Ferroelectric transition and structural modulation in Sr2Na(Nb1−xTax)5O15 tungsten bronze ceramics

Abstract

Effects of B-site Ta5+ substitution on the ferroelectric transition and crystal structure modulation were investigated for Sr2Na(Nb1−xTax)5O15 (x = 0–0.5) tungsten bronzes. The weakened ferroelectricity and enhanced relaxor behaviors were observed with increasing Ta, while the room temperature crystal structure changed from non-centrosymmetric to centrosymmetric. Raman spectra showed a decreasing intensity of the internal vibrations with increasing Ta, revealing different bonding mechanisms between Nb/Ta cations and O anions with reduced covalence by Ta-substitution, which resulted in weakened polarization. Two sets of superlattice reflections were detected in the selected area electron diffraction patterns: commensurate modulation for all compositions (type I), described by the wave vectors q1=14(a0∗+b0∗)+12c0∗ and q2=12(a0∗+b0∗); extra reflections of 1/2[0 0 l] at some local areas for Ta-containing compositions (type II), indicating the possible ordering of B-site cations. The distinct bonding state between the B-site cation and the O anion dominates the crossover from ferroelectric to relaxor behaviors in the present compounds.

Ferroelectric-Relaxor Crossover and Energy Storage Properties in Sr2NaNb5O15-Based Tungsten Bronze Ceramics

“…In this case, the functional relationship of T m with f changes from the power law to V–F with the increase of the Zr content, and finally conforms to the Arrhenius law . A similar trend of T F is found in Ta-doped SNN and some other tungsten bronzes, and it is used as a measure of ferroelectricity/relaxor strength. , Therefore, vacancies are destructive to the order of long-range ferroelectrics. Compared with GT, the PNRs in GV remain highly dynamic even under cooling.…”

“…31 A similar trend of T F is found in Ta-doped SNN and some other tungsten bronzes, and it is used as a measure of ferroelectricity/relaxor strength. 19,32 Therefore, vacancies are destructive to the order of long-range ferroelectrics. Compared with GT, the PNRs in GV remain highly dynamic even under cooling.…”

“…21,22 However, the modification of relaxor−ferroelectric crossover of SNN under a rich theoretical background is rare, and it is mainly based on B-site cation substitution. 9,19 It must be noted that no clear frequency dispersion was observed, although Zheng et al…”

“…For instance, Ba 4 Sm 2 Zr 4 Nb 6 O 30 is a classical ferroelectric, while Ba 4 La 2 Zr 4 Nb 6 O 30 is a relaxor with a P r value that is nearly zero . The influence mechanisms of B-site cations on the relaxor behavior were also proposed, such as the reduction of the second-order Jahn–Teller distortion (SOJT), which was caused by weak bonding, and the percolation of polar oxygen octahedra in a non-polar matrix. − Recently, the importance of crystallographic vacancies for additional structural disorders and relaxors was emphasized again. , However, the modification of relaxor–ferroelectric crossover of SNN under a rich theoretical background is rare, and it is mainly based on B-site cation substitution. , It must be noted that no clear frequency dispersion was observed, although Zheng et al claimed to obtain a Ca-doped SNN relaxor for energy storage . It is necessary to further explore the potential relaxor behavior regulation mechanism to evaluate the possibility of SNN as an energy storage capacitor, especially the dielectric layer of the BME-MLCC.…”

Enhancing Energy Storage Performance in Lead-Free Bismuth Sodium Niobate-Based Tungsten Bronze Ceramics through Relaxor Tuning

“…The peaks below the 200 cm –1 vibration mode are related to the relative displacement of the A-site cation and NbO 6 octahedron. According to Figure a,b, the Raman peaks near 250 and 630 cm –1 gradually move to a lower wavenumber with the increase of x , indicating that the interaction between the B and O in the BO 6 octahedron is weakened, which is due to the greater electronegativity of Nb (1.6) than that of Ta (1.5) and the greater covalence of Nb–O than that of Ta–O. , In other words, the introduction of Ta 5+ weakens the bond energy of B–O, leading to the deterioration of the ferroelectricity and the enhancement of the relaxor behavior of SBNN- x Ta ceramics with the increase of x . − Therefore, Ta substitution introduces local component disorder and some local weak dipoles, which break the long-range ferroelectric order and enhances the relaxor behavior. Figure c shows the values of the full-width at half-maximum (fwhm) of the 250 and 630 cm –1 vibration peaks with the increase of x .…”