Ferroelectric And Relaxor Ferroelectric Behaviors In BaO-Ln₂O₃-TiO₂-Ta₂O₅ Ceramics (Ln=La, Sm) With Tungsten-bronze Structure

Dielectric ceramics with tungsten bronze structure have been studied widely for their excellent ferroelectric,piezoelectric,pyroelectric and nonlinear optical properties.In order to search new high permittivity dielectric ceramics for high frequency applications,the Ba_pLa_6-pTi_8-pTa_2+pO₃₀and Ba_pSm_6-pTi_8-pTa_2+pO₃₀(p=3,4, 5)tungsten bronze ceramics are investigated systematically in the present work.The relations among composition,crystal structure and dielectric characterization are elucidated in association with the relaxor ferroelectric nature.In Ba_pLa_6-pTi_8-pTa_2+pO₃₀(p=3,4,5)system,Ba₃La₃Ti₅Ta₅O₃,Ba₄La₂Ti₄Ta₆O₃₀ and Ba₅LaTi₃Ta₇O₃₀crystallize in single-phase tetragonal tungsten bronze structure after calcination at 1573 K.All the three compositions are paraelectric at room temperature.They indicate a high dielectric constant(139～170),a low dielectric loss (10^-4～10^-3)and a small temperature coefficient.Ba₃La₃Ti₅Ta₅O₃₀ceramics show typical relaxor ferroelectricity,whereas no anomaly is observed for Ba₄La₂Ti₄Ta₆O₃₀ and Ba₅LaTi₃Ta₇O₃₀in the whole measuring temperature range(123～573 K).This indicates that Ba₄La₂Ti₄Ta₆O₃₀and Ba₅LaTi₃Ta₇O₃₀are paraelectric down to 123 K. The difference of dielectric constant in Ba_pLa_6-pTi_8-pTa_2+pO₃₀(p=3,4,5)series might originate from the distribution of A and B ions.Ba_pLa_6-pTi_8-pTa_2+pO₃₀(p=3,4,5) ceramics have considerable tunability and are suitable for high frequency capacitors and temperature compensation capacitors.In Ba_pSm_6-pTi_8-pTa_2+pO₃₀(p=4,5)system,Ba₄Sm₂Ti₄Ta₆O₃₀and Ba₅SmTi₃Ta₇O₃₀crystallize in P4bm tetragonal tungsten bronze structure at room temperature.Paraelectric-ferroelectric transition is observed in Ba₄Sm₂Ti₄Ta₆O₃₀at 332 K,while the dielectric constant of Ba₅SmTi₃Ta₇O₃₀shows broad peaks and strong frequency dispersion,which suggests the typical relaxor ferroelecity and can be fitted using Vogel-Fulcher equation.These dissimilar dielectric responses can be ascribed to the different occupancy of A1 and A2 ions and the subsequent different crystal strcture.Both compounds indicate weak ferroelectricity.The P_s,P_r and E_c of Ba₄Sm₂Ti₄Ta₆O₃₀are 3.13μC/cm²,0.777μC/cm² and 10.5 kV/cm,respectively.The P_s,P_r and E_c of Ba₅SmTi₃Ta₇O₃₀are 3.02μC/cm²,1.22μC/cm² and 17.5 kV/cm, respectively.Generally,the composition,crystal structure and dielectric behavior of quaternary systems Ba_pLa_6-pTi_8-pTa_2+pO₃₀(p=3,4,5)and Ba_pSm_6-pTi_8-pTa_2+pO₃₀(p=4, 5)are evaluated thoroughly.The phase transition and ferroelectricity in the above tungsten bronze compounds are discussed in detail.