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Abstract:
Low energy electron diffraction from a clean and carbon contaminated Pd{110} surface has been studied over the temperature range 100-800 K and evidence for a phase transition on the clean surface near 250 K is found. Residual carbon on or in the surface layer causes shifts in the energies of peaks in intensity versus energy spectra which correspond to an average outward expansion of about 4% for the first 3-4 interplanar spacings. Carbon segregates to the surface for temperatures in the range 400-600 K. The clean Pd{110} surface is obtained only after extensive cleaning cycles and shows a (1 × 1) diffraction pattern over the entire temperature range. The possible phase transition is indicated by subtle changes in the temperature dependence of both the diffraction beam intensities and widths. The diffraction beam widths increase below T ≈ 250 K, indicating a decrease in average domain size by about 25% during the transition. The transition is sensitive to small amounts of impurities.
