Resolving Competing Phonon Modes in the Relaxor-type Ferroelectric Cd₂Nb₂O₇

Cd₂Nb₂O₇ undergoes a relaxor-type ferroelectric transition between 204K and 196K, drawing attention as one of the few stoichiometric relaxor ferroelectrics. Characterization of the phase transition has proved challenging due to competing contributions from two Γ-centered phonon modes (Γ₄^- and Γ₅^-). Global crystal structure refinements favor Γ₅^- as the primary mode, whereas first principles simulations indicate that Γ₄^- is more unstable. We use first principles calculations to compute energy as a function of mode amplitude and order parameter direction, discovering that the more unstable Γ₄^- mode also exhibits a highly isotropic energy surface. These results, along with x-ray pair distribution function measurements, point to a resolution of the competition between phonon modes, suggesting that Γ₄^- dominates the local structure whereas the more anisotropic Γ₅^- mode dominates globally.