Hugoniot overtake sound-velocity measurements on CsI

Abstract

Shock-wave overtake experiments have been used to confirm that CsI melts along the Hugoniot curve at a pressure of roughly 30 GPa. The experimental results give the pressure dependence of the bulk sound velocity in the fluid along the Hugoniot curve to 150 GPa, from which the pressure dependences of both the adiabatic bulk modulus and the Grüneisen parameter γ can be calculated. These quantities show normal behavior, with no obvious indication of the band closure and metalization which are predicted for this region of the Hugoniot curve. The results of these experiments are combined with the shock-wave optical pyrometry data of Radousky et al. [Phys. Rev. B 31, 1457 (1985)] to obtain results for the variation along the Hugoniot curve of both the constant-volume heat capacity (which increases by 30% from the melting line to 90 GPa) and the isothermal bulk modulus. The large values of $C_V$, which are roughly twice the classical value for a solid, are the only indications of unusual behavior for CsI. Thermodynamic properties along the Hugoniot curve are consistent only with the diamond-anvil isotherm which used solid xenon as a pressure-transmitting medium.