Isotope effects at the order-disorder phase transition of the (2 × 2)−2H structure on Ni(111)

Abstract

The decrease in critical temperature of the order-disorder phase transition of the 2 × 2−2H phase observed when pure hydrogen is exchanged with pure deuterium gives clear evidence for an isotopic dependence of the lateral interactions in this adsorbate system. The variation of relative HD concentration at constant average coverage of 0.5 results in non-monotonic behavior of T c which can be explained by a finite heat of dissolution of D in H. Both findings can be qualitatively understood as corrections to a simple lattice gas model which take into account the zero point motion by simple perturbation theory.