Authors

Abstract

The relaxor ferroelectric transition in Cd2Nb2O7 is thought to be described by the unusualcondensation of two -centered phonon modes, 4 and 5. However, their respective roles haveproven to be ambiguous, with disagreement between ab initio studies, which favor 4 as the primarymode, and global crystal refinements, which point to 5instead. Here, we resolve this issue bydemonstrating from x-ray pair distribution function measurements that locally, 4 dominates, butglobally, 5 dominates. This behavior is consistent with the near degeneracy of the energy surfacesassociated with these two distortion modes found in our own ab initio simulations. Our first-principlescalculations also show that these energy surfaces are almost isotropic, providing an explanationfor the numerous structural transitions found in Cd2Nb2O7, as well as its relaxor behavior. Ourresults point to several candidate descriptions of the local structure, some of which demonstratetwo-in/two-out behavior for Nb displacements within a given Nb tetrahedron. Although this suggeststhe possibility of a charge analog of spin ice in Cd2Nb2O7, our results are more consistent with aHeisenberg-like description for dipolar fluctuations rather than an Ising one. We hope this encouragesfuture experimental investigations of the Nb and Cd dipolar fluctuations, along with their associatedmode dynamics.