The Effect Of Vacancy Defect & Substitutional Doping On The Structure And Mechanical Properties Of CaB₆

Metal hexaborides?MB₆?are a series of important structural&function materials with excellent properties including high melting point,high strength and superior chemical stability.Calcium hexaboride?CaB6?among the hexaboride show low density and high hardness as a promising ceramic material.Besides,the low bandgap for CaB6 makes it a new semiconducting material for spin electronics.At present,a lot of investigations for metal hexaborides have been carried out on the synthesis,preparation and performance analysis.The first-principle calculations are used in this paper to estimate the electronic structure,mechanical and optical properties for some kind of divalent alkaline-earth hexaborides and lanthanum hexaboride theoretically,and analyze the effort of microscopic mechanism in the vacancy or substitutional doping defect on the macroscopic properties.This paper's calculation utilizes CASTEP package based on the density functional theory.The main contents are as following parts.The lattice parameters,band structures,bonding orbitals,density of states,mechanical properties and stabilities for four kind of divalent alkaline-earth hexaborides and lanthanum hexaboride are estimated using first-principle calculation in the present work.Magnesium hexaboride is not a stable phase.A striking similitude for the band stuctures of M?Ca,Sr,Ba,La?B6 near the fermi level can be observed.The first three of these belongs to direct-gap semiconductor,with the bottom of the conduction band and the top of the valence band located at X point of brillouin zone.The band gap of CaB6 is 0.715 eV,and the gap for SrB6,BaB6 decreases in order.LaB6 show properties of metals.The optical properties of metal hexaborides are associated with band stuctures.The interband transition characteristics are analyzed in combination with dielectric function,absorption spectrum,etc.The effect of vacany defect in CaB₆ on the stability is calculated.Both the Ca atom vacany and B atom vacany defect can cause a sharp rise of the formation energy.The vacancy defect has relatively great influence on the stability.The Ca vacancy formation energy is three times larger than B vacancy.CaB₆ is more prone to form B vacancy than Ca vacancy.Finally we examined the mechanical properties for M?Ca,Sr,Ba,La?B6 or doing CaB6(M?Mg,Sr,Ba,La?_xCa_1-xB₆)demonstrate a linear relationship between doping level and correlated mechanical parameters.The analyses of DOS and bonding orbitals suggest that the main exposition of the above phenomenoa depend on the electrons in the d-orbitals.Ca,Sr and Ba atom have no d-orbitals electrons,and the gained extra electrons in d-orbitals reinforce the weaker B–B?II?bond strength.The increasing of torsional strength for the bond causes the rise of shear moduli in macroscopic propert.The extra gained electrons in La atom 5d-orbital with valence electron 5d¹6s² to form hexaborides affect the shear moduli corresponds with Ca,Sr and Ba atoms.The one original 5d-orbital electron composes delocalized bond in the whole system to reinforce the material's bulk modulus and crosses the fermi level as metal properties.The Young's moduli in the[100]orientation index decreases with the growth of M atoms radii for the reason that the increasing in lattice constants stretches both the B–B?I?and B–B?II?bond length.