Rare-Earth 4f/5d Electrons And Their Bonding Characters In Modulation Both On Structure And Properties Of Conductive Materials

The unique 4f5d electrons of rare earth elements have resulted in the wide application of rare earth functional materials in the fields of optics,electricity and magnetism.Lanthanide contraction as a controller for tuning the microstructures of materials can make the structure of materials change "quasi-continuously",which provides a particular platform for revealing the relationship between structure and properties of functional materials.In this thesis,based on density functional theory,firstly,the bonding properties of 4f5d electrons in PrO2 are studied with consideration of strong-correlated effects,and then with lanthanide contractions,the relationships between structure and properties of several electrical functional materials are revealed here.It is found that 4f electrons in PrO2 are divided into two parts:localized and delocalized,and under the change of chemical environment,the two parts can be converted to each other via 5d electrons as electronic channels.It also reveals the basic micro-mechanism both of static and dynamic Jahn-Teller distortion.In investigation of 1111-type Fe-As-based superconductors REFeAsO,it is found that fine tuning of the microstructures can balance the electron-hole pair,which is beneficial to the increase of superconducting temperature.The contribution of rare-earth 4f electrons to magnetic coupling can not be ignored and has a positive effect on Fermi surface.Finally,the effect of rare earth on the modification and strengthening mechanism of ionic conductive material CeO2 was studied.It is found the charge difference gradient in the system is related to the bonding strength of 4f5d electrons,and an intrinsic driving force for oxygen vacancy migration.In addition,the transport behaviors of oxygen ions and protons in the cathode materials of solid oxide fuel cells,La2NiO4 and La3Ni2O7 with Ruddlesden-Popper structure are also studied in this thesis.In the first chapter,we give a general introduction for our research background-We review the electron strucute of the rare earth elements,their physical and chemical properties,the research situations of rare earth functional materials,the strong correlated behaviors and their physical scenes related to the 4f/5d electrons.According to the problems in the basic research of rare-earth,our research purposes and significance for the present paper are presented,and then the main works are given in brief.In the second chapter,we introduce the fundamental knowledge about the density functional theory,including its basic approximations,the common theoretical model,the form of exchange-correlation functional,and basis functions and its treatment.Meanwhile,the common uses of the simulation software are briefly introduced.In the third chapter,we reveal the 4f5d electron bonding property and its relevance to the peculiar physical properties within PrO2.The electron Coulomb repulsion and spin-orbit coupling effects are both taken into consideration in the density functional theory calculations,which in combination with Wannier function methods.According to the first principles calculations,the strong-correlated'behavior of 4f/5d electrons,including the static and dynamic Janh-Teller effects,the magnetic coupling and the coupling between spin and orbital ordering are discussed Thus,an unprecedented and transparent description with a vivid comprehension of the behavior of 4f/5d electrons is given in this work.In the fourth chapter,we use first-principles calculations based on density functional theory by accounting for both electronic correlation and spin-orbit coupling effects on rare earths and Fe to study the underlying microscopic mechanism as the microscopic structural distortions in REFeAsO is tuned by lanthanide contraction or strain.Herein,two highlights are put forward.One is that an optimal structural regime that will not only initialize but also optimize the orbital fluctuations due to the competing Fe-As and Fe-Fe crystal fields is found.It is considered that the key structural features in REFeAsO,such as ?As-Fe-As bond angle intrinsically induce the modification of the Fermi surface and dynamic spin fluctuation.On the other hand,we found that the rare-earth 4f electrons also play important roles on the high superconductivity transition temperature whose behavior might be similar to that of the heavy-fermion superconductors.To sum up,the high-Tc for these 1111-type iron-based superconductors is considered to originate from the synergistic effects of local structures and 4f electrons.In the fifth chapter,a series of aliovalent rare-earth doped ceria(LnxCe1-xO2-?,Ln = lanthanides)served as SE is comprehensively and comparatively calculated,through which the determinant factors for oxygen vacancy formations and their migration activity are figured out at atomistic level via the first-principles calculation with the consideration of electronic correlations.Initially,it is found that the oxygen vacancy formation energies of the Ln-doped ceria are largely reduced in contrast to the undoped ceria(CeO2-?),which is obviously agreed with the previous literatures.Then,the migration activity of oxygen vacancy in LnxCe1-xO2-? is closely correlated to the association energies of Ln-Vo,in which the different 4f5d bonding property for different Ln ions should be taken into account.Additionally,the analysis of charge difference gradient(CDG)is revealed to be the intrinsic driving force for oxygen vacancy migration.We hope that our investigation provides a microscopic insight into the oxygen vacancy defect physics,and also is benefit to design more advanced relevant functional materials.In the sixth chapter,we systematically investigate both oxygen migration and proton diffusivity for transition-metal(Mn,Fe,Co and Cu)doped La2NiO4/La3Ni2O7 using first-principles calculation.The results show that the double-layered La3Ni2O7 exhibits better transport behaviors than single-layered La2NiO4,which consistently corresponds to the experimental results.Transition-metal doping has remarkable influence on the oxygen/proton transport for La2NiOa/La3Ni2O7.Furthermore,according to the in-depth electronic analysis,direct links between the barriers of oxygen/proton migration and microelectronic properties have been established:the migration activity of oxygen ion is closely relevant to the degree of metal-O bonding and charge difference gradient forming along the oxygen migration pathway,and the faster proton diffusion in the Co/Cu doped La2NiO4 and Mn/Fe/Co/Cu doped La3Ni2O7 are attributed to their weak dopant-proton association and large capacity of'electron pocket' around Fermi level.Therefore,our study presents a microscopic understanding of oxygen/proton migration in La2NiO4/La3Ni2O7-based perovskites and provides the design principle for high performance cathode materials.