摘要： Two-dimensional (2D) oxide materials have tremendous potential in fundamental research and cutting edge technologies owing to their outstanding physical and chemical properties, which makes them excellent candidates for a wide range of applications including power harvesting, hydrogen storage, fuel cells, gas sensors, advanced electronic and spintronic devices, and nanocatalysis.1,2 To gain a fundamental understanding of the novel properties, afforded by the reduced dimension of oxide nanostructures, structurally well-defined model systems have been utilized, typically in the form of ultrathin oxide films (with a thickness of one to few atomic layers) epitaxially grown on single-crystal metal surfaces. Emergent phenomena in their geometric architecture, electronic and vibrational structure, chemical nature and magnetic behavior have been recognized,3–8 rendering oxide nanolayers vast and unforeseen opportunities in science and technology. To date, most of the studies have been focused on binary 2D oxides, but the increased interest in more complex oxide materials, such as ternary or multicomponent oxides with a broader range of functionalities, requires an adequate understanding of their properties at the nanoscale, which is still scarce. Even for high technologically important ternary oxides, such as the perovskites, there are only few studies reporting on the preparation of 2D oxide layers on metal supports, which are found to display unique structural behavior with no counterpart in the bulk.9,10 One reason for this is that the preparation of ternary oxide nanolayers with well-defined structure and stoichiometry is more challenging than for binary oxides, which requires the development of new fabrication strategies, where suitable thermodynamic and kinetic parameters have to be optimized in a narrow multiparameter space to obtain structures with desired stoichiometry and 2D morphology. Moreover, the elucidation of their structural properties at the atomic level is experimentally and theoretically more demanding than for the binary oxides.