Atomic-Scale View at the Segregation of Alkali Metals\ntoward the KTaO<sub>3</sub>(001) Perovskite Surface

Perovskites exhibit outstanding performance in applications such\nas photocatalysis, electrochemistry, or photovoltaics, yet their practical\nuse is hindered by the instability of these materials under operating\nconditions, specifically caused by the segregation of alkali cations\ntoward the surface. The problem arises from the bulk strain related\nto different cation sizes, as well as the inherent electrostatic instability\nof perovskite surfaces. Here, we focus on atomistic details of the\nsurface-driven process of interlayer switching of alkali atoms at\nthe inorganic perovskite surface. We show that the (001) surface of\nKTaO<sub>3</sub> cleaved at room temperature contains equally populated\nTaO<sub>2</sub> and KO terminations, while the uncompensated polarity\nof these terminations promotes diffusion of KO from the subsurface\ntoward the topmost surface layer at temperatures as low as 200 °C.\nThis effect is directly probed at the atomic scale by Atomic Force\nMicroscopy and the chemical properties of the resulting surfaces are\ninvestigated by the adsorption of CO and H<sub>2</sub>O. The experiments\nindicate that KO segregation is associated with the formation of K\nand O vacancies in the near-surface region, which is further supported\nby depth-dependent X-ray Photoelectron Spectroscopy measurements and\nDensity Functional Theory calculations. Our study shows that the KO\nsegregation influences the surface reactivity both toward CO and water,\nwhich was probed at the atomic scale.