Structural and dynamic phase transitions of NaNbO3 from first-principles calculations

The crystal structures and phase transitions of NaNbO3 are analyzed with density functional theory through GGA-PBEsol and LDA approximations. The phonon dispersion curves from the high-symmetry cubic perovskite phase are reported to have many unstable branches, predisposing to several combinations of phase transitions to various distorted structures. Lattice parameters are optimized and results compared with existing experimental and theoretical data. The indirect band gap is observed for the cubic phase. Many phases, observed and unobserved experimentally, are analyzed mode by mode. Our work reveals that, more than in PbZrO3 , given their all important potential wells, none of the individual modes condensed deletes all other but it is their coupling which plays a key role in the condensation of the ground state sodium niobate. This work points out the importance of instabilities at R and M points of the first Brillouin zone and along the line T (connecting the M and R points) to stabilize the low energy phases. Within GGA-PBEsol approximation, the ground state is rhombohedral FE/AFD R3c, while in LDA it is rather the orthorhombic FE/AFD Pmc21 structure that stabilizes the lowest energy in this compound. In both calculations we find small energy difference between the three lowest energy phases Pbcm (AFE/AFD), Pmc21 (FE/AFD[110]), and R3c (FE/AFD[111]), which is a key characteristic of antiferroelectricity. The GGA-PBEsol approach gives more sensible results than LDA approximation.