Electrochemical Control of a Superheavy Domain-Wall Lattice in an Adsorbed Halide Monolayer

Abstract Specific adsorption of halides at electrified metal/electrolyte interfaces provides a model platform for understanding how two-dimensional interfacial solids accommodate electrochemically induced misfit. A central unresolved question is whether potential-driven increases in adsorbate density are accommodated by homogeneous lattice compression or by insertion of a domain-wall, or solitons, lattice─a distinction that is particularly difficult to resolve under liquid-phase conditions. Here we use in situ electrochemical scanning tunneling microscopy to show that the centered-rectangular striped phase of iodine on Au(111) is best described as a potential-tunable superheavy domain-wall lattice rather than a uniformly compressed overlayer. Increasing electrode potential drives continuous uniaxial electrocompression, shortening the stripe period from approximately (8a) to (4.5a) and increasing the coverage from 0.374 to 0.404 ML before the transition to the rotated-hexagonal phase. Reciprocal-space analysis links this evolution to soliton insertion, while atom-resolved imaging shows that the walls are not abrupt but remain strongly relaxed over several rows. Apparent noninteger wall indices arise from coexistence of neighboring integer stripe spacings within the STM field of view, rather than from fractional structural units. Local nearest-neighbor analysis further reveals that the iodine–iodine distance distribution broadens as the wall spacing decreases, showing that higher electrochemical compression produces a more heterogeneous wall-related strain field. These results establish the centered-rectangular phase of I/Au(111) as an atomically resolved example of potential-controlled soliton formation at a solid–liquid interface. More broadly, they show that electrochemical interfaces can accommodate potential-driven coverage changes through insertion of relaxed domain walls, providing a mechanistic picture for adsorbate ordering, strain relief, and phase evolution in two-dimensional interfacial solids.