Hybrid Improper Ferroelectricities In Sr-based Ceramics With Ruddlesden-Popper Structures

As functional materials that respond to external fields in a unique way,ferroelectric ceramics bear great application potential in areas like detectors,sensors and electrocommunication.Moreover,the proposal of the conception,"magnetoelectric coupling",renders novel ferroelectrics as a focus in the academic field.To explore and discover more novel ferroelectrics,Sr3-xCaxSn2O7(x = 0,0.1,0.2,0.3)and Sr3-xBaxZr2O7(x = 0,0.1,0.2,0.3),whose crystal structures are both Ruddlesden-Popper layered perovskites with n = 2,are investigated in this thesis.Their crystal structures,origin of ferroelectricity and phase transition behaviors are thoroughly analyzed.Dense Sr3-xCaxSn2O7(x = 0,0.1,0.2,0.3)ceramics are synthesized through Spark Plasma Sintering(SPS)process.They are made up with infinite stacks of two layers of perovskites and one layer of rock salt(AO).Their space groups are polar A21am.These ceramics respond to external electric field as ferroelectrics,which present the nature of hybrid improper ferroelectricity,induced by the oxygen octahedral rotations that are responsible to the anti-ferroelectric displacements of A-site ions.Ca ions,with smaller radius than Sr ions,strengthen the ferroelectricity in the materials.The largest remnant polarization value is 0.12 ?C/cm2 when x = 0.3.The ferroelectric transition temperature decreases with increasing Ca substitute,with the largest value of 563 K and lowest of 398K.The thermal hysteresis appeared during DSC and dielectric tests indicate a typical first-order ferroelectric transition behavior.Based on the results of variant-temperature XRD and Raman measurements,the paraelectric space group of this material is confirmed as I4/mmm.Sr3-xBaxZr2O7(x = 0,0.1,0.2,0.3)ceramics are also two-layered Ruddlesden-Popper structures,with a polar space group of A2iam,and they are typical hybrid improper ferroelectrics,whose ferroelectricity originates from the rotations and tilts of oxygen octahedral.According to this mechanism of ferroelectricity,to incresease tolerance factor of the system is a way to decrease its coercive field.So Ba2+ ion,whose radius is larger than Sr2+ ion,is chosen in this case as the substitution to decrease the coervive field of this system,along with its ferroelectricity,theoretically.But the experimental results show otherwise.Reasons for this anomaly could be that ferroelectric hysteresis loops detected are not fully developed.The substitution of Ba2+ions cause changes in the ferroelectric transition temperatures,by decreasing them as x increases in a linear way.The highest transition temperature in this group of materials is 682K,and the lowest is 53 7K.DSC results and dielectric response in these materials reveal the typical nature of first-order improper ferroelectric transition behavior.