Room Temperature Ferroelectricity in Na_1−xSr_x/2◻_x/2NbO₃ through the Introduction of Cationic Vacancies

Abstract

The present study relates structural and microstructural modifications in the Na1−x Sr x/2◻ x/2NbO3 solid solution with variations in the dielectric behavior when changing the chemical composition as an unprecedented investigation in lead-free ferroelectric compounds. X-ray diffraction, electron diffraction, and high resolution electron microscopy have been used as techniques to obtain a complete knowledge of the structural and microstructural evolution of the system as a result of the doping process. In this paper, we focus our attention in the relevant structural transition which occurs in the 0 < x ≤ 0.2 composition range correlated to the change in dielectric behavior observed from the electric characterization performed. The microstructural study performed in this system in correspondence with the exhaustive electric characterization provides evidence about the evolution suffered by the system in which the changeable A-vacancy concentration might play a crucial role by tuning the electric behavior.

Phase coexistence in NaNb(1−x)TaxO3 materials with enhanced dielectric properties

“…Therefore, a clear structural change from the four-fold superstructure to the two-fold superstructure seems to occur as a result of the doping process. This structural change might be associated with a change in the electrical properties from the antiferroelectric to the ferroelectric behavior similar to that previously observed for NaNbO 3 when doped with alkaline elements 22,23 where the two-fold phase is obtained as a unique phase for a certain threshold value, thus giving rise to a homogeneous ferroelectric material. It has been recently shown that, before the stabilization of the two-fold superstructure, the coexistence of structural domains of both the four-fold and the two-fold superstructure occurs, with a definitive influence on the final dielectric behavior of the materials.…”

“…It has been recently shown that, before the stabilization of the two-fold superstructure, the coexistence of structural domains of both the four-fold and the two-fold superstructure occurs, with a definitive influence on the final dielectric behavior of the materials. 23 In this sense and provided the relevance of the microstructural features, we have performed an exhaustive TEM characterization of the NaNb (1Àx) Ta x O 3 system by combining Selected Area Electron Diffraction (SAED) and High Resolution Electron Microscopy (HREM). From this study, the intergrowth of structural domains of the O2a c  4a c  O2a c and the O2a c  2a c  O2a c unit cells was proved in the 0.1 # x # 0.33 composition range, as shown in the high resolution image corresponding to the x ¼ 0.2 composition in Fig.…”

“…The ferroelectric phase can be induced by applying an electric field 20 or through the formation of adequate solid solutions. Several works have succeeded in the stabilization of ferroelectric materials through the partial substitution of Na in the A sublattice [21][22][23] of the antiferroelectric NaNbO 3 . However, the tunability of the dielectric properties through the doping process in the B sublattice of NaNbO 3 oxide has aroused less attention in the literature.…”

Strain-induced phase transitions in epitaxial NaNbO3thin films grown by metal–organic chemical vapour deposition

“…Bulk NaNbO 3 exhibits six phase transitions between 20 and 1000 K which result in an antiferroelectric orthorhombic P phase at room temperature (Megaw, 1974;Mishra et al, 2011). By application of low electric fields or by chemical doping it can easily be transformed into a ferroelectric state (Yuzyuk et al, 2004;Saito et al, 2004;Torres-Pardo et al, 2008). Also, the deposition of strained thin films offers the possibility of stabilizing the ferroelectric phase (Yuzyuk et al, 2010;Tyunina & Levoska, 2009).…”

The influence of Mn substitution on the local structure of Na0.5Bi0.5TiO3 crystals: Increased ferroelectric ordering and coexisting octahedral tilts

“…4 This could be due to a more complete poling enabled by enhanced resistivity 4 and also, in part, from changes in the phase stability or domain structure. 5 Doping is known to modify the crystal structure and improve material properties: examples include La and K in PZT, [36][37][38][39] Sr and Ta in NaNbO 3 , [40][41][42][43] and Ca and Sm in BiFeO 3 . [44][45][46][47][48] Crystals of Mn-doped NBT have been reported to have: (i) enhanced resistivities, remnant polarizations, coercive fields, and d 33 values; (ii) a slight decrease in the phase transformation temperatures; and (iii) a decrease in the size of the T ferroelastic domains.…”