Dielectric relaxations, ultrasonic attenuation, and their structure dependence in Sr₄(La_xNd_1-x)₂Ti₄Nb₆O₃₀ tungsten bronze ceramics

The dielectric anomalies and their structure dependence were evaluated and discussed in Sr₄(La_xNd_1-x)₂Ti₄Nb₆O₃₀ ceramics, together with the analysis of ultrasonic velocity shift and attenuation spectra in the low-temperature range. The room-temperature structure was confirmed as the tetragonal in space group P4bm for all compositions. One diffuse ferroelectric peak and three relaxor ferroelectric peaks corresponding to the commensurate/incommensurate modulation of oxygen octahedra, polar clusters of A-site ion ordering, and B-site ion ordering, respectively, were observed in the composition with x = 0.25. With decreasing the radius difference between A1- and A2-ions (increasing x), the dielectric relaxations, especially the one originating from the polar clusters of A-site ion ordering, tended to increase significantly and overlap the diffuse ferroelectric peak, which was completely overlapped for x ⩾ 0.75. This process just reflected the increased disordering degree of both A- and B-site ions, and the analysis of ultrasonic attenuation strongly supported the above conclusions on dielectric relaxations and their structural origins. The ultrasonic attenuation peak at approximately 100 K corresponded to the freezing process of the dielectric relaxations, and the fluctuation with composition of the ultrasonic attenuation peaks between 150 and 260 K suggested the possible structure variation.