A theoretical investigation of the high temperature phase change behavior of hydrogen in niobium is described. In an effort to understand the interplay between the hydrogenndash;hydrogen interactions considered and the appearance of the agr;, agr;prime;, bgr;, and egr; phases, a series of latticehyphen;gas model calculations of the phase diagram is performed. It is shown that the pairwise interactions at the first three shells beyond the hard core must satisfy certain constraints in order to obtain the agr;rarr;agr;prime;rarr;bgr; sequence of phases at high temperatures. A set of interactions which gives the best predictions for the agr;ndash;agr;prime;ndash;bgr; phase boundary is described. It does not appear possible to produce a set of pairwise interactions of range 21/2awhich simultaneously yields the agr;, agr;prime;, bgr;, and egr; phases. This indicates that multibody interactions are of nonnegligible significance at higher hydrogen concentrations.
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