ORDER-DISORDER TRANSITION OF OXYGEN ON (111) NICKEL.

We have studied the order-disorder transition of the double-spaced oxygen structure on (111) Ni by LEED, AES, and work function measurements, with particular emphasis on the profiles of the half-order diffraction spots. This appears to be the same transition reported somewhat earlier for this face left bracket C. Davisson and L. H. Germer, Phys. Rev. 30, 705 (1927) right bracket . At room temperature, ordering occurs at a critical coverage about half that corresponding to uniform coverage by the double-spaced structure. The transition temperature increases with coverage to a limiting value of about 415 K. At lower coverages, the sticking coefficient decreases below T//c, but additional oxygen can be incorporated, which gradually extinguishes the half-order spots. Work function measurements show the bonding of the ''excess'' oxygen to be different from the ordered. It is shown that the order-disorder transition can be identified with an abrupt onset of broadening of the half-order spots.