Dynamic Atomic Off-centering and Its Selective Coupling with Phonons in KTa_(1-x)Nb_xO₃

The mechanism of ferroelectric phase transitions (e.g. displacive or order-disorder) in simple compounds like BaTiO₃ and KNbO₃ has remained debated for decades. Finally solving this question requires simultaneously resolving atomic dynamics, local crystal distortions and complex correlated disorder. To address this challenge, we map the energy and momentum resolved dynamic structure factor S(Q, E) with neutron scattering and we perform DFT-accuracy machine-learning MD (MLMD) simulations, to rationalize the spatial and temporal correlations of atomic disorder and the coupling with phonons. Here, we focus on correlated atomic off-centering in KTaO₃ and KTa_(1−x)Nb_xO₃ (KTN) and find an intermediate picture between displacive and order-disorder scenarios. Our experiments show two-dimensional diffuse sheets from correlated local disorder in KTN, contrasting with displacive KTaO₃. Our first-principles and MLMD simulations reveal that correlated 1D chains of off-centered B-site atoms in KTN are energetically favorable, and show how the correlated disorder selectively couples with the ferroelectric instability as well as transverse acoustic modes. These results provide insights into these model systems and offer general guidance for related systems whose atomistic behavior remain debated.