Research On Theoretical Calculation,preparation And Properties Of Single Crystals Rare Earth Borides

Rare earth borides are excellently thermionic cathode materials due to high melting points,high hardness,low work function,low volatility,strong resistance to ion bombardment and high chemical stability.They are widely used in various electron beam analyzer,particle beam accelerator,precise electronic analytical instrument and dynamic vacuum electronic devices.Up till now,the researches mainly focus on the praperation of doping rare earth borides single crystals and the improvement of electronic emissioin by finding the optimum doping proportion and element.However,the investigations of work function,electronic structure and the mechanism of doping are few.In this paper,we systematically study the electronic structure,work function and thermionic emission properties of binary and doping rare earth borides by experiments combining with the first-principles calculations.Firstly,we studied the electronic structures and work function of binary rare earth hexaborides by first-principles method based on density functional theory.The band structure and DOS presents that the wide range of Ln-5d state are main contributors in the region of conduction band,and provide plenty of free electrons for electronic emission.The large DOS near the Fermi level is beneficial to electronic emission.The work function becomes lower when the surface has larger amount of rare earth atoms.The work functions on different surfaces satisfy the relation of?(100)<?(210)<?(110)<?(111).The work functions of bulk binary rare earth hexaborides with(100)surface obey the relation of?(LaB₆)<?(CeB₆)<?(PrB₆)<?(NdB₆)<?(SmB₆)<?(GdB₆)<?(EuB₆).The 1×1×?nanosturcture have lower work function compared with bulk sturcture.The work functions of nanowires binary rare earth hexaborides with(100)surface obey the relation of?(LaB₆)??(CeB₆)<?(GdB₆)<?(PrB₆)<?(NdB₆)<?(SmB₆)<?(EuB₆).Secondly,to reveal the physical nature of doping,we calculated the work function and electronic structure of(La_xCe_1-x)B₆,(Ce_xPr_1-x)B₆ and(La_xM_1-x)B₆(M=Ca,Sr or Ba,x=0,0.25,0.5,0.75 or 1).It is found that(La_0.75Ce_0.25)B₆,(Ce_0.75Pr_0.25)B₆,(La_0.5Ba_0.5)B₆,(La_0.75Sr_0.25)B₆ and(La_0.75Ca_0.25)B₆ have lowest work function,respectively.It indicates that proper doping can render a relatively low work function.The bands intercepting the Fermi level are dominating by the d-orbitals of rare earth atoms which are spatially extended and hybridized with the p-orbitals of B.Doping rare earth or alkaline earth elements partly distorts the lattice structure,increasing the total energy and Fermi energy.Thus the work function decrease.Thirdly,we prepared the large-size and high-quality single crystal LaB₆,GdB₄ and GdB₆ by spark plasma sintering(SPS)combining with optical floating zone method.The lattice structure,crystal orientation,crystal quality and emission properties of single crystals are systematically characterized.The polycrystal GdB₄and GdB₆can be rapidly prepared by SPS at relatively lower temperature using the powders of Gd H₂ and B.The high-quality GdB₆ single crystal without impurities was prepared by accurately controlling the temperature and composition of the molten zone.The average effective work functions of LaB₆ and GdB₆are 2.65 e V and 2.99 e V,respectively.The thermionic emission results are basically accordant with the calculated values.GdB₄and GdB₆show antiferromagnet ordering transformation at 42 K and 16 K,respectively.GdB₄presents anisotropic positive magnetoresistance ratio of 6100%at T=5 K and H=14 T.Finally,we also prepared the large-size and high-quality single crystal of doping rare earth hexaborides by SPS combining with optical floatingzone method.Thermionic emission results present the average effective work function of(La_0.75Ce_0.25)B₆is 2.61e V.The work functions of different crystal faces of(Ce0.4La0.2Pr0.2Nd0.2)B6 and(Ce_0.25La_0.25Pr_0.25Nd_0.25)B₆both obey the ralation of?(100)<?(310)<?(210)<?(110)<?(111).The average effective work functions of(Ce_xLa_yPr_yNd_0.05Gd_0.05)B₆(x=0.5,y=0.2 or x=0.6,y=0.15 or x=0.7,y=0.1)are 2.62 e V,2.66 e V and 2.67 e V,respectively.It shows that doping rare earth hexaborides have lower work function compared with CeB₆.The thermionic emission results are basically accordant with the calculated values.