Origin of antiferroelectricity in NaNbO₃

The stabilization and origin of antiferroelectricity in the antiferroelectric (AFE) materials have always been the tools to facilitate the AFE/FE engineering. However, the mechanistic understanding of the driving forces, especially in the electronic level, is still elusive. Here, combining density functional theory calculations and symmetry analysis, following the pseudo-Jahn-Teller effect (PJTE) theory, we investigate both the stabilization and origin of antiferroelectricity in the AFE perovskite NaNbO3. Utilizing the potential energy surface and effective Hamiltonian, it is observed that the cooperative couplings play a critical role to stabilize the AFE phase. Moreover, considering adiabatic potential energy surface cross sections at F on the basis of the PJTE, the origin of the cubic-AFE phase transition at F is observed as the coupling of (T-2u, T-2g) electronic states, inducing the A5 mode, whereas both the (T-1u, T-2g) state and (T-2u, T-2g) state couplings via the Gamma(-)(4) mode are the reason behind the cubic to ferroelectric phase transition at Gamma.