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

The crystal structures and phase transitions of NaNbO3 are analyzed with density fonctional theory through generalized gradient spproximation-PBEsol (GGA-PBEsol) and local density approximations (LDA). 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. The coupling between the modes and the strain relaxation play a key role in the condensation of the ground state of sodium niobate. The instabilities at R and M points of the first Brillouin zone and along the line T (connecting the M and R points) are very important for stabilizing the low-energy phases. Within the GGA-PBEsol approximation, the ground state is rhombohedral ferroelectric/antiferrodistortive (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, there is only a small energy difference between the three lowest-energy phases Pmc21 (FE/AFD[110]), Pbcm (antiferroelectric/AFD), and R3c (FE/AFD[111]), which is a key characteristic of antiferroelectricity. The GGA-PBEsol approach provides more sensible results than the LDA approximation.