Polymorphism in Micro-, Submicro-, and Nanocrystalline NaNbO₃

Abstract

NaNbO(3) powders with various particle sizes (ranging from 30 nm to several microns) and well-controlled stoichiometry were obtained through microemulsion-mediated synthesis. The effect of particle size on the phase transformation of the prepared NaNbO(3) powders was studied using X-ray powder diffraction, Raman spectroscopy, and nuclear site group analysis based on these spectroscopic data. Coarsened particles exhibit an orthorhombic Pbcm (D(2h)(11), no. 57) structure corresponding to the bulk structure, as observed for single crystals or powders prepared by conventional solid-state reaction. The crystal symmetry of submicron powders was refined with the space group Pmc2(1) (C(2v)(2), no. 26). The reduced perovskite cell volumes of these submicron powders were most expanded compared to all the other structures. Fine particles with a diameter of less than 70 nm as measured from SEM observations showed an orthorhombic Pmma (D(2h)(5), no. 51) crystal symmetry. The perovskite formula cell of this structure was pseudocubic and was the most compact one. A possible mechanism of the phase transformation is suggested.

Competing antiferroelectric and ferroelectric interactions inNaNbO3: Neutron diffraction and theoretical studies

“…[31][32][33][34][35][36] In spite of the extensive experimental 4-30 and theoretical 31 -36 studies, there are numerous controversies surrounding the phase diagram of NaNbO 3 with conflicting reports in the literature 29 on the existence of ferroelectric orde ring at low temperature. Recent x-ray diffraction and Raman spectroscopic studies suggest that the observation of noncentrosymmetric ferroelectric phases in NaNbO 3 is strongly influenced by the particle size 37 , but these are again not clearly understood. As m aterial properties are strongly influenced by the associated crystal structure, a ccurate characterization of the phase diagram of NaNbO 3 is essential for design of new sodium niobate based ceramics for applications.…”

Consolidation, Microstructure and Crystallography of Dense NaNbO₃ Ceramics with Ultra-Fine Grain Size

“…It should be noted in any case that all samples contain different fractions of these ultrafine NaNbO 3 grains, which are believed to crystallize in the new polymorph Pmc2 1 because of their small size. 12 The lamellar substructure could be explained through the formation of ferroelastic or ferroelectric domains, since in this crystal structure the center of inversion, which is normally observed in bulk coarse NaNbO 3 is absent. The exact nature and the origin of the substructure and the lamellae, however, are still unclear and need further detailed studies, e.g.…”

“…One should note that the structure with Pmma space group is stable for a particle size below 70 nm while the submicron structure Pmc2 1 is stable when the particle size is below 600 nm. 12 In XRPD patterns, the two structures can be distinguished by the splitting of the 022 220 reflections 2uŋ40c as well as by the position of the 201 102 peaks 2uŋ36.1c for the Pmc2 1 structure and 2uŋ36.4c for the Pmma structure . According to these remarks, one can conclude that the sinterforged sample is mainly composed of submicron sized grains.…”

“…Recently we reported on a new polar polymorph in pure ultra-fine particles of NaNbO 3 -powders 12 showing enhanced piezoactivity compared to coarse particles of the same composition 13 due to the stabilization of a noncentrosymmetric crystal structure. Coarse NaNbO 3 is known to crystallize in the orthorhombic space group Pbcm, 14 which is not polar due to the presence of an inversion centre in the unit cell of the lattice.…”

“…Basically the results from XRD and Raman spectroscopy for the present ceramics indicated good agreement with those for powders. 12 To clarify phase stabilization mechanisms, difference of surface properties, which is attributed to the presenceabsence of grain boundaries, should be also taken into consideration.…”