Liquid-Solid Phase Equilibria in the Hydrogen-Deuterium System [chapter]

David White, J. R. Gaines
1965 Low Temperature Physics LT9  
The experimental investigation of excess thermodynamic properties 1 of liquid solutions of the hydrogen isotopes shows that the deviations from Raoult's law are positive. The magnitudes of the deviations for hydrogen-deuterium mixtures are in accord with the theory of Prigogine, Bingen, and Bellemans, 2 which predicts a continuous range of liquid and solid solutions down to approximately 2°K. However, the recent experiments of Kogan, Lazarev, and Bulatova 3 clearly show a phase separation in
more » ... se separation in the hydrogen-deuterium system at approximately 16.4 °K. The phase diagram shows a peritectic at this temperature corresponding to the coexistence of a liquid solution in equilibrium with two solid solutions of different composition. The results of recent nuclear magnetic resonance experiments 4 in hydrogendeuterium solid mixtures are not in agreement with those of Kogan and collaborators. 3 In order to clarify the phase behavior of mixtures of hydrogen and deuterium in the liquid and solid state, we have re-examined this system calorimetrically. The calorimeter employed in these experiments has already been described in detail. 5 Two types of measurements were made: (1) Heat capacity measurements of several solid mixtures of parahydrogen and normal deuterium from approximately 8°K to their melting points (in order to determine whether or not a phase separation occurs in the solid). (2) Heating curves of several mixtures of parahydrogen and normal deuterium (i.e., temperature-time measurements for a constant rate of energy input into the calorimeter) from the solid to the liquid state (in order to establish the phase boundaries in the region where the liquid and solid coexist). The heat capacities of the solid mixtures are shown in Fig. 1 . The two solid lines represent the heat capacities of pure normal deuterium and pure parahydrogen. It can be seen from Fig. 1 that, to a reasonable approximation, the heat capacities of the mixtures can be represented as a linear combination of the heat capacities of the pure components. This type of behavior can be ascribed either to a continuous range of nearly ideal solid solutions or to a mechanical mixture of the pure components. We believe the former represents the actual physical situation. The heating curves for several mixtures are shown in Fig. 2 . The points A correspond to the discontinuities determined by numerical analysis of the data. The low temperature discontinuity should correspond to the melting point of the mixture, the high temperature one to the freezing point. The phase diagram constructed from points A is shown in Fig. 3 by the broken lines. It should be possible
doi:10.1007/978-1-4899-6443-4_100 fatcat:2hkx727z45cr7axot3bdb644ue