First-principles Study On Multiferroicity Of Organic Charge-transfer Complexes TTF-CA And TTF-QBrCl₃

The multiferroic materials, in which the ferroelectricity, ferromagnetism and ferroelasticity coexist,have led to a wealth of physical properties and a broad application in terms of multi-function devices.In recent years,people have strong interest and enthusiasm to investigate the single-phase materials in which magnetic and ferroelectric coupling together.However,mostly common single-phase multiferroic materials are transition metal oxides,which have strong interaction among spin,lattice and charge degree of freedom.It may constraint on the deeper research of the physical mechanism in those materials.Furthermore,research on single-phase perovskite-type multiferroic fail to achieve greater breakthroughs recently,people begin to shift some of the research into organic multiferroic materials.Among these organic multiferroic materials,the electron donor-acceptor charge-transfer complexes attract more attention,but the magnetoelectric coupling physical mechanism of them still require further research.This thesis is based on the first-principle of density-functional theory,and make efforts to study the magnetoelectric coupling physical mechanism of the two kinds charge-transfer complexes,TTF-CA and TTF-QBr Cl₃,and explore their potential multiferroic properties.This thesis consists of several chapters. The first chapter gives a brief introduction of the research background and mainly progress of multiferroic materials.The second chapter introduces first-principles and related methods of density-functional theory which we used in the our calculation,as well as the related calculation softwares.In the third chapter,we make the calculation and analysis of phase transition, band structure, spin-polarized density of states of TTF-CA,and explore its antiferromagnetic and ferroelectric properties.The calculation results show that TTF-CA has a magnetic ground state of collinear antiferromagnetic configuration,and the magnetism directly influence on electron polarization.TTF-CA is a good "electronic ferroelectric" multiferroic material,highlighting the magnetoelectric coupling properties.In the fourth chapter,we make the calculation and analysis of TTF-QBr Cl₃,which has similar lattice structure and neutral-ionic transition with TTF-CA,and we continue to explore whether TTF-QBrCl₃ has a similar multiferroic properties.So we make the calculation and analysis of phase transition,band structure,spin-polarized density of states,antiferromagnetic and ferroelectric properties of TTF-QBr Cl₃.The calculation results also show that TTF-QBr Cl₃ also has remarkable magnetoelectric coupling properties,it’s a good "electronic ferroelectric" multiferroic material as we expected.