Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in CsLn₂Ti₂NbO₁₀ Dion-Jacobson Phases

The Dion-Jacobson (DJ) family of perovskite-related materials have recently attracted interest due to their polar structures and properties, resulting from hybrid-improper mechanisms for ferroelectricity in n = 2 systems and from proper mechanisms in n = 3 CsBi₂Ti₂NbO₁₀. We report here a combined experimental and computational study on analogous n = 3 CsLn ₂Ti₂NbO₁₀ (Ln = La, Nd) materials. Density functional theory calculations reveal the shallow energy landscape in these systems and give an understanding of the competing structural models suggested by neutron and electron diffraction studies. The structural disorder resulting from the shallow energy landscape breaks inversion symmetry at a local level, consistent with the observed second-harmonic generation. This study reveals the potential to tune between proper and hybrid-improper mechanisms by composition in the DJ family. The disorder and shallow energy landscape have implications for designing functional materials with properties reliant on competing low-energy phases such as relaxors and antiferroelectrics.