Mean-field-like structural phase transition of H in Fe/V(001) superlattices

We present the hydrogen (H) solubility isotherms in biaxially compressed and elastically constrained quasi-two-dimensional V lattices. The relative partial enthalpy and entropy, as determined from the isotherms, show a temperature dependence for [H/V] <= 0.1 (atomic ratio). This has not previously been reported for H in bulk V and indicates the presence of strong H-H correlations. For [H/V] <= 0.1, the H-H interaction was determined to be roughly three times more attractive than in bulk V. The compressibility of the H lattice-gas is derived, and the asymptotic behaviour when approaching the critical point is shown to be in accordance with mean-field theory. The critical temperature and the critical concentration for transition were determined to be T-C = 251 +/- 1 and [H/V] approximate to 0.0365, respectively. The effects of the extension of the lattice and the elastic constraints of the V layers on the H-H interaction and critical behaviour are discussed.