Octahedral Tilting and Ferroelectric Order in Tetragonal Tungsten Bronze-Like Dielectrics

The onset of classic ferroelectric behavior in tetragonal tungsten bronze-like dielectrics Ba₂La_xNd_1-xNb₃Ti₂O₁₅ was attributed to a strong coupling between local polar displacements and a certain type of octahedral tilting. The ferroelectric phase transition in these systems is associated with a transformation of an incommensurate tilted structure to a distinct commensurate superstructure. The driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion that reside in perovskite type channels decreases. No classical ferroelectric transition is observed (down to 100 K) for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.