Preparation Of Hybrid Improper Ferroelectric Ca₃Ti₂O₇ Ceramics By Sol-Gel Method And The Influence Of Bi Element Doping On Its Physical Properties

The discovery of hybrid improper ferroelectrics(HIF),in which the primary order parameter during the phase transition process is not the spontaneous electric polarization anymore,has widely caused concern in recent years,because HIF has potential possibility in creating room-temperature multiferroics with strong magnetoelectric coupling effect.The HIF widely exist in Runddlesden-Popper(R-P)structures.As a member of the hybrid improper ferroelectrics and a perovskite-related compound,Ca3Ti2O7 draws our attention.Firstly,we synthesized Ca3Ti2O7 ceramics through a tartrate sol-gel method,and investigated their crystal structure.UV-vis absorption spectra,first principles calculations,photoluminescence spectrometer and electric properties.Through X-ray diffraction of Ca3Ti2O7 ceramic,it is found that the xerogel-turning temprature was not a determining factor for the synthesis of pure phase of Ca3Ti2O7,while calcination temperature and calcination time are determining factors for the synthesis of pure phase.When the calcination temperature is 1420-1440 0C,pure phase Ca3Ti2O7 can be obtained.The obtained value of the band gap from UV-vis measurements is 3.56 e V which is similar to the theory calculation(3.35 e V).Ca3Ti2O7 ceramics show a switchable diode effect with high driving electric field.The underlying mechanism for the observed effects were proposed based on the interplaying between mobile charged defects and bound charges.It is found that,the ferroelectric hysteresis of our Ca3Ti2O7 ceramics prepared by a sol-gel method unveils an ultralow coercive field of 23 kV/cm,which is nearly one orders of magnitude lower than the bulk CTO single crystals and CTO ceramics prepared by a standard solid-state reaction process.The abatement of coercive field is due to the defects of our Ca3Ti2O7 ceramics.The coercive field is critical for material design and device performance.So,the low coercive field(EC)of our pure Ca3Ti2O7 ceramic is very beneficial for industrial production.Secondly,we selected the Bi3+ as dopant to substitute Ca2+ ions and prepared(Ca1-xBix)3Ti2O7(x=0,x=0.01,x=0.03)ceramics via a tartrate sol-gel method.And their structure,UV-vis absorption spectra,photoluminescence spectrometer and electric properties were investigated.The XRD results revealed that all the(Ca1-xBix)3Ti2O7 samples with x=0.01,x=0.03 were synthesized successfully.And with the increase of Bi3+ content,the XRD diffraction peaks of(Ca1-xBix)3Ti2O7 ceramics shift to high angles.The coercive field decreases with the increase of Bi dopant,and(Ca1-xBix)3Ti2O7(x=0,x=0.01,x=0.03)ceramics also show a more obvious switchable diode effect in the current-voltage curves with high driving electric field.