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

Abstract

We present the hydrogen (H) solubility isotherms in biaxially compressed and elasticallyconstrained quasi-two-dimensional V lattices. The relative partial enthalpy andentropy, as determined from the isotherms, show a temperature dependence for[H/V]⪅0.1 (atomic ratio). This has not previously been reported for H inbulk 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 inbulk V. The compressibility of the H lattice-gas is derived, and the asymptotic behaviourwhen 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 beTC = 251 ± 1 and , respectively. The effects of the extension of the lattice and the elastic constraints of the Vlayers on the H–H interaction and critical behaviour are discussed.