Hybrid Improper Ferroelectricity In Ca₃Ti₂O₇-based Ceramics

Ferroelectric materials have extensive applications in novel electronic devices.Recently,the field of ferroelectric materials has been widely expanded by introducing the concept of improper ferroelectrics.Among them,Ruddlesden-Popper?R-P?layered perovskite structure material,in which ferroelectricity originates from the oxygen octahedral distortion,has been a hot spot of current research because its intrinsic electrical-controlled magnetism character paves up a novel way to achieve multiferroic at room temperature.In the present work,we prepared Ca_3-xLa_xTi_2-xAl_x O₇?x=0.0,0.1,0.2,0.3?ceramics and Ca₃Ti_1.8Al_0.1B_0.1O₇?B=Nb,Ta?,Ca₃Ti_1.9Al_0.1O_6.95.95 ceramics based on Ca₃Ti₂O₇,then investigated the influence of ionic substitution on the crystal structures,ferroelectric and dielectric properties,phase transition behaviors,as well as the relationship between ferroelectric behavior and its physical origin.At room temperature,the crystal structure of Ca_3-xLa_xTi_2-xAl_xO₇?x=0.0,0.1,0.2,0.3?ceramics belongs to a polar A2₁am space group.The ferroelectric orders originate from a^-a^-c⁺,an oxygen octahedral distortion mode,which generated by the coupling of in-plane rotation?a⁰a⁰c⁺?and out-of-plane tilt?a^-a^-c⁰?of the oxygen octahedra.Furthermore,the antiferroelectric displacements of A-site ion are generated,breaking the spatial inversion symmetry of the crystal.The tolerance factor increases with increasing x.The maximum value of remanent ferroelectric polarizations appears in Ca₃Ti₂O₇ ceramic,almost 0.91?C/cm².The remanent polarizations of the samples decrease with increasing x,while the coercive fields remain to be stable,which may be related to the declined octahedral tilt mode and the unexpected rigid rotation mode,respectively.Curie temperatures decline linearly as x increases,both the DSC test and the dielectric response of the samples indicate a first-order improper ferroelectric transition in the vicinity of phase transition.The variable-temperature XRD results reveal that the ferroelectric-paraelectric transition is from orthorhombic A2₁am to tetragonal I4/mmm at the Curie temperature,and two phases coexist in a region below the Curie temperature.Ca₃Ti_1.8Al_0.1B_0.1O₇?B=Nb,Ta?,Ca₃Ti_1.9Al_0.1O_6.95 ceramics also form a polar A2₁am space group at room temperature.The maximum value of remanent ferroelectric polarizations appears in Ca₃Ti_1.8Al_0.1Nb_0.1O₇,which is enhanced to about 1.26?C/cm².However,the ferroelectric polarization of Ca₃Ti_1.8Al_0.1Ta_0.1O₇ falls down to 0.38?C/cm².On the one hand,the substitution of Ta⁵⁺results in the decrease of the octahedral tilt angle?_T to an even larger extent than that of the Nb⁵⁺.On the other hand,Nb⁵⁺has a stronger ferroelectric activity than Ta⁵⁺.The rigid coercive fields could be adjusted by the stable?_R.Caused by a small increase of the tolerance factor of about2.89%,Curie temperatures of Ca₃Ti_1.8Al_0.1Nb_0.1O₇ and Ca₃Ti_1.8Al_0.1Ta_0.1O₇ remain basically unchanged.While the tolerance factor of Ca₃Ti_1.9Al_0.1O_6.95.95 increases by 16.4%as a consequence of using the smaller ionic radius Al³⁺to replace Ti⁴⁺cations and the Curie temperature declines with substitution of Al³⁺cations.Obvious endothermic and exothermic peaks,as well as thermal hysteresis,were observed in DSC thermal analysis.Combined with the dielectric anomalies observed in the dielectric response,which could prove a typical first-order improper ferroelectric phase transition in the ceramics.