First-principles Study Of The Electronic Structure Of La_1-xSr_xMnO₃

Perovskite manganese oxide La_1-xA_xMnO₃ has attracted the attention of scholars due to its special electronic properties and magnetic properties,and is widely used in spintronic devices,resistive sensors and so on.In this group of materials,La_1-xSr_xMnO₃exhibits properties such as colossal magneto-resistance?CMR?and metal-insulator transition due to the delicate coupling between charge,spin,orbital,and lattice freedom.However,there are few studies on optical properties of La_1-xSr_xMnO₃.Morever,in the actual preparation and application process,there are usually external conditions?such as oxygen vacancy defects,pressure,magnetic field strength and oxygen pressure?to affect structure,electrical properties,magnetic properties and optical properties of La_1-xSr_xMnO₃ materials.In this paper,we calculated the geometric structure,electronic structure and optical properties of La_1-xSr_x MnO₃ materials.Then,the effects of oxygen vacancy defects,pressure and uniaxial strain on electronic structure and optical properties of La_1-xSr_xMnO₃ materials were studied.The main findings are as follows:1.Six exchange-association functions?GGA-PBE?GGA-RPBE?GGA-PW91?GGA-WC?GGA-PBESOL and LDA-CAPZ?are used to optimize La_1-xSr_x MnO₃?1/6?x?5/6?strong correlation materials.According to the energy minimum principle,we selected the LDA?CA-PZ?algorithm,and U=1.6 eV is utilized to calculate the materials.The results show that with increase of Sr content,the energy of materials is becoming lower and lower,which indicates that Sr replaces the La atom in the A position affects the stability of the structure.The results of band structure reveal that half-metallic of material disappears when x=1/6 and x?1/2 materials are metallic.When 1/6<x<1/2,the minority spin band structure has bandgaps,which indicates that matericals are half-metallic.The position of the valence band maximum of the band gap is determined by O-2p orbital,meanwhile the position of the conduction band minimum is determined by Mn-3d orbital.And as x component increases,band gap gradually decreases,while the conductivity increases.For optical properties of the materials,the absorption of visible and infrared light for materials is gradually enhanced with increasing Sr content.The static refractive index also has an increasing tendency with increase of x,and when photon energy is higher,k tends to be zero,n tends to be a constant.2.We select La_0.75Sr_0.25MnO₃?LSMO?materials with x=0.25 and remove one oxygen atom per layer to obtain z=0,z=c/8,z=c/6,z=c/4,and z=c/2La_0.75Sr_0.25MnO_3-?(LSMO_3-?)crystal structures.The results show that bond angles becomes smaller and rotation angles becomes larger;at the same time,Mn-Mn spacing of the z=0?z=c/6 and z=c/2 LSMO_3-?defect models is smaller than that of remaining defect models,which due to the direct contact between the two Mn atoms to produce a strong d-d orbital interaction.The effect of oxygen vacancy defects on structure causes changes in band structure and density of states.In minority spin band structure of materials,only the band gap of z=0 LSMO_3-?is smaller than the that of ideal LSMO structure,so its conductivity increases.Furthermore,the conductivity of z=c/6 and z=c/2 LSMO_3-?defects are lower than that of ideal structure due to the increase in the bandgaps.In partial density of states,Mn-t_2g states of the z=0?z=c/6 and z=c/2LSMO_3-?defective structures are wider than that of LSMO.For the calculation of oxygen vacancy formation energy,it is known that z=c/2 LSMO_3-?vacancy has the lowest formation energy,and oxygen vacancies are most easily formed.In the visible light range,z=0 LSMO_3-?has the strongest absorption and z=c/4 LSMO_3-?has the weakest absorption.3.Ferromagnetic FM La_0.75Sr_0.25MnO₃?LSMO?has the lowest energy and is the most stable in entire pressure variation range.An anti-ferromagnetic phase transition happened while tensile force is above 19.7GPa,from the original stable AFM2 magnetic to the AFM1 magnetic.However,under the strain condition,LSMO crystal structures undergo a transformation of AFM1 metallicity-FM half metallic-AFM2 metallicity-AFM1 metallicity.For electronic structure of the material,when the pressure is lager than or equal to 9 GPa,FM La_0.75Sr_0.25MnO₃ loses half-metallic.At the same time,its conductivity is enhanced.The disappearance of half-metallic is mainly due to the Mn-3d state crossing the Fermi level.However,there is no obvious change in the band structure of AFM LSMO.Under compression or tensile strain,the ferromagnetic phase is the most stable in the range of-6.5%⁵%,and the minority bandgap increases firstly and then decreases,corresponding to the decrease in the electrical conductivity before the enhancement.As pressure increases,the absorption of the infrared light by the material gradually weakens;after pressure reached 9 GPa,the absorption of infrared light by the FM phase and AFM phase LSMO enhanced with increasing pressure.Under the strain conditions,the light absorption of LSMO materials gradually decreases with the increase of compressive strain in the infrared range.As the tensile strain increases,the absorption of light in the visible and infrared regions of the AFM magnetic phase LSMO is gradually weakened.