Ab Initio Molecular Dynamics Simulations of the Ferroelectric-Paraelectric Phase Transition in Sodium Nitrite

This paper reports on the first ab initio molecular dynamics study of the ferroelectric Sodium Nitrite, shedding light on its order-disorder phase transition. The remnant polarization P_r was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in excellent agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature T_c between 400 K and 450 K in good agreement with the experimental value of 437 K. In addition, the anomalous lattice behavior (shrinking of the c-axis) during the phase transition is reproduced. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analysing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a c-axis rotation of the Nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline order-disorder ferroelectrics.