Multiple steps of UPD film formation

The system Cu—Au(l 11) is an example of the formation of a compact and completely discharged UPD film in two steps. The first peak potential represents a phase structure a, which, at a second potential, is transformed into the phase structure fi. The relevant free energy from the formation of the compact layer of metal B is then obtained by the sum of the free energy of formation of phase a and the free energy of transformation of phase a into phase fi. 

The summation must take into account the phase stoichiometry. The formation of each phase refers to the transportation of one mole of metal B into the new structure. The final value of the Gibbs energy of formation is defined for one mole UPD metal in a two-dimensional form. 

The Gibbs energy of formation of the a-phase in the high voltage region is obtained from the first peak potential. This corresponds to the formation reaction 

The Gibbs energy of formation is calculated for one mole Cu transferred in the neutral cell reaction from the bulk into the UPD phase (Eq. (4.66)). 

From the potential of the second peak, the Gibbs energy of transformation of the a-phase into the j -phase can be calculated. The transformation of the a-phase to the jS-phase is described by the reaction 

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