n=2 Ruddlesden-Popper Sr_(3)B_(2)Se_(7)(B=Zr,Hf)非常规铁电性的第一性原理研究

First-principle investigation of hybrid improper ferroelectricity of n=2 Ruddlesden-Popper Sr_(3)B_(2)Se_(7)(B=Zr,Hf)

近年来,层状钙钛矿材料中存在的非常规铁电性为新型铁电体设计提供了新的途径.基于第一性原理,本文系统研究了具有Ruddlesden-Popper (RP)(n=2)结构的Sr_(3)B_(2)Se_(7) (B=Zr, Hf)化合物的基态结构、电子结构和非常规铁电性.研究表明, Sr_(3)B_(2)Se_(7) (B=Zr, Hf)基态均为具有A2_(1)am极性相的直接带隙半导体;其非常规铁电性来源于BSe_6八面体的两种旋转模式的耦合.而且,因具有较强的铁电极化值与可见光吸收带隙, Sr_(3)B_(2)Se_(7) (B=Zr, Hf)有望成为新一代铁电光伏材料.

Recently,perovskite ferroelectric photovoltaic materials have been studied extensively.Traditional photovoltaic device usually uses the internal electric field formed by PN junction to realize the separation of photogenerated carriers to form the photovoltaic effect,while ferroelectric material,due to the existence of spontaneous polarization,can spontaneously realize the separation of photogenerated electrons and holes without the formation of PN junction,presenting the ferroelectric photovoltaic effect.Chalcogenide perovskite with suitable band gap and visible light absorption is expected to be a new generation of ferroelectric photovoltaic materials.However,its application is limited due to the lack of ferroelectric properties.Hybrid improper ferroelectricity(HIF)in layered perovskites has opened a new way for developing the new ferroelectrics.In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force,the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders.In this work,we study the ground state,electronic structure and hybrid improper ferroelectricity of n=2 Ruddlesden-Popper(RP)Sr_(3)B_(2)Se_(7)(B=Zr,Hf)based on the first principles.The total energy calculations and phonon spectrum analysis show that the ground state of Sr_(3)B_(2)Se_(7)(B=Zr,Hf)is of A2_(1)am polar phase.The hybrid improper ferroelectricity originates from the coupling between two rotation modes of BSe6 octahedron.Electronic structure calculations show that Sr3Zr2Se7 and Sr3Hf2Se7 are semiconductors with direct band-gaps,which are around 1.56 eV and 1.84 eV,respectively.The ferroelectric polarization values calculated by the Berry phase method are around 12.75μC/cm^(2) and 9.69μC/cm^(2),respectively.The contribution of each atomic layer to the ferroelectric polarization is investigated when the Born effective charge method is used.The results show that the polarization of Sr_(3)B_(2)Se_(7)(B=Zr,Hf)mainly comes from the Sr-Se atomic layers.To sum up,Sr_(3)B_(2)Se_(7)(B=Zr,Hf)show strong ferroelectric polarization and good visible light absorption characteristics and they are expected to be candidates of a new generation of ferroelectric photovoltaic materials.