High temperature equilibrium measurements of the yttrium–hydrogen isotope (H2, D2, T2) systems.

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Abstract
Equilibrium pressures as a function of concentration and temperature were measured for the systems, yttrium–hydrogen, yttrium–deuterium, and yttrium–tritium. Temperatures were varied from 700–1000 °C and concentrations in the solid yttrium phase ranged from H/Y or D/Y atom ratios of 0.01 to 2.0 and T/Y ratios of 0.03 to 1.1. Equilibrium isotherms have been plotted and Van’t Hoff isopleths calculated for these systems. Relative partial molal enthalpies and entropies have been tabulated for the reactions of the hydrogen isotopes with yttrium as a function of atom ratio in the solid. In the low concentration range (H/Y=D/Y=T/Y<0.3), the data obey Sieverts law where the concentration of hydrogen isotope in yttrium is proportional to the square root of the hydrogen isotope pressure. Sieverts constants have been derived. At constant atom fraction the tritium compound has the highest equilibrium pressure and the protium compound the lowest. Expressions for the isotopic hydrogen pressure ratios have been derived and agree well with a simple theoretical treatment in which the dissolved hydrogen atoms are regarded as independent oscillators held in tetrahedral potential wells in the yttrium lattice.