First-principles calculations of structural properties of NaNbO3,

MACHADO, RODRIGO; SEPLIARSKY. MARCELO; STACHIOTTI, MARCELO

Bordeaux

Congreso; 12 European Meeting on Ferroelectricity; 2011

The perovskites include many compounds with ferroelectrics phases. NaNbO3 is the one with the most complex sequence of phase transitions among them. Its many phases include a low temperature ferroelectric and a room temperature antiferroelectric. It was reported that these two phases can coexist in a range of temperature. Although the extensive experimental and theoretical studies, there are numerous controversies surrounding the phase diagram of NaNbO3. In this work we report total energy calculations for different crystal structures of NaNbO3 over a range of unit cell volumes using the all-electron full-potential (L)APW method with two different exchange-correlation functionals: the Hartree-Fock local-density approximation (LDA) and the Wu-Cohen generalized gradient approximation (GGA-WC). We found that LDA not only produces the well known underestimation of the equilibrium volume of the system but it also predicts a wrong antiferroelectric ground-state for the Sodium Niobate. In contrast, with the GGA-WC functional we obtain a cell volume in great agreement whit the experiments and an improvement on the stability of the ferroelectric phase relative to the antiferroelectric (figure 1). The small energy difference between the two phases explains the coexisting ferroelectric and antiferroelectric structures observed in experiments. We then use the GGA-WC functional for the calculation of the phonon dispersion curves of the cubic high-temperature phase to search for the presence of competing structural instabilities in the whole Brillouin zone. This curves displays several modes with imaginary frequencies. In particular, we found octahedral tilts instabilities at M and R points and a ferroelectric instability at the Γ point. The octahedral tilts corresponding to the soft modes at the M- and R-points of the Brillouin zone are the primary order parameters, which induce the first two structural phase transitions from the cubic phase. We also studied comparative calculations of the ferroelectric and octahedral tilts instabilities in NaNbO3 and KNbO3 as a function of volume, and octahedral tilt in KxNa1−xNbO3 in function of K concentration in order to provide insights for a qualitative understanding of the structural behavior of KxNa1−xNbO3 solid solutions