Theoretical Study On Perovskite Interface Properties,Electrochemical Activity And D-electronic Properties Of Alkaline Earth Metals

Perovskite is a kind of crystals based on octahedrons connected by common vertices,and its chemical expression is generally AMX₃.Because of the various types of perovskite materials,the symmetrical diversity brought about by the distortion of the octahedron,and the large number of symbiotic structures lead to a wealth of physical properties.Nowadays,perovskite materials are an essential part of studying materials science,including ferroelectric,piezoelectric,polarization,superconductivity,giant magnetoresistance,catalysis,photovoltaics,etc.Perovskite materials are widely used in all aspects of society,such as perovskite solar cells that have been widely studied in the energy field,and manganates that have received widespread attention in the field of information storage.Perovskite materials can be divided into two categories according to their types,one is inorganic perovskite materials,and the other is organic-inorganic hybrid perovskite materials.From the perspective of first principles,this work conducts theoretical analysis and simulation on the interfacial polarization of perovskite ferroelectric heterojunction Bi Fe O₃/SrTiO₃ and the anode surface of Ta-doped solid oxide fuel cell.First,the discovery of two-dimensional electron gas(2DEG)on the interface(La O+/TiO₂~0)of heterostructure composed of polar and non-polar perovskites has induced the heterostructure of perovskite attracted wide attention,such as La Al O₃/SrTiO₃.Recently,two-dimensional hole gas(2DHG)has also been observed on the La Al O₃/SrTiO₃(Al O₂^-/Sr O~0)interface through experiments.Bi Fe O₃/SrTiO₃ is chemically similar to La Al O₃/SrTiO₃,and its polar/non-polar Fe O₂^-/Sr O~0 interface corresponds to La Al O₃/SrTiO₃.However,unlike La Al O₃,Bi Fe O₃ exhibits spontaneous ferroelectric polarization.Because of this characteristic,we studied the influence of the polar/non-polar interface on the iron polarization in Bi Fe O₃ in this work.We calculated the geometric structure and electronic structure of BFO/STO,and analyzed the positions of 2DEG and 2DHG in the material from the density of states.Based on the modern polarization theory,the combination of ferroelectric and polar/non-polar interface can produce new physical phenomena,including the ferroelectric domain wall induced by the interface effect,unidirectional ferroelectric polarization,and 2DEG and 2DEG at the domain wall.The formation of 2DHG.In addition,it is found that the 2DEG at the ferroelectric domain wall has spin-polarized properties.We also simulated the application of structural strain on the BFO/STO superlattice,and confirmed that it has a significant effect on the in-plane polarization component of the BFO block,while it has a smaller effect on the out-of-plane polarization.This study shows the possible relationship between interface chemistry and interface physics:interface chemistry(such as changes in valence)can induce interface physics(such as electrostatic interactions at the interface),thereby affecting the overall properties of the superlattice.Secondly,it was found experimentally that the perovskite La_0.8Sr_0.2Fe_0.95Ta_0.05O_3-?was doped with Ta at the B site of the La_0.8Sr_0.2Fe O_3-?(LSF)perovskite solid fuel cell.(LSFTa05)and La_0.8Sr_0.2Fe_0.9Ta_0.1O_3-?(LSFTa10),after introducing O vacancies,has not only improved stability compared with the previous LSF,but also improved electrochemical performance when used as a fuel cell electrode.But the principle has not yet been figured out.We study the reaction mechanism from the density of state,and understand the difficulty of the reaction by calculating the enthalpy of formation and the formation energy of O vacancies.The mechanism of stability and conductivity of ferrites with different Ta content is studied by DFT calculation.Our work provides theoretical support for experiments and clarifies the reasons for this phenomenon.It provides useful inspiration for the development of stable perovskite anodes for SOFC.Finally,since it has recently been reported that alkaline earth metal(AEM)complexes have electronic properties similar to those of transition metal complexes,their d orbitals are involved in the bonding process.Although this surprising discovery has changed people's understanding of the nature of AEM,the mechanism of d orbital involvement remains unclear.Therefore,we use first-principles calculations to study calculations with VASP as the main and Gaussian as the supplement.Through the density of states of the system,analyze how different orbitals participate in the reaction,calculate the binding energy,excitation energy and hybridization energy,and use the crystal field effect analysis to put forward the mechanism of the participation of d orbitals in the AEM complex,in which the crystal field effect induces The electronic transition.We explained the effect of crystal field effect on the formation mechanism and how the electronic feedback affects its stability.In addition,AEM complexes are usually unstable,and our theoretical calculations also predict more stable AEM complexes with the participation of d electrons.