Structural Evolution in Paraelectric–Antiferroelectric Phase Transitions of Tungsten Bronze-Type K2NdNb5O15

Abstract Tungsten bronze (TB)-type K2RNb5O15 (R: rare-earth element) compounds exhibit antiferroelectric (AFE) properties and are promising for energy-storage and high-voltage applications. However, the structural origin of their antiferroelectricity remains unclear, hindering the establishment of rational design principles. Here, we investigated the structural evolution of K2NdNb5O15 from its aristotype P4/mbm phase to the AFE phase by combining high-resolution synchrotron X-ray and neutron powder diffraction experiments with machine-learning-assisted molecular dynamics (MD) simulations. The diffraction data revealed three successive phase transitions between 300 and 600 K, with a previously undetected intermediate phase lying between the aristotype P4/mbm and AFE phases. The MD simulations and Rietveld analysis consistently identified the soft modes that drive the structural phase transitions from the P4/mbm phase to the AFE phase. Condensation of a NbO6 octahedral tilting mode gives rise to the intermediate phase, whereas out-of-phase twisting of NbO6 octahedra and antipolar displacements of Nd and Nb cations emerge together at the transition to the AFE phase. These findings resolve the multiple structural distortions responsible for the antiferroelectricity in K2NdNb5O15 and provide microscopic insight into the design of TB-type AFEs.