Layered intercalation ferroelectricity induced by asymmetric ion coordination: a mini-review

Two-dimensional (2D) van der Waals ferroelectricity has attracted significant interest, which offers opportunities to address many issues that hinder practical applications of ferroelectrics in nanoelectronics. Recently, the family of 2D ferroelectrics has been broadened by emerging systems where ferroelectricity is induced by intercalation of metal ions in van der Waals layers. The intercalated tetra-coordinate cations break the symmetry and give rise to large vertical polarization with a high Curie temperature of up to 800 K. Such ferroelectricity has been experimentally verified in systems like CuCrS₂, CuCrSe₂ and AgCrS₂, and various high-performance devices based on them have been constructed. Such ferroelectricity may also be used to control various properties like magnetism, chirality of magnetic topology and superconductivity. These ferroelectric systems may also exhibit exotic effects like unprecedented electrostrain, negative piezoelectricity and thermal expansion. In particular, many of them are also ion conductors, leading to unconventional quantized ferroelectricity violating Neumann's principle, which has been experimentally confirmed. The potential of these materials to unravel new science and technology has stimulated considerable interest in intercalation ferroelectricity, making it a fast developing field.