Scanning tunneling microscopy and low energy electron diffraction studies of mixed O + N layers on Rh(llO) reveal a competition between («xl) N-driven and (1x2) O-driven added/missing row reconstructions. At low coverage [001] N-Rh rows prevail, forming islands with a (3x1) periodicity. At moderate coverage separate (2xl)N and (lx2)0 islands are formed. At high coverage the growing (lx2)0 islands displace those of (2xl)N. The structural transformations involve a transition from a structure consisting of separate (2x 1)N and (1 x2)0 islands to a mixed <:(2x2) N + O layer on a (1 x 2) reconstructed surface. PACS numbers: 68.35.Bs, 61.14.Hg, 68.35.Dv, 68.35.Rh The (110) planes of most fee transition metals are unstable and often undergo restructuring in the presence of adsorbates. The potential of scanning tunneling microscopy (STM) to reveal details of reconstructive interactions in single component systems has already been demonstrated, e.g., for O with Cu(llO) [1-4], Ni(llO) [5], Rh(llO) [6], N with Cu(llO) [7], Ni(llO) [8l, and Rh(l 10) [91, S and H with Ni(l 10) [5]. "Real" systems usually contain more than one adsorbate, where each can favor a different type of reconstruction. However, structural information at the atomic level for coadsorbate systems is scarce. One example where such information has been given is from an STM study of O coadsorption over (2x3)N layers on Cu(llO) [10]. Here we address the complex structural changes induced by mixed adatom layers produced by dissociative molecular adsorption. Since dissociative adsorption is a very common phenomenon it is important to understand the impact of adlayer composition on the surface structure and the driving forces behind the structural changes taking place in multicomponent adsorbate systems.Recently, we have studied the interaction of atomic oxygen and nitrogen with Rh(llO) [6,9,[11][12][13][14][15][16][17][18][19]. O induces a (1 XAI) reconstruction of the Rh(l 10) surface where n is an integer changing from 2 to 5 in the coverage range 0.5-1.0 monolayer (ML). STM studies have shown that the (1 XAT) periodicity is due to the absence of every nth [iTO] row [6]. In contrast, N induces a (nx\) added row reconstruction with /7=2 at saturation coverage of 0.5 ML [9]. Mixed O + N layers show a quite complex behavior. When equal amounts of O and N are deposited several ordered phases can be observed. The sequence of low energy electron diffraction (LEED) patterns with increasing N+O coverage from 0 to 1 ML is (3x1) with streaking in the [001] direction, mixed (2x1) +(1x2), and c(2x4) [15,16]. The models proposed to explain these structures involve (3x1) and (2x1) N-induced and (1x2) O-induced reconstructions [15], but no detailed structural studies have been performed to justify these models.In this Letter we report direct observations of the complex structural behavior of mixed O + N layers on Rh(l 10). STM images of the various structures reveal a competition between the (nx\) N-induced and (1x2) O-induced reconstructions, governed by factors such as co...
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