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

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.

Figure 1

Longitudinal wave velocities of l-Cs as a function of pressure for the present work ($T=500\mathrm{K}$, two runs, black and red open circles; dotted line, as guides for the eyes), plotted along with low-pressure ultrasonic data on l-Cs at 500 K from Ref. [28] (black line) and ultrasonic measurements on solid-Cs at 300 K from Ref. [29] (crosses). The maximal uncertainties associated with the relative sound velocity $\mathrm{\Delta }{v}_l/v_l$ is less than 0.5% [22, 26].

Figure 2

Reflectivity change of l-Cs at 500 K as a function of the optical probe-pulse time delay. The modification of the optical reflectivity due to the acoustic strain has been recorded around 2 GPa using two types of detection: reflectometry (a) and interferometry (b). Same but around 4 GPa are given in (c) and (d) respectively. The pulse shape qualitatively varies (shifted in phase by about $\pi$) only in the vicinity of 4 GPa and using a reflectometric detection.

Figure 3

Density ($\rho$) of l-Cs at 500 K as a function of pressure deduced from present measurements, and compared to previous studies (x-ray-diffraction results from Falconi [17] and x-ray-absorption from Hattori [19]). The maximal uncertainties associated with the relative densities $\mathrm{\Delta }\rho /\rho$ is estimated at about 2% [22, 26]. The two black stars correspond to solid densities [29] at the pressure where two maxima are observed on the melting line, where both solid and liquid phases have the same density. We here emphasize the clear agreement between our data and those from Hattori [19].