CO₂-Driven Polarity Compensation Mechanism for Stabilizing High-Index Facets in KTaO₃

In ionic crystals, the simultaneous control of polarity compensation and exposure of high-crystallinity surfaces has long been a critical bottleneck for modulating their interfacial electronic and spin properties. Using the typical ionic crystal KTaO₃ (KTO) as a model system, it is demonstrated that supercritical carbon dioxide (SC CO₂) treatment is an effective solution to this challenge. As the SC CO₂ pressure increases from 12 to 20 MPa, the surface of KTO gradually transforms from a rough, low-index (001) facet into high-crystallinity, high-index polar facets, specifically ( $0\overline{1}1$ ), ( $0\overline{2}1$ ), and (111). Density functional theory (DFT) calculations indicate that this transformation primarily arises from the lower adsorption energy of CO₂ on high-index facets, making these CO₂-adsorbed high-index surfaces thermodynamically more stable. Notably, CO₂ induces magnetic moments via polarity compensation mechanisms without introducing oxygen vacancies (O_v), thereby enhancing macroscopic magnetism with increasing pressure. This finding challenges the conventional view that magnetic moments in nominally nonmagnetic oxides are induced solely by O_v. Therefore, the research demonstrates that SC CO₂, as a green and scalable treatment strategy, can expose high-crystallinity, high-index facets through polarity compensation, thus offering a versatile platform for oxide facet engineering and polarity compensation studies.