Local atomic structure distortions in the Dion-Jacobson ferroelectric CsBiNb2O7

Despite recent interest in Bi-containing layered-perovskite ferroelectrics due to high ${T}_{c}$, their rational design is constrained by a poor understanding of local atomic structures. Here, we use neutron total scattering and first-principles theory to examine the local atomic structure in Dion-Jacobson layered-perovskite ferroelectric $\mathrm{CsBi}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$. We show that the instantaneous short-range atomic structure can be described by a Debye-Einstein model that accommodates atomic motions due to acoustic and low-energy optic phonon modes. We determine structural distortions due to localized atomic dynamics with extremely high resolution. These are in addition to ground-state relative ionic displacements. The magnitude of dynamic variations in ionic positions points toward strong electron-phonon coupling in this material.