Theoretical Study Of The Electronic Structure, Optical And Elastic Properties Of M 2 O 5 (M=V, Nb, Ta)

According to the strong electronic localization,transition metal oxides M₂O₅?M=V,Nb,Ta?exhibit sorts of excellent and novel properties,such as high dielectric constant,low dielectric loss,broad-band gap,high film hardness,good chemical and thermal stabity,etc.These transition metal oxides are attractive candidates for several applications,especially in the field of catalysis,condenser,dye-sensitized solar cell,catalyst,electrochromism,secondary batteries,and so on.However,the preparation process under different temperature and pressure make the M₂O₅?M=V,Nb,Ta?exist lots of phase structure.In general,the reported experiments thoroughly studied the common structures.Nevertheless,due to the complicated symmetry,a series of monocline structure ?-V₂O₅,?-Nb₂O₅ and B-Ta₂O₅ often be neglected.Thus,it's important to explore these traditional structures,and the new reconstituted one also be analyzed in this paper.Firstly,based on density functional theory?DFT?,this paper uses the first principles to predict the fundamental properties of ?-V₂O₅,?-Nb₂O₅ and B-Ta₂O₅,namely m-M₂O₅?M=V,Nb,Ta?.Taking advantage of the local density approximate?LDA?and generalized gradient approximation?GGA?functions of CASTEP module,this paper also investigates the electronic structure,optical and elastic properties,and the thermal conductivity for the three kinds of monocline structure m-M₂O₅?M=V,Nb,Ta?.The electronic structure plays an important role in exploring the application in optical device.Therefore,the scissors operator is added to guarantee the accuracy of electronic structure and optical properties.And on this basis,we obtain the changes of band gap under different pressures.Results indicate that the band gap follow this rule:?-V₂O₅?2.81 eV?< ?-Nb₂O₅?3.91 eV?< B-Ta₂O₅?4.21 eV?.This shows the m-M₂O₅?M=V,Nb,Ta?are all indirect bandgap semiconductors.When the pressure increases,the bandgap changed significantly,and the critical points appear.Corresponding to band structure,the wave crest in the density of states reflects the strong bonding strength and sp hybridization.Then,the optic constants of dielectric constants,absorption coefficient,reflectivity,complex refractive index,complex conduction,and loss function,which are closely related to the electronic structure,are discussed completely.On account of the microcrack and lattice distortion appeared in the synthesis process,which always influence the excellent performance of materials.Thus,the calculated elastic properties imply the high elastic modulus and anisotropy for m-M₂O₅?M=V,Nb,Ta?.In addition,the thermal conductivities make the m-M₂O₅?M=V,Nb,Ta?enough to apply for the materials of heat barrier coat.In chapter 4,the atom Ta of traditional structure b-Ta₂O₅ are replaced by V and Nb from the ?B group.The traditional and rebuild structure are unified name as b-M₂O₅?M=V,Nb,Ta?.In this section,the theoretical part is the same as above.However,when calculate the electronic structure and optical properties,the Norm-conserving pseudopotentials?NCPP?and hybrid functional HSE06 are adopted for comprehensive study.The results indicate that the valence band maximum?VBM?of b-V₂O₅ crosses the Fermi level,which reveal the conductivity of materials.But the VBM doesn't overlap the conduction band minimum?CBM?.This phenomenon means conductivity changes as the external environment changes.In constrast,the VBM and CBM ofb-Nb₂O₅ and b-Ta₂O₅ correspond to the same point,which are called the direct bandgap semiconductors.With the increase of pressure,the changes of bandgaps are gentle,and the crure of b-Ta₂O₅ shows different trajectories.In the calculation of optical properties,the relationship between the dielectric function,absorption spectrum,refractive index,and electronic structure can be observed.The electronic inter-band transition and band transition occur in the ultraviolet region.While the elastic properties show elastic modulus,shear modulus,and Young's modulus of b-M₂O₅?M=V,Nb,Ta?are larger than m-M₂O₅?M=V,Nb,Ta?,and the same as the degree of anisotropy.Similarly,m-M₂O₅?M=V,Nb,Ta?has a small minimum thermal conductivity,which can be used as thermal insulation film materials and thermal barrier materials.