Investigating the ferroelectric phases of sodium niobate: A computational approach

This study investigates the ferroelectric phases of NaNbO₃ using density functional theory (DFT) simulations. Special attention is given to the antiferroelectric polymorph Pbcm and the purported polar phases with monoclinic P1m1 and orthorhombic P2₁ma symmetries. The results reveal similarities in the diffraction patterns and Raman spectra of the P1m1 and P2₁ma models, while the Pbcm model exhibits greater distinctiveness. A comprehensive mechanical analysis was conducted, revealing notable anisotropy in mechanical properties and an unusually negative Poisson’s ratio for the R3c symmetry. In terms of ferroelectric properties, only the P1m1, P2₁ma, and R3c structures exhibit non-zero values for piezoelectric charge constants, indicating ferroelectric behavior. The Pbcm space group results from the stacking of two P2₁ma layers by a second-order improper rotation, explaining its antiferroelectric behavior. This work significantly contributes to the literature by providing a detailed understanding of the structural, vibrational, and mechanical properties of various NaNbO₃ phases, highlighting the distinct ferroelectric and antiferroelectric behaviors.