Dielectric characteristics and diffuse ferroelectric phase transition in Sr₄La₂Ti₄Nb₆O₃₀ tungsten bronze ceramics

In this work, Sr₄La₂Ti₄Nb₆O₃₀ ceramics with tetragonal filled tungsten bronze structure were prepared, and the dielectric characteristics and ferroelectric transition were investigated. The room-temperature dielectric constant of the present ceramics showed very weak frequency dependency; it decreased from 570.4 to 561.7 when the frequency varied from 200 Hz to 1 MHz, and a comparatively low-dielectric loss was observed (in the order of 10⁻³). Unlike the situation for Ba₄Ln₂Ti₄Nb₆O₃₀ (Ln = La, Nd and Sm), Sr₄La₂Ti₄Nb₆O₃₀ ceramics showed a diffuse ferroelectric phase transition, and the diffuseness exponent γ was 1.61. Moreover, the obvious frequency dispersion was observed over a broad frequency range from 200 Hz to 1 MHz, and the maximum dielectric constant temperature T_max shifted to higher temperatures with increasing frequency. The dielectric data agreed closely with the Vogel–Fulcher relationship. We concluded that tungsten bronze Sr₄La₂Ti₄Nb₆O₃₀ has the relaxor ferroelectric nature.