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

Effects of B-site Ta5+ substitution on the ferroelectric transition and crystal structure modulation were investigated for Sr2Na(Nb1-xTax)(5)O-15 (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 q(1) = 1/4 (a(0)* + b(0)*) + 1/2 c(0)* ansd q(2) = 1/2(a(0)* +b(0)*); 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.

Automated summary

In this study, the effects of B-site Ta5+ substitution on Sr2Na(Nb1-xTax)(5)O-15 tungsten bronzes were investigated, revealing weakened ferroelectricity and a change in crystal structure with increasing Ta content. Raman spectra showed different bonding mechanisms resulting in reduced covalence and weakened polarization. Electron diffraction patterns suggested the possible ordering of B-site cations, which dominate the crossover from ferroelectric to relaxor behaviors in the compounds.