Study On The Preparation And Dielectric Properties Of Ruddlesden-popper Phase A₃B₂O₇ Ceramics

On the basis of successful first-principles predictions of hybrid improper ferroelectric,a number of new ferroelectrics have been experimentally comfired.Among them,the n=2Ruddlesden-Popper phase A₃B₂O₇ is an ideal structure for the exploration of the room temperature ferroelectric materials due to its special superlattice configuration,and it has become a hot spot in the research of new type of hybrid ferroelectric in recent years.To seek new hybrid improper ferroelectricity,firstly,the phase composition and dielectric properties of several A₃B₂O₇（A=Sr,Ba,B=Zr,Sn,Ti）ceramics were studied.The results show that four kinds of pure Ruddlesden-Popper phase compounds were obtained by soild reaction,which were Sr₃Zr₂O₇,Sr₃Sn₂O₇,Sr₃Ti₂O₇,Ba₃Sn₂O₇.Through the measurement of the dielectric properties,we find that Sr₃Sn₂O₇ may be a new kind of room temperature hybrid improper ferroelectric,and Ba₃Sn₂O₇ shows obvious relaxor ferroelectric characteristics.Secondly,the preparation process,structure,dielectric and ferroelectric properties of Sr₃Sn₂O₇ ceramics were studied.The phase purity was characterized by XRD,and the microstructure and composition were analyzed by SEM and EDS.The dielectric constant as the function of the temperature at different frequencies showed a dielectric peak at about132℃,and the polarization-electric field measurement at room temperature showed a typical hysteresis loop.The results indicate that the density of Sr₃Sn₂O₇ ceramics second sintered at 1550℃was about 96%.It has an orthorhombic structure with a=20.6823（4）?,b=5.7411（1）?,c=5.7159（1）?,dielectric constant value about 7 at room temperature and it shows significant ferroelectricity with the Curie temperature at about 132℃.Finally,the phase composition and dielectric properties of A-site Ba²⁺、Ca²⁺substitution(Sr_1-xBa_x)₃Sn₂O₇、(Sr_1-xCa_x)₃Sn₂O₇ and B-site Zr⁴⁺substitution Sr₃(Sn_1-xZr_x)₂O₇were studied.The results indicate that A-site Ba²⁺substitution(Sr_1-xBa_x)₃Sn₂O₇ can form contionuum soild solution,the maximum substitution of A-site Ca²⁺is less than 0.3,and the maximum substitution of B-site Zr⁴⁺was 0.15.The substitution of Ba²⁺reduced the ferroelectric phase transition temperature of Sr₃Sn₂O₇,and the substitution of Ca²⁺and Zr⁴⁺increased the ferroelectric phase transition temperature of Sr₃Sn₂O₇.Their change rates of Curie temperature per tolerance factorΔT_c/Δt are-1.2×10⁴℃、-3.2×10⁴℃、-0.9×10⁴℃.The results indicate that the tolerance factor is one of the most important factors affecting the ferroelectric phase transition temperature in hybrid improper ferroelectric.