Ab initio molecular dynamics simulations of the ferroelectric-paraelectric phase transition in sodium nitrite

Abstract

This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric 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 outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature ${T}_{c}$ between 425 and $450\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in excellent agreement with the experimental value of $437\phantom{\rule{4pt}{0ex}}\mathrm{K}$. In addition, the anomalous lattice behavior (shrinking of the $c$ axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. 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 analyzing 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 ferroelectrics, using a design principle inspired by ${\mathrm{NaNO}}_{2}$.