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

Abstract

Raman spectra of sodium niobate (NaNbO 3 ) 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 NbO 6 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.

Low‐temperature Raman spectroscopic studies in NaNbO3

“…Extensive Raman studies combined with X-ray diffraction (XRD) and dielectric measurements confirmed the presence of an incommensurate phase in NaNbO 3 in the temperature interval 410-460 K. [2] Low wavenumber Raman studies on NaNbO 3 single crystal revealed the occurrence of a quasi-elastic peak, and the phase transition was attributed to the order-disorder process due to the relaxation response of Nb ion in the temperature interval 513-713 K. [7] Shen et al, [8] on the basis of the temperature variation of phonon modes during cooling and heating cycles of measurement, suggested that both FE and AFE phases coexist in the temperature range 40-180 K during cooling cycle. However, it is not clear from these studies [2,8] how the temperature range of coexistence of both phases was arrived, as no detailed intensity analysis had been done. Temperaturedependent Raman scattering studies in NaNbO 3 by Lima et al [10] suggested that the orientation of NbO 6 group plays an important role in stabilising the low-temperature FE phase.…”

Electronic and Optical Properties of Sodium Niobate: A Density Functional Theory Study

“…ere are also reports about a room-temperature phase transition into a FE phase (P2 1 ma) induced by an applied electric field [6,7], by nanoparticle growth [8] or by growth as a strained thin film [9], respectively. A full list of recent experimental data can be found in [2,[10][11][12].…”

Development of Lead-Free Nanowire Composites for Energy Storage Applications

“…Results demonstrated an increment in the dielectric permittivity of the ISRN Nanomaterials nanocomposites as the volume fraction increased, due to the high dielectric constants of both PTO and NNO, which are about two orders of magnitude higher than that of PVDF matrix [28,29]. Note that 0% volume fraction indicates pure PVDF samples here.…”