CO ₂ ‐Driven Polarity Compensation Mechanism for Stabilizing High‐Index Facets in KTaO ₃

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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>0</mml:mn> <mml:mrow><mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mn>1</mml:mn></mml:mrow> </mml:mrow> <mml:annotation>$0{{\bar 11}}$</mml:annotation></mml:semantics> </mml:math> ), ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>0</mml:mn> <mml:mrow><mml:mover><mml:mn>2</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mn>1</mml:mn></mml:mrow> </mml:mrow> <mml:annotation>$0{{\bar 21}}$</mml:annotation></mml:semantics> </mml:math> ), 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.