B-cation effect on relaxor behavior and electric properties in Sr2NaNb5−xSbxO15 tungsten bronze ceramics

Sr2NaNb5−xSbxO15 ceramics (SNNS, x=0.0, 0.3, 0.5, 1.0) with ‘filled’ tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of introducing Sb concentration in B-sites on the microstructure, dielectric and ferroelectric properties were investigated in detail. With increasing of Sb concentration, the crystal structure of SNNS ceramics distorted slightly from the TB orthorhombic Ccm2 phase to tetragonal P4bm phase, and then to paraelectric P4/mbm phase at room temperature. The smaller ionic radius of Sb5+ (0.60 Å) compared with that of Nb5+ (0.64 Å) contributed to the shrinkage of crystal structure. It was found that accompanying with the introduction of Sb, some unique dielectric and ferroelectric behavior emerged for SNNS ceramics. Only the samples with x<0.5 showed additional dielectric anomaly in low-temperature range, which was attributed to the incommensurate superstructure ICS-associated dielectric dispersion process caused by local structural fluctuation. With increasing of Sb concentration, Tc shifted to lower temperatures, and the diffuse phase transition and relaxor behavior appeared to be exceptionally induced around Tc at higher Sb concentration. The underlying mechanisms of relaxor behavior induced by different Sb concentration in B-sites for SNNS ceramics were clarified in detail. Moreover, the ferroelectric properties deteriorated and even disappeared at higher Sb concentration, because the ceramics showed P4/mbm phase structure as a result of decreasing of Tc to lower than room temperature.