Theory of subsurface occupation, ordered structures, and order-disorder transitions for hydrogen on Pd(111)

The embedded-atom method (EAM) is applied to the calculation of the structure and phase diagram of H/Pd(111). Many-atom interactions are included inherently in the EAM, leading to substrate-mediated adatom interactions. The EAM predicts the occupation of both surface and subsurface sites forming two ({}3 ×{} {}3 )R30^∘{} structures at coverages of CTHETA=(1/3) and (2/3) monolayers. Zero-point energy corrections to the classical energies are required. In addition, the EAM is combined with Monte Carlo simulations to predict the critical temperatures of the order-disorder transformation. The symmetries of the predicted structures agree with the experimental low-energy electron diffraction patterns. The predicted critical temperatures of 120 and 110 K at CTHETA=(1/3) and (2/3) monolayers, respectively, are in excellent agreement with the experimental values of 85 and 105 K.