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 ${\mathrm{NaNbO}}₃$. 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 $\mathrm{\ensuremath{\Gamma}}$ on the basis of the PJTE, the origin of the cubic-AFE phase transition at $\mathrm{\ensuremath{\Gamma}}$ is observed as the coupling of (${T}_2u, {T}_2g$) electronic states, inducing the ${\mathrm{\ensuremath{\Delta}}}₅$ mode, whereas both the (${T}_1u, {T}_2g$) state and (${T}_2u, {T}_2g$) state couplings via the ${\mathrm{\ensuremath{\Gamma}}}₄^{\ensuremath{-}}$ mode are the reason behind the cubic to ferroelectric phase transition at $\mathrm{\ensuremath{\Gamma}}$.