Isothermal Compressibility and the Structure Factor of Liquid Alkali Metals

Data and calculations are presented which indicate that the isothermal compressibility of alkali metals is determined mainly by the repulsive part of the ion-ion potential. The speed of sound in rubidium was measured over the temperature range 56-260^∘{} C, and the isothermal compressibility, κT, was calculated. An effective hard-sphere diameter, σ, was determined by using the experimental value of κT in the compressibility relation obtained from the hard-sphere solution of the Percus-Yevick equation. Agreement with the values of σ obtained by Ashcroft and Lekner from neutron diffraction data is good. The hard-sphere structure factor based on values of σ obtained from compressibility data was used to calculate the temperature dependence of the resistivity of liquid rubidium. Finally, using the hard-sphere model, the concentration dependence of the compressibility of Na-K alloys was calculated.