First-principles Study On The Physical Properties Of Novel Organic-inorganic Hybrid Perovskite Halide Ferroelectrics

Organic-inorganic hybrid perovskite（OIHP）materials are a new class of composite materials formed by the self-assembly of organic and inorganic molecules.crystals with a perovskite structure are formed in the octahedron.This hybrid material combines the advantages of organic and inorganic components,both structural diversity,mechanical plasticity,high polarizability provided by organic components,and thermal stability,semiconducting,and magnetic and dielectric properties provided by inorganic components.performance.These excellent properties make organic-inorganic hybrid perovskite materials have potential applications in photovoltaic cells,new memory and optoelectronic devices.In this paper,we investigate the ferroelectric,magnetic,piezoelectric,optical absorption and carrier mobility properties of organic-inorganic hybrid perovskites in different dimensions（quasi-one-dimensional,one-dimensional,three-dimensional）.The main researches are as follows:（1）We systematically investigated the ferroelectric,optical,and electrical properties of quasi-one-dimensional OIHP（MV）AI₃Cl₂（MV=methylviologen,A=Bi,Sb）.The polarization mechanism of ferroelectric（MV）AI₃Cl₂ was explored,and it was found that the ferroelectric polarization of（MV）AI₃Cl₂ mainly originated from the distortion along the direction of the octahedral chain.Due to the loosely coupled ferroelectric chains in the quasi-one-dimensional structure,（MV）Bi I₃Cl₂ and（MV）Sb I₃Cl₂ have relatively low energy barriers for polarization conversion when the ferroelectric polarization is reversed by 180°.The calculated results predicted that the upper storage densities of（MV）Bi I₃Cl₂ and（MV）Sb I₃Cl₂ are 14.9 Tb/cm²and 15.0Tb/cm²,respectively,which are expected to be applied in high-density ferroelectric memory devices.The strong anisotropic optical absorption of（MV）AI₃Cl₂occurs in the visible region,and its maximum power conversion efficiency is>23%.The high anisotropic carrier mobility of（MV）AI₃Cl₂ facilitates the separation of electron-hole pairs.（MV）AI₃Cl₂ has a positive piezoelectric effect.Therefore,using strain engineering to improve the power conversion efficiency and carrier mobility of（MV）AI₃Cl₂ provides a design for the experiments of ferroelectric photovoltaic cells.（2）Since the quasi-one-dimensional ferroelectric OIHP（MV）AI₃Cl₂ contains heavy metal elements Bi,Sb and I elements,we study its k_x-k_y plane Rashba spin texture.The Rashba constant increases with the enhancement of ferroelectric polarization.In addition,the Rashba constant increases along the z direction.Electric field and strain engineering can effectively tune the Rashba spin splitting of（MV）AI₃Cl₂,which is expected to be applied in spin field effect transistors and photovoltaic cells.（3）We explore the ferroelectric,optoelectronic,and piezoelectric properties of three-dimensional（EATMP）Pb₂X₆[EATMP=（2-aminoethyl）-trimethylphosphanium]（X=I,Br,Cl）.The（EATMP）Pb₂X₆ space group is Pc,with biaxial ferroelectricity.The ferroelectricity mainly originates from the organic molecular EATMP rotation and the inorganic octahedral Pb₂X₆ distortion.（EATMP）Pb₂X₆ exhibits strong light absorption anisotropy and high carrier mobility.The photoelectric conversion efficiency of（EATMP）Pb₂I₆ is as high as 16%.It has good stability in mechanical properties.This provides a good theoretical basis for ferroelectric photovoltaic cells.（4）We systematically study the magnetic and ferroelectric properties of multiaxial multiferroic organic-inorganic hybrid perovskite material（TMCM）TMCl₃（TMCM=trimethylchloromethylammonium,TM=V,Cr,Fe,Co,Ni）.（TMCM）TMCl₃ exhibits strong ferroelectric polarization and antiferromagnetism,further confirming its multiferroic properties.The ferroelectricity originates from the rotation of organic molecules,and the magnetism originates from the 3d orbital spin splitting of transition metal elements.Therefore,if the ferroelectric polarization is used to control the magnetic phase transition in（TMCM）TMCl₃ to realize the electronically controlled magnetism,it is expected that the application of multiaxial multiferroic（TMCM）TMCl₃in information storage.