Irregular stacking sequence in the initial growth of ultrathin Rh films on Ru(0001)

Rhodium grows on single-crystal Ru(0001) surface initially layer by layer, adapting the in-plane lattice parameters of Ru(0001) (pseudomorphic growth). Using quantitative low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we determined the atomic structures of 1-monolayer (ML), 2-ML, and 3-ML Rh films on Ru(0001). The 1-ML Rh film continues the hexagonal-close-packed (hcp) stacking sequence of Ru(0001) (...ABAb), while the 2-ML Rh film already adopts its face-centered-cubic (fcc) stacking sequence (...ABAbc) if the topmost Ru layer is included into the stacking consideration. Three MLs of Rh form a fcc stacking sequence of ...ABAbac, requiring a massive restructuring of the second Rh layer when adding one ML of Rh onto the 2-ML Rh film