Hydrogen and deuterium in vanadium as lattice gases with repulsive short-range interactions

Pressure-composition isotherms at temperatures between 150 and 5500C for solutions of hydrogen and deuterium in vanadium and the temperature dependence of the heat capacity in a series of V-D alloys have been determined in the single phase regions (α-phase) of the V-H(D) phase diagrams. The concentration dependences of the partial molar enthalpies Δanti HH(D) and entropies Δanti SH(D) of hydrogen and deuterium have been derived from the solubility data. For both isotopes Δanti HH(D) decreases with increasing H or D concentration x = [H(D)]/[V] at low x and increases a higher H or D content, having a minimum near x≅0.4. The concentration dependence of Δanti SH(D) shows a large negative deviation from the ideal configurational entropy for random occupation of tetrahedral interstitial sites. The heat capacity of V-Dx contains a substantial excess contribution, which remains when estimated electronic and vibrational contribution are subtracted from the experimental data. The experimental results are analyzed by means of Monte Carlo simulations of a lattice gas with repulsive short range interactions between hydrogen (deuterium) atoms. The essential features of the experimental results are described by a lattice gas model which includes hard core interactions (blocking effect) and finite repulsive interactions outside the hard core. (orig.)