Modulated phases in NaNbO3:: Raman scattering, synchrotron x-ray diffraction, and dielectric investigations

Raman spectra of sodium niobate (NaNbO3)were obtained in all phases and revealed a significant disorder in the high-temperature U, T2 and T1 phases and a complicated folding of the Brillouin zone at the transitions into modulated S, R, P and N phases associated with the competitive zone-boundary soft modes (in-phase and out-of phase octahedral tilts) along the M-T-R line. An extensive Raman study combined with x-ray diffraction (XRD) and dielectric measurements confirmed the presence of the incommensurate (INC) phase in sodium niobate. XRD experiments revealed the invar effect in the temperature interval 410-460 K corresponding to the INC phase associated with rotations of the NbO6 octahedra modulated along the b-direction. Our experiments suggest that the phase P consists of three phases: monoclinic (P-m )between 250 and 410 K, INC between 410 and 460 K, and orthorhombic (P-o) between 460 and 633 K. At the low-temperature transition to the ferroelectric rhombohedral N phase all folded modes originating from the M- and T-points of the Brillouin zone abruptly disappear, Raman spectra in the N phase become much simpler and all peaks were assigned.