Perovskite ferroelectrics are widely used in modern science and technology due to their exceptional properties. Phase transitions in perovskite are accompanied by changes in lattice dynamics characteristics. These changes can be used to identify phase transitions and also serve as important factors responsible for the performance variations caused by phase transitions. Accordingly, understanding the evolution of lattice dynamics as a result of phase transitions is essential for establishing the relationship between the phase structures and properties of perovskites. Potassium tantalate niobate ($\text{K}{\mathrm{Ta}}_{1-x}{\mathrm{Nb}}_x{\mathrm{O}}_3$, abbreviated as KTN) is a perovskite ferroelectric with the advantage of a tunable phase structure. In this study, we investigate the evolution of the KTN crystal lattice dynamics during phase transitions by combining density-functional-theory calculations and experimental Raman spectroscopy. The changes in symmetry and intrinsic vibrational displacement due to phase transitions are explored and further demonstrated through vibrations of the intermediate structures between transitions. Additionally, ab initio molecular dynamics calculations are employed to investigate the structural characteristics and order-disorder behavior of paraelectric KTN, which is responsible for the Raman intensity. Our results provide standards for determining symmetry or complex multiphase structure via Raman spectroscopy. Moreover, the findings are crucial foundations for establishing the phase-structure-property relationship, which will hopefully guide research on the origins and optimization of the performance of KTN crystals and other perovskite systems.

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• Ferroelectricity
• First-principles calculations
• Lattice dynamics
• Phase transitions