The First Principles Study Of Two Iron-based Selenides

The iron-based materials have attracted enormous interest in the field of condensed matter physics,due to its diversity of crystal structures,abundant physical properties and wide application prospects.This type of iron compounds has many interesting physical properties,such as ferroelectricity,(anti)ferromagnetism and superconductivity,which have great application potential for magnetic storage,low power device,medical,transportation and so on.In this thesis,the methods of first principles have been employed to study the electronic and magnetic properties of iron-based family and related compounds,and to predict the physical properties of new synthetic materials.The thesis is organized as below:In the first chapter,the research backgrounds,including two kind structures of iron-based compounds and related physical properties,have been briefly introduced.For the iron-based materials,the previous researches focused on the superconductivity,but there are some other interesting phenomena in those materials.In this thesis,we primarily review the magnetism,ferroelectricity,multiferroicity and superconductivity,and thus hope to search for more iron-based compounds with these physical properties.In the second chapter,the theoretical backgrounds of our calculations,including the fundamental laws of the crystal field theory,the density functional theory and the Berry Phase theory,and the VASP package have been briefly introduced.In the third chapter,KFe2Se2,the parent compound of iron-based superconductor(122 type),has been investigated in detail.Using symmetry analysis,we find that each Fe-Se layer is polar.Comparing to the neutron experiments of BaFe2Se3,we construct three possible Block antiferromagnetic orders in KFe2Se2-Then the magnetic point group analyses indicate that only Block-B antiferromagnetic configuration is ferroelectric and others are antiferroelectric.Furthermore,by first principles calculations,firstly,we confirm the magnetic ground state of KFe2Se2 is the Block-type antiferromagnetic state.Secondly,due to the magnetostrictive effect,Fe-Se layer should be multiferroic and this layered system can be either ferroelectric with a finite macroscopic ferroelectric polarization P or anti-ferroelectric with offsetting P,depending on the stacking pattern of magnetic blocks along the c-axis.Thirdly,for the Block-B structure,the net P calculated using the standard Berry phase theory is along the c-axis and its magnitude is about 0.48 ?C/cm2.Finally,according to the projected density of states,the hybridization between Fe and Se suggests the partial covalence of Fe-Se bonds.In the fourth chapter,BaFe2X2O(X = S/Se)with ?-BaFe2S3 structure has been studied.Using first principles calculations,the same collinear AFM-3 magnetic configuration and large band gap have been confirmed for the whole family of those compounds,and then the most suitable Hubbard U and crystal structure have also been obtained by tuning U.Furthermore,according to the calculated density of states,we find the partial covalence of Fe-X bonds.Then the optical property has also been investigated,and we observe that the optical absorption coefficient is large for these materials,providing a broad application potential for optical devices.In the fifth chapter,we summarize our present work and propose the future work.