DYNAMIC LOW-ENERGY ELECTRON-DIFFRACTION INVESTIGATION OF LATERAL DISPLACEMENTS IN THE TOPMOST LAYER OF PD(110)

Based on He-atom diffraction evidence for an order-disorder transition on Pd(110) at 230 K, other authors have proposed a model in which the atoms of the topmost layer are displaced laterally by 0.7 angstrom al ong the [001] direction, with the directions of the displacements corr elated at T < 230 K, but largely uncorrelated at T > 230 K. To test th is model, we have examined the proposed ordered phase by dynamical low -energy electron-diffraction analysis. Our results favor a nonlaterall y displaced geometry, with a contraction of 4.4+/-1.5% in the first in terlayer spacing and an expansion of 1.5+/-1.5% in the second interlay er spacing, but do not conclusively rule out the possibility of latera l displacements < ca. 0.13 angstrom. Nevertheless, lateral displacemen ts of the magnitude required to account for the He-atom diffraction re sults are clearly not present.