Adlayer formation on C-plane (0001) and R-plane ( 1 1 ‒ 0 2 ) Al 2 O 3 surfaces.

Adlayers on C-plane (0001) and R-plane terminated surfaces of corundum phase aluminum oxide were synthesized by annealing mixtures of two oxide powders, aluminum oxide with an additive. Using high-angle annular dark field scanning transmission electron microscopy, the adsorbed layers were characterized, and image simulations aided interpretation of the results. The adlayers were pseudomorphic, one atomic layer thick and with a fractional site occupancy. Atomic positions of the adlayer atoms relaxed and changed relative to the bulk structure, where there is evidence that the magnitude of the relaxation is sensitive to the ionic radius of the adsorbate. The pseudomorphic adlayer structure formed for different elements including, but not limited to, the lanthanides (i.e., Ge, Ba and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm).