FIGURE 2. In-situ structural characterization of the passivation layer.

a, Diffracted intensity along q in units of (1, - 1,0) for varying electrochemical potential; the statistical error bar is below the symbol size. A Bragg peak from the epitaxial passivation layer is growing at (1.89, - 1.89,0); in addition the (2, - 2,0) substrate peak is observed. b, Experimental reciprocal space map in the (1,1, - 2)/(1,1,1) plane characteristic for the potential regime from -100 mV to 270 mV. q is given in units of 1/3(1,1, - 2). The Bragg reflections from the passivation layer and the Cu₃Au substrate are observed and from the shift of the Bragg peak positions between the substrate and the film along q, it can be inferred that an epitaxial, single-crystalline overlayer with inverted CBA stacking is forming. c, Integrated intensities of the ultrathin passivation layer along q (q = (1.89, - 1.89,0)); the red line represents the best fit to the data based on the structural model depicted in Fig. 1b. Solid lines indicate the q position for a layer with CBA stacking, dashed lines the q position for an ABC stacking.