First-principles Study Of Doping And Strain Effects In BaTiO₃-based Ferroelectric Metal

Ferroelectric metals,also called polar metals,were proposed by Anderson and Blount in1965.Two supposedly taboo properties-ferroelectricity and conductivity can coexist.Until2013,Shi et.al.of the Chinese Academy of Sciences proved experimentally that Li Os O3single crystal is a ferroelectric metal material.The coexistence of metallicity and ferroelectric structure will produce many unique physical properties,such as non-centrosymmetric superconductivity,nonlinear optical response,magnetoelectric effect,and high anisotropic thermal power response.Recently,although people have made a series of progress on the coexistence mechanism of ferroelectricity and metallicity in ferroelectric metals,there are still controversies about the origin of ferroelectric metals,and the application research on ferroelectric metals is extremely rare.In this paper,using the first-principles calculation based on density function theory,systematic research on several realization ways of Ba Ti O3(BTO)based ferroelectric metals,that is,carrier doping,element doping and the construction of polar ferroelectric superlattices(SL).The coexistence mechanism of ferroelectricity and metallicity in ferroelectric-based ferroelectric metals and the strain effect regulation of the stability of ferroelectric metals are explored,and the application of ferroelectric metals in the magnetoelectric effect and thermoelectric response has been investigated and analyzed.Our research provides a theoretical basis for the doping and strain effects of ferroelectric-based ferroelectric metals.The main innovations obtained are as follows:(1)Proper carrier and atom doping can make ferroelectrics become ferroelectric metals.Taking the typical lead-free perovskite ferroelectric BTO as the research object.To explore the coexistence mechanism of ferroelectricity and metallicity in ferroelectric metals through the carrier electrostatic doping and element doping BTO.When the electron concentration is low,the electrons preferentially occupy the dxy orbital,and the polarization intensity is basically unchanged.However,the polarization stability is reduced,which is due to the anti-bonding state between Ti 3d and O 2p orbitals after electron doping.When the electron doping concentration further increases,the screening effect of the increasing dxz/dyz orbital electrons on polarization becomes more and more significant.When the electron concentration exceeds the critical electron concentration nc=0.254 e/u.c.,the ferroelectricity disappears.When the hole doping concentration is low,the holes preferentially occupy the px/py orbitals.The Ti 3d and O 2p orbitals are in a bonding state by holes doping,and the ferroelectricity is slightly increased.Like electrons,gradually increasing pz orbital holes enhance the screening effect on polarization,and the critical hole concentration|pc|=0.443h/u.c.For real atoms M(M=Al,Cr,Fe,Ni,V,and Nb)doped with BTO,it can be found that the ferroelectric and metallic properties can coexist in the Al and Nb-doped BTO system.In addition,the Nb-doped BTO system can also realize the coexistence of ferroelectric and ferromagnetic.While other doping systems,the electrons are localized near the doping atoms,which makes the electrons cannot itinerant.This has important guiding significance for the doping control of the ferroelectric properties of ferroelectric-based ferroelectric metals.(2)In-plane biaxial compression strain can effectively enhance the converse magnetoelectric effect in ferroelectric metals.Because Nb doped BTO ferroelectric metal has ferroelectric,ferromagnetic and metallic properties.Therefore,taking the traditional magnetic atom Fe-doped BTO(FBTO)system and non-magnetic atom Nb-doped BTO(NBTO)ferroelectric metal as the research objects,the FBTO system is used as a reference.To reveal the origin of the converse magnetoelectricity effect(CME)and its strain regulation in the NBTO polar metal.Unlike the FBTO system where the magnetic moment is mainly contributed by the eg orbital of Fe atoms,the magnetic moment of the NBTO system is mainly contributed by the t2g orbital of Ti(Nb)atoms and highly depends on the polarization.Although the FBTO system has strong ferroelectricity and ferromagnetism,the magnetic moment changes very little during the polarization switching((35)M=0.031?B).The weak ferroelectricity and ferromagnetism in the NBTO system,and the change of magnetic moment is also very small((35)M=-0.024?B).However,the in-plane biaxial compressive strain increases the ferroelectricity but weakens the ferromagnetism.When the in-plane compressive strain is?<-2%,since electrons mainly occupy dxy orbitals and tend to be two-dimensional(2D),the magnetic moment of the NBTO system is zero.However,the switching process of the polarization causes a strong converse magnetoelectric effect,and the magnetic moment changes significantly.Especially when?=-4%,the magnetic moment change of the NBTO system is(35)M=0.85?B,which is dozens of times that of the unstrained NBTO system,showing the extraordinary ability of electrically controlled the magnetism.These results are very useful for designing new ultra-fast and low-power information storage devices with strong electrically controlled magnetism.(3)A suitable interface structure can make the superlattice a ferroelectric metal and can be controlled by strain.Construct KNO/BTO SL with different interface structures,study the ferroelectricity and metallicity of KNO/BTO SL,and analyze the possibility that KNO/BTO SL is polar metals.Subsequently,the in-plane biaxial strain and the bulk value were compared to explore the effect of carrier screen and covalent bond synergy on ferroelectric properties.Since the difference between the minimum conduction band or the valence band maximum between BTO and KNO is very small,the symmetrical NN-type and PP-type SL can as ferroelectric metals,that is,ferroelectricity and metallicity can coexist in each unit cell,and exhibits n-type and p-type conductivity,respectively.In the NN-type SL,the compressive strain makes the ferroelectric properties closer to the bulk value under the same strain,while the tensile strain makes the ferroelectric properties far away from the bulk value under the same strain.When the compressive strain?=-3%,the dxz/dyz orbital electrons are zero,and with no screening effect of electrons.Due to the electron doping weakening the covalent bond,the ferroelectricity still decreases.In the PP-type SL with compressive strain,the pz orbital holes are zero,and the covalent bond is enhanced by hole doping,and the ferroelectricity increases.When?=1%,the pz orbital holes have a strong screen effect on polarization.Because the hole doping enhances the covalent bond effect,it can be found that the polarization properties are still Enhanced.When?>1%,the ferroelectricity decreases.This indicates that constructing a reasonable interface structure in the perovskite superlattice can become a ferroelectric metal,and the subsequent strain effect clarifies that the polarization properties is affected by the combined effect of carrier screening and covalent bonding.(4)Proper carrier doping and strain can improve the thermoelectric properties and anisotropy of ferroelectric metals.Since strain can control orbital carrier occupation in ferroelectric metals,taking BTO,carrier-doped BTO,and KNO/BTO SL as the research objects,the thermoelectric anisotropy is studied.Due to the lattice asymmetry and non-centrosymmetric structure,there is strong in-plane anisotropy of electrical and thermal conductivity in BTO.Compressive strain enhances polarization properties and leads to an increase in anisotropy conductivity.The application of tensile strain weakens the polarization properties and reduces the conductivity anisotropy.In addition,because BTO is an insulator,it has very large in-plane and out-of-plane Seebeck coefficients but does not have much thermal power anisotropy.For carrier-doped BTO,when low-concentration doping,the carrier screen effect is small,the polarization properties are basically unchanged,the conductivity anisotropy increases,and the hole doping makes the conductivity anisotropy increase more.When the carrier doping concentration makes the ferroelectricity disappear,the thermoelectric performance tends to be isotropic.For the KNO/BTO SL,due to the uneven distribution of electrons in the BTO and KNO regions,the unstrained KNO/BTO SL has stronger conductivity anisotropy than the carrier-doped BTO.When compressive strain?=-3%,dxz/dyz orbital electrons are zero,free electrons are completely restricted to move in the plane,and the anisotropy of conductivity and electronic thermal conductivity are as high as 6.8×10~5 and4.7×10~5,respectively,which means SL has huge anisotropic electrical properties.In addition,the compressively strained KNO/BTO SL also has very strong thermal power anisotropy.This shows that ferroelectric metals have broad application prospects in the thermoelectric field.