First-principles Study On The Structure And Performance Of Cation Doped KSr₂Nb₅O₁₅

Potassium strontium niobate(KSr₂Nb₅O₁₅,KSN)is a kind of ferroelectric materials with tetragonal tungsten bronze structure.It has the obvious advantages of strong spontaneous polarization,high Curie temperature,high dielectric constant,and optical birefringence,which makes KSN broad application in producing ferroelectric,dielectric and optoelectronic devices.However,during the preparation of KSN ceramics,K⁺and Sr²⁺are easily volatiled at high temperature,which affects the composition,microsturture and performance of the ceramics.Fortunately,due to the different structural voids in the KSN lattice structure,the electrical and optical performance of the KSN ceramics can be improved through doping ions and controlling cation vaccancies.In this thesis,the first-principles method is used to study the influence of A-site cations(La³⁺,Bi³⁺and Y³⁺)and B-site cations(Ti⁴⁺,Ta⁵⁺and W⁶⁺)doping as wll as the cation vacancies,on the structure and performance of KSN.And the performance tailoring mechanism of cation doping and vacancies for KSN is explored.Firstly,the KSN crystal structure model is established,different exchange correlation methods are used to optimize the lattice parameters of the defect-free KSN.The electronic structure,thermodynamic properties,electrical properties,optical properties,mechanical properties,and anisotropy of the defect-free KSN are calculated.The results show that the band gap of the defect-free KSN is 2.198 e V.As the temperature increases,the specific heat capacity,entropy,internal energy,thermal expansion coefficient,and the changes in volume of the KSN increase,while Helmholtz energy and Gibbs energy decrease.The Debye temperature of KSN is 452.61 K.The electrical and optical properties of KSN were calculated by using HSE06 and PBEsol,respectively.It is found that the performance curves calculated by these two methods had the same trend,and the results calculated using HSE06 were lower than those calculated by PBEsol.The electrical and optical properties show a certain anisotropy,with relative higher dielectric constant,absorption coefficient,refractive index,extinction coefficient and reflectance at[001]direction.Within 0-2 e V,the KSN shows transparent optical performance,and within 2-5e V,the KSN mainly shows refractivity,and within 5-8e V,KSN mainly shows reflectivity.The absorption is mainly shown within 100-400nm range,and it is UV-visible absorption.Secondly,the effects of A-site cations(La³⁺,Bi³⁺and Y³⁺)and B-site cations(Ti⁴⁺,Ta⁵⁺and W⁶⁺)doping on the crystal structure,electronic structure,chemical potential,defect formation energy,thermodynamic properties,electrical properties,and optical properties of KSN were investigated through the first-principles method.The results show that either the A-site or B-site of KSN lattice is doped with cations,the crystal structure will change from the original tetragonal phase to the orthorhombic phase because of the Nb-O octahedron distortion.A-site doping decreases the band gap,while B-site doping increases the band gap.The formation enthalpy,the defect formation energy,the specific heat capacity,the entropy,the internal energy,and the thermal expansion coefficient of doped KSN are obtained by the first-principles calculation.The electric and optical properties’calculation results show that except for the KSN-Ta,the dielectric constants of KSN-A and KSN-B increase with doping different cations.In addition,in the 0-5e V region,the doped KSN material is dominated by refraction,and the refractive property of the doped material is enhanced.The optical properties of KSN is dominated by reflection within 5-8e V,and the reflection ability is weakened with doping different cations.The absorption spectrum is mainly in the 100-400nm range,showing ultraviolet-visible absorption.Thirdly,the effects of A-site cation vacancies(K⁺vacancies,Sr²⁺vacancies,coexistence of K⁺vacancies and Sr²⁺vacancies)and B-site cation vacancies(Nb⁵⁺vacancies,at Nb1 site and Nb2 site)on the lattice structure,electronic structure,electrical and optical properties of KSN are investigated through the first-principles method.The results show that the presence of vacancies cause serious distortion of the Nb-O octahedron of KSN which results in the phase transition from tetragonal to orthorhombic,and the reduced band gap.After the cation vacancy at the A-site is generated,the specific heat capacity,entropy,and thermal expansion coefficient decrease with increasing temperature.The presence of cation vacancies at the A-site of KSN increases the dielectric constant.KSN-Sr shows the largest dielectric constant,which is 20.150 times than that of the defect-free KSN.As for the cation vacancies present in the B-site of KSN,KSN-Nb1 has larger dielectric constant than that of KSN-Nb2.KSN with cation vacancies mainly shows refraction in the 0-4 e V region,and reflection in the 4-8 e V region.The absorption spectrum is in the 100-400nm range,showing ultraviolet-visible absorption.Finally,the mechanism of cation substitution and cation vacancy on the dielectric properties of KSN is explored,the polarizability and polarization strength of the substitution and cation vacancy of KSN are studied,the Madelung constants,Born index,dipole moment,spontaneous polarization and internal electric field strength of KSN,KSN-A and KSN-B are calculated.The results show that the microscopic polarizability of KSN-A and KSN-B increases,and the microscopic polarizability is in the range of（4.970-11.546）×10^-39F·m².The microscopic polarizability of KSN with cation vacancies is in the range of（7.716-13.345）×10^-39F·m²,which is in good agreement with the experimental results.The polarization of defect-free KSN is in the range of 19.04-35.41μC/cm²,and the local electric field intensity is in the range of 2.32-4.30（10MV/mm）;the polarization of 20.19-80.76μC/cm²in the presence of doped cation is determined.Similarly,the polarization range18.11-58.07μC/cm² of the cation vacancies existed KSN is determined.And it is found that the magnitude of KSN polarization is mainly caused by the action of Nb-O bonds.The dielectric properties of KSN can be regulated by doping cations through the following mechanism.On one hand,the band gap of cation doped-KSN changes,and extrinsic lattice void defects are generated,which causes distortion of the lattice cell and increases the polarization response.On the other hand,suitable cation vacancies and oxygen vacancies form defect dipoles,increasing the induced dipole moment and spontaneous polarization,and enhancing the dielectric constant of the KSN material.