High Temperature — High Pressure Apparatus for Neutron Diffraction on Fluids: Structure Factor of Expanded Fluid Rubidium

The paper describes a new experimental set‐up for neutron scattering experiments on fluid systems at high temperatures and pressures. This technique has been applied for the investigation of the static structure factor S (Q) of expanded fluid rubidium up to 1970 K and 150 bar. The first results obtained up to these conditions show a strong decrease of the intensity of the first peak in S (Q) and a pronounced increase of the scattering at small angles with reducing densities. Within experimental errors no shift in the position of the first peak is found above 900 K. These observations together with the corresponding behaviour of the Fourier transform of S (Q) indicate, that with expansion the distance of nearest neighbours changes only a little, whereas the number of nearest neighbours decreases by about a factor of two, if the density is reduced from 1.2 g cm −3 at 900 K to 0.6 g cm −3 at 1970 K. It is the first time that such a change in the microscopic structure has been studied experimentally for a fluid metal over a wide range of temperatures and densities. The correlation between the increase in S (O) and the change in the mean interatomic interaction is briefly discussed.