ISOTOPE EFFECTS AT THE ORDER-DISORDER PHASE-TRANSITION OF THE (2X2)-2H STRUCTURE ON NI(111)

The decrease in critical temperature of the order-disorder phase trans ition of the 2 x 2-2H phase observed when pure hydrogen is exchanged w ith pure deuterium gives clear evidence for an isotopic dependence of the lateral interactions in this adsorbate system. The variation of re lative 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 understo od as corrections to a simple lattice gas model which take into accoun t the zero point motion by simple perturbation theory.