Hydrogen equilibrium separation factors in metal/hydrogen systems, a synopsis on the basis of the localized harmonic oscillator model

Abstract

AbstractThe isotopic equilibrium separation factor between gas phase hydrogen and hydrogen in metals is calculated for H‐D mixtures and H‐T mixtures between 150 K and 1000 K. Parameter for the calculations is the vibrational energy of hydrogen in interstitial lattice positions. The interstitial hydrogen isotopes are treated as localized Einstein‐oscillators of three‐fold degeneracy. The influence of lower point‐symmetries of interstitial positions is discussed by way of the Ta/hydrogen system as an example. The separation factors evaluated relate to tracer conditions as established with vanishing concentrations of deuterium (tritium). It is shown, how the separation factor for optional deuterium (tritium)‐concentrations in the gas phase is obtained from the tracer separation factor in question. — Special attention is drawn to the fact that metal/hydrogen systems, which dissolve hydrogen in tetrahedral interstitial positions, invert the direction of the isotope effect at a certain temperature. Inversion temperatures are given as a function of the local mode energy. Systems with H in octahedral (tetrahedral) positions are discussed with respect to technical utility. — The calculations are examined by means of experimentally determined separation factors and in the light of recent neutron vibrational spectroscopy work on metal/hydrogen systems.