Interpretation of Hydrogen Solutions in bcc Early Transition Metals

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Abstract
Marked departures from ideal thermodynamic behavior have been observed in dilute solutions of hydrogen in niobium and have been attributed to clustering of hydrogen atoms in the solutions. An attempt has been made in this paper to formulate a general physical picture of hydrogen solutions in the early transition metals and to show how this could give rise to the observed behavior in the niobium–hydrogen and other related hydrogen systems. Factors which are important in these systems are the very large number of interstices available to hydrogen in the body‐centered cubic metal structure and the particular electronic structure of the early transition metals. A distinction has been drawn between dilute solutions in which dissolved hydrogen atoms may interact with each other by means of scattering of conduction electrons and more concentrated solutions in which the electronic structure of the metal is completely altered by the presence of a high concentration of hydrogen atoms in the metal. The transition from one type of behavior to the other would appear to occur in range 2–10 at.% hydrogen. The paper is largely concerned with effects in dilute solutions and with the conditions governing the possible existence of pairs of associated hydrogen atoms in them. A complete analysis of this problem in terms of many‐body theory would appear, at the moment, to present considerable theoretical difficulty, and, therefore, a less rigorous model has been used.