Growth and Partial Reduction of Sm₂O₃(111) Thin Films on Pt(111): Evidence for the Formation of SmO(100)

We investigated the growth and partial reduction of Sm 2 O 3 (111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm 2 O 3 (111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm 2 O 3 (111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O–Sm–O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm 2 O 3 (111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm 2 O 3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm 2 O 3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm 2 O 3 (111). We show that the SmO(100) and Sm 2 O 3 (111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm 2 O 3 (111) domains as well as the proposed Sm 2 O 3 (111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm 2 O 3 (111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.