Calculation Design And Property Regulation Of Novel Perovskite Niobate Dilute Magnetic Semiconductor

Oxide materials with two-dimensional magnetic properties and transition metal element doped dilute magnetic properties,could achieve multi-scale control of electronic transport and optoelectronic properties due to the ability of magnetic-electric coupling,which is different from traditional semiconductor electronic transport materials.The exploration of novel physical properties and the development of new dilute magnetic material systems have long been the hotspots of research in this field.Perovskite oxide is the cornerstone for realizing all-oxide electronic devices.Its two-dimensional magnetic properties and doped dilute magnetic properties have been extensively studied in recent years.However,these researches are mainly focused on the titanate-based and ferrite-based perovskite oxide superlattice materials or bulk materials,which are easy to prepare.There are few reports on the research of perovskite niobate materials,of which the novel physical and chemical properties are worthy to be studied.In this thesis,Two perovskite niobate of KNb O₃ and BaNbO₃are studied.The composite molten salt method are used to prepare materials.The calculation and experimental research methods include DFT calation,SEM,XRD,TEM,XPS,UV-spectra,vibration magnetometer.Two-dimensional KNb O₃ slab model and BaNbO₃crystal model doped by transition metal elements are constructed.The novel physical properties of KNb O₃ crystal with reduced dimensionality and the influence of element doping on the dilute magnetic properties of BaNbO₃ crystal are calculated and studied.Doped BaNbO₃ diluted magnetic semiconductor is developed with the combination of calculation and experiment in screening and preparing.Its crystal structure,electronic structure and magnetic properties are characterized and analyzed,and the polyferroicity is predicted and prospected.Theoretical calculation studies show that the state and polarity of the in-plane could be controlled by the growth orientation of two-dimensional KNb O₃crystal,and realizes the transformation from two-dimensional free electron gas of the（001）_pc-oriented to two-dimensional free hole gas of the（111）_pc-oriented.The symmetry breaking of the electronic structure at the interface leads to the Zeeman-type spin polarization split in the two-dimensional KNb O₃ crystal.In the（001）_pcorientation model,the spin-split p and d orbitals hybridized and canceled each other’s electronic states near the Fermi level,thus presenting an incomplete spontaneous polarization state.In the（111）_pc orientation model,near the Fermi level is occupied by the spin-split O＿2p orbitals,which presents a 100%spontaneous polarization state when the sample thickness is less than 2 nm,showing perfect semi-metal properties.At the same time,the Zeeman-type spin-polarized splitting has a linear relationship of 1/r with the decrease in sample thickness,and its surface electric polarization has a linear relationship of lnr with the decrease in sample thickness.Calculation and experiment results have shown that the existence of oxygen vacancies causes weak magnetism in the intrinsic BaNbO₃.Fe,Co,and Ni are effective doping elements for BaNbO₃ tending to occupy the B site.The crystal structure changes from cubic to tetragonal caused by oxygen vacancies.The composite molten salt method are used to prepare BaNbO₃ and Fe,Co,Ni-doped BaNbO₃ powders.XRD and TEM characterization results verified the calculation prediction of phase transformation.Calculation and experiment both supports the change of lattice constant caused by doping,which decreases in Fe-doped crystal,increases in Co-doped crystal.The lattice constant of Ni-doped crystal presents to have a decrease in a and b axis direction and increase in c axis.Element content analysis found that the doping amount of Ni can reach up to28%,which is several times that of Fe～4%and Co～2%.This is because Ni balances oxygen vacancy defects and has the closest ionic radius to Nb.Ni-doped BaNbO₃ crytal model has modified based on the content of Ni.Nb ions present a mixed state of+4 and+5 valence in Ba Ni_xNb_（1-x）O_3-δconfirmed by XPS experiment.Calculation study has shown that this is caused by the difference in the degree of electron loss of Nb atoms with different symmetries caused by Ni doping and oxygen vacancies.Magnetic property tests show that Fe,Co,and Ni doping can increase the room temperature magnetic properties of BaNbO₃ powder materials,but Co and Ni doping is significantly better than Fe,which is consistent with the theoretical calculation results.Among them,Ba Ni_xNb_（1-x）O_3-δhas the highest M_sat～2.54 emu/g,M_r～0.228 emu/g,and small H_c～99.14 Oe.Its magnetization is two orders of magnitude of pure BaNbO₃ powder.The above experimental results well verify the prediction results of the theoretical model.