Tuning between Proper and Hybrid-Improper Mechanisms for Polar Behavior in Cs TiNbO 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 = 2 systems and from proper mechanisms in = 3 CsBiTiNbO. We report here a combined experimental and computational study on analogous = 3 Cs TiNbO ( = 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.