Preparation,Structure And Properties Of Niobite A₆B₁₀O₃₀ Ceramics

The tungsten-bronze niobates are a very important family of dielectric and ferroelectric materials.These compounds play an important role in integrated ferroelectrics industry.According to its good compatibility and controllability of tungsten--bronze structure,through combination design of positive ion in and outside the oxygen octahedron,we prepared a series of new compounds and ceramics with filled tungsten bronzes structure in AO-Pr₂O₃-TiO₂-Nb₂O₅(A=Ba,Sr)and BaO-Ln₂O₃-Fe₂O₃-Nb₂O₅ systems.We also analyzed their crystal structure,microstructure,phase transition characteristic,dielectric and ferroelectrie properties by x-ray powder diffraction (XRD),scanning electron microscope(SEM),dielectric and ferroelectric properties measurements to provide the foudmatial data in exploring new dielectric ceramics on the one hand and discovering new lead-free ferroelectric materials on the other hand.Firstly,A_4+nPr_2-nTi_4+nNb_6-nO₃₀(A=Ba,Sr;n=0,1)ceramics are prepared by high temperature solid-state reaction route.The results showed that the structure of A_4+nPr_2-nTi_4+nNb_6-nO₃₀(A=Ba,Sr;n=0,1)niobates belonged to tetragonal tungsten bronze structure.Ba₅PrTi₃Nb₇O₃₀ceramics was in paraoelecctric phase in the room temperature,the dielectric constant of the Ba₅PrTi₃Nb₇O₃₀ceramic was very large (＞210),but the temperature coefficient of dielectric constant was also very large (＞-2750ppm°C^-1),so it might have potential application in temperature-compensating capacitors.Ba₄Pr₂Ti₄Nb₆O₃₀,Sr₄Pr₂Ti₄Nb₆O₃₀,Sr₅PrTi₃Nb₇O₃₀ceramics Only appeared phase transition for one time,which indicated the phase transition belonged to ferroelectric(P4bm)- paraoelecctric(4/mmm)phase transition with the Curie temperature of 123℃,104℃and 90℃respectively,the transition exhibited diffusive phase transition with diffusion coefficient of 1.61,1.16 and 1.78,so they might become new lead-free ferroelectric material.Then,Ba₄Ln₂Fe₂Nb₈O₃₀(Ln=La,Nd,Sm))ceramics are also prepared in BaO-Ln₂O₃-Fe₂O₃-Nb₂O₅ system by high temperature solid-state reaction route.The results showed new compounds and ceramics were all phase pure and belonged to tetragonal tungsten bronze structure.Each diffraction peak was be indexed. Ba₄Ln₂Fe₂Nb₈O₃₀(Ln=La,Nd,Sm)ceramics belong to tetragonal tungsten bronze structure and ferroelectric(P4bm),and the Curie temperature were 30℃,62℃and 138℃.Their Curie temperature reduces along with the Ln³⁺ionic radius increasing, the phase transition peak manifested diffusive characteristic and their diffusion coefficient were equal to 1.30,1.57 and 1.65.Ferroelectric properties measurements further confirmed they might become new lead-free ferroelectric material.