Effects of Gd-substitution on structural, and impedance spectroscopic study of Sr2Sm1−xGdxTi2Nb3O15 tungsten bronze ceramics

Abstract

In the present work, we have studied the effect of Gd-substitution on the structural, dielectric and electrical properties of Sr2Sm1−xGdxTi2Nb3O15 (x = 0 and 0.5) tungsten bronze ceramics prepared by a high-temperature solid-state reaction technique. The formation of the phase of tetragonal tungsten bronze was verified by the Rietveld refinement using X-ray diffraction data. With increasing Gd concentration, the crystal structure of the Sr2Sm1−xGdxTi2Nb3O15 ceramics has slightly deformed from the tetragonal P4bm phase to the paraelectric P4/mbm phase. Scanning electron microscopy (SEM) of ceramics shows high densification and homogeneous distribution of grains of different sizes over the total surface. The crystallite size increases with Gd-substitution from 6.19 nm for Sr2SmTi2Nb3O15 to 10.83 nm for Sr2Sm0.5Gd0.5Ti2Nb3O15. The sample Sr2SmTi2Nb3O15 shows a dielectric anomaly of ferroelectric paraelectric type at 332 °C, and has non-relaxor type of diffuse phase transition. In addition, the dielectric properties deteriorated and even disappeared for the sample Sr2Sm0.5Gd0.5Ti2Nb3O15, because the ceramic presented a P4/mbm phase structure as a result of the decrease in Curie temperature (Tc) below room temperature. The electrical property of Sr2Sm1−xGdxTi2Nb3O15 ceramic has been investigated by non-destructive complex impedance spectroscopy (CIS) as a function of frequency at different temperatures.