Competition between Structural Phases in Mixed Cr/Fe Oxide Films on Pt(111): How Dimensionality Governs the Stability of Oxide Alloys

Low-temperature scanning tunneling microscopy (STM), combined with density functional theory (DFT), is employed to probe the atomic-scale mixing of Cr and Fe into ultrathin oxide films on Pt(111). The CrOx and FeOx parent bi-stacks are first identified via their specific (2×2) and (1×1) STM signatures, respectively. In Cr-rich mixed oxides, a (2×2) honeycomb surface remains the primary structural motif, although the presence of defects and ad-structures indicates partial Fe incorporation into the topmost oxide plane. With increasing Fe concentration, regions with characteristic (1×1) termination emerge, consistent with the formation of close-packed FeO ad-layers located above a Cr-rich oxide base. DFT calculations reveal two competing oxide phases under these preparation conditions. Whereas oxygen-rich, spinel-type bi-stacks dominate at Cr-rich compositions, Fe-rich films favor an oxygen-lean h-LuMnO3-type structure, not observed in bulk Cr/Fe oxides. The interplay between the two mixed oxide phases is governed by the availability of tetrahedrally versus octahedrally coordinated cation sites, as favored by the Fe and Cr ions, respectively. The structure of such low-dimensional oxide systems can thus be controlled by the Cr/Fe ratio and oxygen conditions during preparation, paving the way for phase engineering of oxide heterostructures and adaptive interfaces with potential for multifunctional, catalytic, and sensing applications.