Origin of antiferroelectricity in NaNbO₃

To figure out the origin of antiferroelectricity in NaNbO₃, we performed in-depth investigation combining symmetry analysis and density functional theory (DFT) calculation. The primary distortion modes driving the phase transition from the paraelectric (PE) Pm-3m phase to the antiferroelectric (AFE) Pbcm phase are identified to be R₅-, T₂ and Δ₅. The adiabatic potential energy surface reveals that the R₅- and T₂ modes lead to more energy gain than the Δ₅ mode. It also shows that the coexistence of R₅- and Δ₅ modes, and T₂ and Δ₅ modes cooperate to stabilize the AFE phase as well as the trilinear coupling term. To elucidate the electronic driving force of the PE-AFE phase transition, symmetry analysis on the possible couplings between the electronic and vibrational states is carried out following the pseudo Jahn-Teller effect (PJTE) theory, and the adiabatic potential energy surface cross sections are evaluated based on the unfolded band structures. It is observed that the Δ₅ mode is created by the coupling of T_2u and T_2g electronic states by PJTE, while R₅- and T₂ modes are more responsible to stabilize the AFE phase. This paves the way to identify the descriptors from the electronic structure for the displacive phase transitions and materials design in future studies.