Measurement of the Structure Factor of Liquid Rubidium by Neutron Diffraction up to 1400 K and 200 bar

The structure factor S(Q) of liquid rubidium has been measured by neutron diffraction in the temperature range between 450 K and 1400 K corresponding to a density change from 1.42g/cm 3 to 0.98 g/cm 3 . Measurements have been performed with a diffractometer for elastic diffuse scattering of neutrons at the high flux reactor in Grenoble. To avoid evaporation of the liquid metal at high temperatures; a new technique had to be developed. A detailed description is given of the experimental set‐up which is suitable for neutron diffraction measurements of corrosive liquids at elevated temperatures and pressures. It is the first time that neutron diffraction measurements on a liquid metal have been extended to these combined conditions of high temperature–high pressure, as are reported in this paper. The momentum transfer Q covered in this investigation ranges from 0.2 Å −1 to 2.4 Å −1 . As a function of temperature and density the following significant changes in S( Q ) have been observed: the height of the first maximum decreases from about 2.0 at 450 K to 1.12 at 1400 K; in the same temperature limits S(Q) at the lowest Q ‐values measured increases by a factor of 6; a clear shift in the position of the first maximum from 1.525 Å −1 to 1.43 Å −1 is found. Up to 1200 K three different pressures, saturation pressure, 100 bar, and 200 bar have been studied. Corresponding to the relatively small compressibility of liquid rubidium at these temperatures, no appreciable pressure dependence of S(Q) at the different constant temperatures was observed. The accuracy of the measured structure factor data is chequed at low Q ‐values by a comparison of compressibilities of liquid rubidium determined from recent density measurements with those extracted from our S(Q) ‐values according to the Ornstein‐Zernike relation.