Dielectric Characteristics And Ferroelectric Phase Transition In M₆Ti₂Ta₈O₃₀ And M₆Ti₂Nb₈O₃₀ Ceramics (M=Ba, Sr And Ca)

In order to search new dielectric and ferroelectric ceramics for high frequency and high field applications, the MO-TiO₂-Ta₂O₅ (M=Ba, Sr, Ca) and MO-TiO₂-Nb₂O₅ (M=Ba, Sr) ternary systems with filled tungsten-bronze structure were investigated with respect to the relationship of composition, crystal structure and dielectric properties.In BaO-TiO₂-Ta₂O₅ (M=Ba, Sr, Ca) ternary system, Ba₆Ti₂Ta₈O₃₀ and Sr₆Ti₂Ta₈O₃₀ formed tetragonal tungsten-bronze structure, but Ca₆Ti₂Ta₈O₃₀ the cubic CaTa₂O₆ major phase combined with minor amount of CaTiO₃. Both Ba₆Ti₂Ta₈O₃₀ and Sr₆Ti₂Ta₈O₃₀ ceramics, which were paraelectircs at room temperature, had high dielectric constant, very low dielectric loss at 1MHz together with a relatively small temperature coefficient. However, dielectric properties of the nominal composition of Ca₆Ti₂Ta₈O₃₀ differed very much from that of Ba₆Ti₂Ta₈O₃₀ and Sr₆Ti₂Ta₈O₃₀. The present ceramics indicated a high dielectric constant with a near zero temperature coefficient of dielectric constant as well as a low dielectric loss (at 1 MHz). The dielectric constant was almost a constant, while the dielectric loss and temperature coefficient of dielectric constant were sensitive to the sintering conditions. The present ceramics might be promising in temperature compensated capacitors applications.In MO-TiO₂-Nb₂O₅ (M=Ba, Sr) ternary system, ferroelectrics Ba₆Ti₂Nb₈O₃₀ and Sr₆Ti₂Nb₈O₃₀ formed single phase tetragonal tungsten-bronze structure. The microstructures and dielectric properties of the present materials have been investigated as well as the behavior and features of the ferroelectric phase transition. Similar to perovskite ABO₃, tungsten bronze structure consists of several BO₆octahedra. The close distortion of BO₆ octahedra concluded from the 10^1/2 c/a ratiosis observed in Ba₆Ti₂Nb₈O₃₀ and Sr₆Ti₂Nb₈O₃₀, and two ferroelectrics have the close Curie temperature (155℃ for the former and 160℃ for the latter at lMHz). Deducing from Raman spectra at different temperatures around phase transition and the modified Curie-Weiss law, the ferroelectric phase transition of Ba₆Ti₂Nb₈O₃₀ andmay belong to the displacive transition. The obvious diffuse characteristics of the ferroelectric transition may attribute to the disordered distributions of octahedra, as two kinds of BO6, i.e. TiO6 and NbO6, exist in one cell. Meanwhile, the weak ferroelectric of the present materials are indicated from the P-E hysteresis loops, both with 2Pr of 4C/cm2.Further more, we explored the field dependence of Ba6Ti2Nbg03o and Sr6Ti2Nbg03o. When the dc bias was 2.1kv/cm, we can get the maximum tunability of 2.84% and 0.34% at the frequency of 10kHz near Tm (corresponding to the maximum permittivity) for Ba^T^NbgOso and SreT^NbgCbo, respectively.Generally, the effects of composition upon the crystal structures and dielectric properties were clarified in MO-TiO2-Ta2Os (M=Ba, Sr, Ca) and MO-TiO2-Nb2O5 (M=Ba, Sr) ternary systems. At the same time, the ferroelectrics phase transitions were investigated in the MO-TiC^-Nl^Os (M=Ba, Sr) ternary system.