Effects of A-site cations on the electrical behaviors in (Sr1-xCa (x))(2.)1Na0.8Nb5O15 tungsten bronze ferroelectrics

In order to discuss the correlation of composition-structure-performances, it is essential to study the role of Sr/Ca ratio variations on the physical properties in (Sr1-xCax)(2.1)Na0.8Nb5O15 ceramics with nearly 'filled' TB structure. Similar micromorphology of all obtained SCNN ceramics according to the FESEM results was beneficial to reveal the intrinsic structural factors for the electrical performances. Two different dielectric anomalies appeared in the dielectric spectra for all specimens: (1) the high-temperature anomaly corresponded to the ferroelectric phase transition (ferroelectric 4 mm to paraelectric 4/mmm transition); (2) whereas for the low-temperature anomaly, introducing Ca2+ in the Sr2.1Na0.8Nb5O15 composition could force some Na+ ions to occupy bigger pentagonal A2 sites, which could induce the slight distortion of NbO6 octahedron (considered as the subtle structure effects) to further evolve into the structural phase transition from ferroelectric (4 mm)-to-ferroelastic (mm2) (ferroelastic phase transition) at higher Ca2+ contents. The existence of mm2 phase in SCNN ceramics with x > 0.075 could be further confirmed by XRD refinement and Raman results. This finding is helpful for improvement of the fundamental theory and modulation of the performances in tungsten bronze niobates.