. The main experimental situation aimed at is monochromatic scattering in combination with image plates (Daresbury SRS 9.1). Comparisons will be made to position sensitive detectors and to polychromatic scattering detected with solid state detectors. Illustrations will be provided from our recent work on liquid and glassy methanol [1].

[1] M.J.P. Brugmans and W.L. Vos, J. Chem. Phys. 103, 2661 (1995), and to be published.