Thermodynamics of liquid cesium‐cesium hydride mixtures at high pressures and temperatures

Abstract

AbstractThe hydrogen pressure in equilibrium with liquid phases of the system cesium‐cesium hydride has been measured between 575 and 750°C at pressures between 30 and 600 bar. The results support the discussion of metal‐nonmetal transition in this system. Apparatus and procedure have been described earlier. An autoclave was used with an outer and an inner compartment with equilibrated hydrogen pressures. A critical solution point was found at 665°C, 157 bar and a mole fraction x(CsH) = 0.68. Below this temperature two liquid phases coexist. Nine isotherms of hydrogen pressure in dependence of cesium‐cesium hydride composition have been determined at sub‐ and supercritical temperatures. The melting temperature of pure cesium hydride has been redetermined to 522°C at nearly 30 bar. A monotectic point exists at 519°C. The width of the two phase region in terms of mole fraction difference, Δx, as a function of subcritical temperature gives a “classical” critical exponent β = 0.5. A careful calculation suggests a “bulge” of the two phase equilibrium boundary curve near x(CsH) = 0.85; this is the region of prominent metal‐nonmetal transition. Activity coefficients and excess functions of Gibbs energy, enthalpy and entropy are presented.