ELECTRIC-FIELD-INDUCED TRANSITIONS ON THE (110) SURFACES OF NOBLE-METALS

Electric-field effects on the transition between the reconstructed and unreconstructed states of (I 10) noble-metal surfaces in contact with electrolytes are investigated theoretically. Two models of different complexity are used to describe the transitions between the 1 X 2 ''missing row'' and the 1 X 1 structures. These are the Ising model and the ''step-excitation'' model which is a variant of a solid-on-solid model, used earlier in the description of Ising and roughening transitions on (I 10) noble-metal surfaces in ultrahigh vacuum. A suggested procedure of parametrization of the model parameters with the surface charge of the metal leads, finally, to a phase diagram in the ''temperature-surface-charge-density'' plane. The diagram shows that the transition between the reconstructed and unreconstructed states, which follows the charging of the surface towards more positive values, passes through a narrow intermediate region of charges where the surface is disordered. This is an ''Ising disorder'' and/or roughening, depending on the type of the model and the relation between basic parameters. This analysis explains why the transition induced by tuning the electrode potential in the electrochemical cell is possible at room temperature, whereas these surfaces in ultrahigh vacuum deconstruct at much higher temperatures; it is in line with the published observations of an intermediate disordered region by in situ x-ray diffraction and scanning tunneling microscopy.