Sound velocity and equation of state in liquid cesium at high pressure and high temperature

Abstract

Liquid cesium (l-Cs) sound velocity at high densities was investigated along a 500-K isotherm using high-pressure picosecond acoustics measurements. At 2.0 GPa, the liquid sound velocity goes through a maximum versus pressure without any change on the reflectivity and interferometry acoustic signals. Upon further compression, a softening of the l-Cs viscoelastic properties is observed from 2.0 up to 4.0 GPa, pressure at which the reflectometric signal is abruptly reversed whereas the interferometric signal remains qualitatively the same. This anomalous behavior could be related to an electronic transformation within the l-Cs state, which here again could reflect what happens at lower temperature within the solid state. If so, such liquid-liquid transition may be driven by the progressive collapse of the $6s$ electronic orbital onto the $5d$ ones. Above 4.0 GPa, the l-Cs sound velocity starts again to increase as commonly expected upon compression.