Phase Transitions in the Chemisorbed 2-DIMENSIONAL Systems, Oxygen on (111)NICKEL.
Abstract
The complete phase diagram of the two-dimensional system of oxygen chemisorbed on the (111) surface of nickel has been determined. The two dimensional system is realized for oxygen coverages of less than half of monolayer and temperatures below 500K. Outside these limits, oxygen diffuses into the nickel lattice. Low energy electron diffraction was used to determine the structure of the oxygen overlayer and the coverage was monitored by Auger electron spectroscopy. Changes in structure and coverage were correlated with work function changes. At low temperatures and coverages oxygen forms ordered p(2 x 2) islands, demonstrating the existence of a long range attractive interaction between adatoms. These islands disorder by a first order transition at temperatures of about 300K. As the coverage approaches saturation of 0.25 monolayers, however, the transition temperature increases abruptly up to 440K, and the chemisorbed overlayer exhibits a continuous reversible order-disorder transition. Short-range order persists above the transition temperature and resulting critical scattering can be observed. At coverages in excess of 0.27 the (1/2 1/2) beams broaden and split. The separation of the split components increases continuously with coverage as a result of antiphase boundaries between domains of the p(2 x 2) structure. For temperatures below 300K an abrupt transition to a well-ordered (SQRT.(3 x SQRT.(3)R 30(DEGREES) structure is observed at coverages of 0.29 monolayers. This phase is surrounded by a first order boundary.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1980
- Bibcode:
- 1980PhDT.......129K
- Keywords:
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- Physics: Condensed Matter