Ferroelectricity with Long ion Displacements in Crystals of Non-Polar Point Groups

In the classical model, ferroelectricity is associated with small ion displacements from paraelectric phases with high symmetry, and ferroelectric crystals must adopt one of the ten polar point groups with low symmetry according to Neumann's principle. In this work, it is proposed that this conclusion is based on perfect bulk crystals without taking the boundaries into account. First-principles evidence shows that ferroelectric polarizations may also be formed in some non-polar point groups as the edges generally break the crystal symmetry. Meanwhile, such polarizations can be maintained at macroscale and are switchable when the transition between multiple equivalent states with high symmetry can be realized via long ion displacements, essentially akin to ion conductors. For example, the switching barriers can be much reduced in sliding ferroelectric bilayer systems or ionic compounds with covalent-like directionality. Such unconventional ferroelectricity can be attributed to the boundaries and long ion displacements, and its existence in several systems like CuCrS2 is supported by experimental observations. It may explain a series of unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics, especially those with high polarizations induced by long ion displacements. Ferroelectric polarization can be formed in some non-polar point groups when the edges break the crystal symmetry. Such polarizations can be maintained at macroscale and are switchable if the transition between multiple equivalent states with high symmetry can be realized via long ion displacements. image