Theoretical Study Of New Ferroelectricity And Multiferroicity In Transition Metal Halides

Transition metal compounds belong to strongly correlated electronic materials,in which the interactions between spin,charge,orbital and lattice degrees of freedom produce a rich variety of electronic phases and physical properties,such as high temperature superconduct,colossal magnetoresistance,charge/orbital ordering,and multiferroicity.Multiferroics,with spontaneous magnetic order and charge dipole order,especially when they are coupling with each other,are not only physically interesting but also potentially useful for applications.On the one hand,despite significant advances on multiferroics in the past decades,it remains challenging to obtain desirable physical properties,i.e.,the coexistence of large polarization,large magnetization,and strong magnetoelectric coupling,at room temperature.On the other hand,for conventional ferroelectric(FE)perovskites with three-dimensional pseudocubic structures,their ferroelectricity is often seriously suppressed in the very thin limit due to the depolarization field and other reasons.As a consequence,new low-dimensional materials with novel FE properties are much needed to fabricate high performance devices.Last but not least,the current research on transition metal compounds is mainly focused on oxide materials,while the research on halides is relatively weak.In this thesis,the density functional theory(DFT)calculations and Monte Carlo(MC)simulations are employed to study exotic magnetism/ferroelectricity/multiferroicity in transition metal halides with promising properties.The main conclusions are summarized as following,1)Charge ordering induced multiferroicity and strong magnetoelectric effects in trirutile-type LiFe₂F₆.Trirutile-type LiFe₂F₆is a charge-ordered material with an Fe²⁺/Fe³⁺configuration.Here,its physical properties,including magnetism,electronic structure,phase transition,and charge ordering,are studied theoretically based on DFT calculations and MC simulations.On one hand,the charge ordering leads to improper ferroelectricity with a large polarization.On the other hand,its magnetic ground state can be tuned from the antiferromagnetic to ferrimagnetic by moderate in-plane biaxial compressive strain.Thus,LiFe₂F₆ can be a rare multiferroic with both large magnetization and polarization.Most importantly,since the charge ordering is the common ingredient for both ferroelectricity and magnetization,the net magnetization may be fully switched by flipping the polarization,rendering intrinsically strong magnetoelectric effects and desirable functions.2)Noncollinear ferrielectricity in two-dimensional dioxydihalides family.Our calculations predict that the monolayer dioxydihalides family MO₂X₂(M:group-VI transition metal,X:halogen)is promising two-dimensinal polar materials with exotic noncollinear ferrielectricity,induced by competing ferroelectric and antiferroelectric soft modes.The d⁰ rule is the driving force for the polar distortions.Noncollinear ferrielectric order persisting up to room temperature is expected.More importantly,the frustration between the FE and AFE modes generates an intrinsically noncollinear dipole texture,which leads to unique physics in the novel Fi E state unveiled here,such as Z₂×Z₂ topological domains,atomic-scale dipole vortices,and negative piezoelectricity.3)Ferroelectricity and piezoelectricity in quasi-one-dimensional WOX₄ halogens.A series of oxytetrahalides WOX₄(X:a halogen element)that form quasi-one-dimensional chains is investigated using first-principles calculations.The crystal structures,electronic structures,as well as ferroelectric and piezoelectric properties are discussed in detail.Group theory analysis shows that the ferroelectricity in this family originates from an unstable polar phonon mode(?)induced by the W's d⁰ orbital configuration.Their polarization magnitudes are found to be comparable to widely used ferroelectric perovskites.Because of its quasi-one-dimensional characteristics,the interchain domain wall energy density is low,leading to loosely coupled ferroelectric chains.This is potentially beneficial for high-density ferroelectric memories:We estimate that the upper limit of memory density in these compounds could reach hundreds of terabytes per square inch.This finding is advantageous for related experiments and high-performance applications.