Ferroelectric Transition and Curie–Weiss Behavior in Some Filled Tungsten Bronze Ceramics

Ferroelectric transitions in filled tungsten bronze ceramics Sr₄R₂Ti₄Nb₆O₃₀, Sr₅RTi₃Nb₇O₃₀ (R=La, Nb, Sm & Eu) and Ba₄Nd₂Ti₄Nb₆O₃₀ are investigated with differential scanning calorimetry (DSC) and the Curie–Weiss law fitting to the dielectric constant. The magnitude of the Curie–Weiss constant C～10⁵ suggests displacement-type ferroelectric transition in the present compounds. The large ΔT difference between dielectric maximum temperature T_m and Curie–Weiss temperature T₀) values indicate the difficult formation of ferroelectric domains or polar nanoregions in the present compounds and also the characteristics of the first order ferroelectric transition. Three categories are suggested for the ferroelectric transition in the above tungsten bronzes. The ferroelectric transition exhibits large thermal hysteresis. According to the DSC results, gradual recovery of the endothermic peak occurs after aging at temperature below the Curie point, indicating the gradual stability of the ferroelectric phase after cooling from the high-temperature para-electric phase. The relationship between the Curie–Weiss law fitting parameters and the nature of the ferroelectric transition is modified for the filled tungsten bronzes.