CONTINUOUS VARIATION OF IODINE ADLATTICES ON AG(111) ELECTRODES - IN-SITU STM AND EX-SITU LEED STUDIES

The structure of the iodine adlattice formed on Ag(111) in aqueous sol utions containing iodide ions has been determined as a function of the electrode potential by in situ scanning tunneling microscopy (STM) an d ex situ low energy electron diffraction (LEED). In KI solutions buff ered with KF-KOH at pH 10, continuous compression of the iodine adlatt ice from square (root 3 x root R-30 degrees), via (root 3qR beta degre es x root 3R-30 degrees), to (root 3 x root 3)R30 degrees was observed as the electrode potential was increased; this was then followed by a n abrupt phase transition into a rotated hexagonal (root 3r x root 3r) R(30 degrees + alpha degrees) phase. Coadsorption of K was observed by Auger electron spectroscopy (AES) at potentials negative of that for the phase transition. In acid solutions (0.1 mM HI) the continuous com pression from (root 3 x root 3)R30 degrees, via c(p x root 3R-30 degre es), into (root 3r x root 3r)R(30 degrees + alpha degrees) was observe d. The Moire pattern observed for the (root 3r x root 3r)R(30 degrees + alpha degrees) adlattice, as well as the flatness of other adlattice s observed in STM images, is explained in terms of lattice registry on the basis of a ''hard ball contact model''.