Crystal structure, spectra analysis and dielectric characteristics of Ba4M28/3Ti18O54 (M= La, Pr, Nd, and Sm) microwave ceramics

High dielectric constant Ba4M28/3Ti18O54 (M = La, Pr, Nd, and Sm) ceramics were synthesized via standard solid-state reaction route. All the samples possessed single-phase orthorhombic tungsten-bronze structure, and the unit cell volumes decreased monotonously when rare-earth ion changed from La to Sm. With the decreasing radius of rare-earth ions, the dielectric constant (epsilon(r)) gradually decreased, which was correlated with the decrease of total ionic polarizability and blue shift of Raman vibration modes. The increase of the quality factor (Q x f) was associated with the increase of packing fraction and the decrease of FWHM of Raman vibration modes (A(g) and B-1g). The temperature coefficient of resonant frequency (TCF) shifted in the negative direction, which was highly related to the decrease of the TiO6 octahedral tilt angle. Infrared reflection (IR) spectroscopy analysis indicated that the infrared vibration modes between TiO6 octahedron and A-site cation (M3+ and Ba2+) at low frequency dominated microwave dielectric polarization in the Ba4M28/3Ti18O54 ceramics.