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Exploring the Stability of Square Planar Carbon in Surface Carbides of Co, Ni, and Beyond


Cobalt and Nickel surface carbides both exhibit an interestingly high degree of stability, even higher than graphite-like carbon species in these transition metals. The anomalous geometry of these carbides has been the source of much debate: carbon prefers square planar (D4h) configuration and forces a spontaneous p4g clock surface reconstruction. A model, motivated by chemical bonding analysis, for these systems is presented that elucidates the unusual structure, stability, and the impetus for reconstruction. Carbon promotes a cluster-like aromaticity on the transition metal (100) surface and this aromaticity is so powerful that electrons are spontaneously taken from the void M4 squares to ensure it. Moreover, this model predicts many new transition metal and main group element surface alloys, which feature high stability, square-planar coordination, aromaticity, and a predictable degree of surface reconstruction.

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