Theoretical Studies Of Ferroelectricity And Magnetoelectric Coupling In Lone-pair Contained Oxides

Ferroic materials characterized by spontaneous symmetry breaking,which are endowed with various energetically and structurally degenerate states and easily manupulated by conjugate external fields,lay material foundation of the information age.Since Landau and his successors proposed the famous free energy term of magnetoelectric coupling?(?)²-(?)²,the coexistence and cross-coupling of primary order parameters,namely,multiferroicity,has attracted great attention.The inter-regulations of these non-conjugate physical quantities greatly expand original monotonous functionality.Especially,the concept of magnetoelectric coupling promotes the unified framework of electromagnetic phenomena in solids.Actually,an easy to realize the magnetoelectric coupling is not straightforward.On one hand,order parameters of ferroelectricity and magnetism obey different symmetry requirements,respectively.On the other hand,microscopic origins of the two above differ.Magnetism originates from electron correlation and relates to non-zero orbital and spin angular momentum,while ferroelectricity is mainly manifested in the long-range correlation of optical phonons caused by dipole-dipole interaction in real space.Due to correlations and entanglements of charge,orbit,spin and lattice degrees of freedom in condensed matter,magnetoelectric coupling physics have already exceeded the above simple picture and contained more abundant physical connotation,including new effects such as excitation,correlation,symmetry,topology,domain and interface coupling.The generation of ferroelectricity is the core problem of magnetoelectric coupling physics and materials.For single-phase multiferroics,the phenomena of ferroelectricity are mainly caused by the lone pair electrons,geometric constraints,charge frustration and magnetic orders.As well,interfacial effects,mechanical coupling,Rashba effect and field effect are also fundemental to composite multiferroics.All these extend the content of ferroelectric physics,and promote their practical application in information functional devices with magnetoelectric coupling.Therefore,it is of great significance to investigate ferroelectric mechanisms and to search multiferroic materials with strong magnetoelectric coupling.Based on density functional theory,analytical model and group theory,we have studied ferroelectric instability,magnetoelectric coupling,metal-insulating properties and other related physical phenomena in lone-pair contained oxides.Lone pair electrons are important ferroelectric mechanisms in traditional ferroelectrics and type-I multiferroics.Their high electronic polarizability and electron-phonon coupling lead to the instability of ferroelectric structures,and are also potential inducements for the phenomena of second harmonic generation,Jahn-Teller distortions and ferroelectric metals.Firstly,we ultilized first-principles orbital selective external potential method,which enables one to apply an external potential to shift the energy level of a specific atomic orbital.The orbital hybridization and ferroelectricity of lone pair electrons in perovskite oxides,represented by ferroelectrics Pb Ti O₃ and type-I multiferroics Bi Fe O₃,are comprehensively studied.The formation of lone pair electrons is related to the orbital hybridization of cations'6s and 6p orbitals as well as anions'2p states in perovskites,and s-p parity matching is the driving force for the reduction of structural symmetry.The electronic hybridization weakens the electrostatic repulsion of short-range electrons,and is conducive to the generation of long-range ferroelectric correlation.Besides,the ferroelectricity of lone pair electrons is an electron-phonon coupling effect.Its contribution to polarization comes not only from the formation of electron dipole moments,but from the mediations of various polar and non-polar lattice vibration modes.Therefore,we have conclusively clarified important roles of lone pair electrons in traditional ferroelectrics and multiferroics.Secondly,we investigate ferroelectric displacements of magnetic ions to defy d⁰ rule,i.e.mutual exclusion of ferroelectricity and magnetism of perovskite oxides,and propose an improper ferroelectric mechanism caused by antiferromagnetic super-exchange,as demonstrated in Bi Fe O₃ with B-site 3d⁵ electronic configuration.This magnetoelectric coupling scenario requires the formation of p-d spin singlet state in the process of antiferromagnetic exchange pairing.In bismuth ferrite,the half-filled 3d⁵ electronic configuration and stereochemical activity of the lone pair electrons trigger strong super-exchange in narrow p-d charge transfer regime,which facilates the formation of spin singlet states and ferroelectricity.The magnetically induced ferroelectricity occurs via collaborativeu² R² coupling with antiferrodistortive Fe O₆modes.As for p-d charge transfer insulator,this mechanism goes beyond the popular belif that spin orbit coupling is the only source towards strong magnetoelectric coupling in single-phase multiferroic materials,thus unveiling underrated strong magnetically driven ferroelectricity in a broad category of collinear antiferromagnets.Finally,we studied the multiferroicity and metal-insulating properties of a van der waals oxide H₂Co Se O₄,and predicted that it is a novel two-dimensional Mott Slater insulator and a potential two-dimensional multiferroic.The electron correlation in narrow band caused by Jahn-Teller effect,as well as antiferromagnetism will collectively open and maintain the band gap in paramagnetic state,indicating that the insulating property is a Mott-Slater type.Exchange frustration may be caused by the nearest-neighbor and next-nearest-neighbor antiferromagnetic super-exchange in combined with the third-and fourth-nearest-neighbor antiferromagnetic direct exchange in tetragonal-like lattice,resulting in fragile two-dimentional antiferromagnetism from magnetic anisotropy.Symmetry analysis shows that H₂Co Se O₄ has abundant polar units,including Co²⁺ions'Jahn-Teller distortion,Se⁴⁺ions'lone-pair electrons and orientationally ordered hydrogen bonds.They break the space inversion symmetry,and cause a chirally interlock structure.As well,nonpolar?₁⁺phonon mode strongly stabilizes polar₁?^-phonon mode with situable ferroelectric switch potential.And Dzyaloshinskii-Moriya interaction introduced by the polar structure will cause the reversal of the direction of net magnetic moment from spin-canting when electric field overturns the polarization,which renders H₂Co Se O₄ a two-dimensional multiferroic.Particularly,the capability of lone pair electrons that cuts off structure in some crystal direction and exposes themselves to outer-surface without dangling bonds,provides a possible route to overcome the depolarization field in traditional oxide ferroelectric films,and also to search and design two-dimensional correlated oxide materials with rich ferroic properties.