Theory and Neutrons Combine To Reveal a Family of Layered Perovskites without Inversion Symmetry

A flurry of recent theoretical studies have predicted the existence of new polar materials among several families of layered perovskites, including the double-layered Dion–Jacobson phases. These predictions have opened up exciting new opportunities for both fundamental studies of the crystal chemistry of Dion–Jacobson phases and their application as components in next-generation memories and low-power electronic devices. However, with some rare exceptions, all known double-layered Dion–Jacobson phases are nonpolar. We use an integrated theoretical–experimental approach to show that several Dion–Jacobson phases that have previously been synthesized and characterized as nonpolar are in fact polar. Additional theoretical calculations reveal that the polar phases of these materials emerge through a hybrid improper or trilinear coupling mechanism. Finally, our work has highlighted the critical role of neutron diffraction in characterizing the structures of double-layered Dion–Jacobson phases, which are typified by subtle oxygen atom displacements not easily resolved using even synchrotron X-ray diffraction.