Study On Physical Properties Of Hybrid Improper Ferroelectricity Of Ca₃M₂O₇（M=Mn,Ti） And Doped System

Electronic devices put forward higher requirements for the integration,low power consumption and high transmission speed due to the development of the information industry.Multiferroic materials have drawn wide attention due to their potential application value.Hybrid Improper Ferroelectric exists in Ruddlesden-Popper structure,which provides a new mechanism for magnetoelectric coupling.Ca3Ti2O7 and Ca3Mn2O7 have attracted much attention due to their unique structure.In this work,we adopted solid-state reaction to prepared the Ca3Ti2O7 ceramic and sol-gel method to prepared the Ca3Mn2O7.First,Na and Rh were selected as dopants to substitute Ca and Ti in Ca3Ti2O7 lattice,respectively.Ca2.91Na0.09Ti2-xRhxO7(x=0.00,0.02,0.04,0.06)were successfully prepared by solid-state reaction method.Ca3Ti2O7 is orthorhombic with A21am space group at room temperature.The remanent polarization and coercive field increase with Rh doping.The larger remanent polarization is due to a larger oxygen octahedral distortion caused by Rh doping,which makes Ca ions produce larger antiferroelectric displacement.The increased oxygen vacancies have a pinning effect on the ferroelectric domain switch and movement at the grain boundary,which increases the coercive field.Therefore,remanent polarization and coercive field are caused by different mechanisms,it is possible to modulate them independently to meet the requirement of application.The bandgap of samples decreases with the increase of Rh content because Rh doping introduces a shallow donor level within the band gap,leading to a decrease of the band gap.We confirm this by analyzing electronic structure based on the first-principles calculation.Then,Fe was selected as dopants to substitute Mn in Ca3Mn2O7.Ca3Mn2-xFexO7(x=0.00,0.02,0.06,0.10)were successfully prepared by sol-gel method.Ca3Mn2O7 is orthorhombic with A21am space group at room temperature by the Rietveld refined of XRD.Ca3Mn2O7 is a G-type antiferromagnetic material accompanied by weak ferromagnetic and spin-glass in low temperature and its Neel temperature is about 110 K.Fe doping changes the Mn-O-Mn angle in the crystal,which affects the super-exchange between Mn ions,and has a regulatory effect on the magnetic properties of the sample.The magnetization coefficient of paramagnetic states follows the Curie-Weiss law.The antiferromagnetic exchange is weakened while the ferromagnetic exchange is strengthened by Fe doping,and the ferromagnetic exchange is the strongest when Fe concentration is lat.%.Electronic structure study reveals that the G-type antiferromagnetic structure is the most stable in a variety of magnetic structures of Ca3Mn2O7.The spin polarization lowers its energy mainly through exchange interaction,which is consistent with the experimental results.Mn4+ are in high spin states,the up-spin states provide more orbitals than down-spin states because the down-spin states of Mn t2g orbitals are raised above the Fermi level.