Chemisorbed Coincidence Lattices on Rhodium

Abstract

The previous low‐energy electron diffraction(LEED) work of the author on coincidence lattices formed by chemisorbed monolayers is extended to include two new structures, one formed by oxygen and the other by CO, on the (100) face of Rh. These structures are analyzed and found to be coincidence lattices, that is, high‐coverage structures which are out of step with the substrate lattice in particular crystallographic directions but come into coincidence with it every several substrate lattice positions. The oxygen structure forms a slightly distorted close‐packed arrangement which is out of step in the Rh [11] direction by 1/7 of the Rh‐Rh separation. The CO structure is out of step in the Rh [10] direction by ⅙ of the Rh face‐centered unit cell. It is concluded that the coincidence lattice is a general type of chemisorbed structure and that contrary to the general presumption, and particularly for high‐coverage structures, the chemisorbed species do not always reside in positions of simple coordination with respect to the substrate.