Structure of liquid rubidium under high pressure

X-ray diffraction for liquid Rb has been measured under pressures up to 6 GPa using synchrotron radiation. Volume dependences of the static structure factor S(Q) and pair distribution function g(r) were obtained in the volume range v (= V/V-0) = 1-0.52 to investigate the effect of the electronic change on the ion-ion pair potential and the structure of liquid metals. The first-peak position of S(Q), Q(1), shows different pressure dependences below and above 3 GPa. Below 3 GPa it varies as expected from a uniform-compression model, i.e., proportionally to v(-1/3). Above 3 GPa Q(1) changes slightly less than expected. The first-peak position of g(r), r(1), changes proportionally to v(1/3), while the first coordination number increases with pressure above 3 GPa. These volume dependences of the structural data are compared with those for other metals under pressure and expanded fluids.