Study On Electric And Thermal Transport Properties Of Rare Earth Hexaborides

Single crystal rare-earth hexaborides(RR6,R=La,Sm,Yb,Ce,Eu,Y)attract more and more attention for complex physical properties,such as superconductivity,semiconductivity,intermediate-valence state and excellent electron emission properties.Lanthanum hexaboride(LaB6),as a thermionic electron emission cathode material,which has low work function,high melting point,low volatility and high chemical resistance,is mostly focused on finding its optimum emission surface and doping R elements in order to improve the electron emission properties in both theoretical calculation and experiment.In addition to its fundamental geometry structure and electronic properties,the reports about phonon,lattice vibration,thermal transport properties and thermal conductivity mechanism of LaB6 are less in recent years.Moreover there are few studies on the effect of hydrostatic pressure on thermal,elastic and mechanical properties of LaB6.In this paper,the electronic structure,phonon,thermal properties,elastic properties and mechanical properties under pressure are systematically studied using the first-principle calculation based on the density functional theory.Firstly,the analysis of calculated electronic structure of LaB6 shows that LaB6 behaves metallic owing to the contribution of La d electrons.The three acoustic branches appear flat along the symmetry path in the Brillouin,while the curves of low-frequency optical branches change apparently which illustrates that the optical branches have the main influence on the lattice thermal conductivity of LaB6.B6 octahedron distortion caused by the vibration of B atoms rather than La atoms determines the lattice thermal conductivity.The thermal conductivity of LaB6 is composed of two parts:one is the lattice thermal conductivity related to the distortion of B6 octahedron owing to vibration of B atoms,and the other is the electronic thermal conductivity determined by La atoms mostly which has the more effect in high temperatures.Secondly,the parts of the conduction bands passed by the fermi level increase with the increasing pressure until there exists a sudden reduction at 50GPa.Then the parts of conduction bands passed by the fermi level decrease in range of 50GPa to 90GPa.Moreover the distribution of density of states near fermi level has the same trend with the band structure and becomes broaden with the pressure increased.The Mulliken charge and bond population analysis show that the ability of La atoms to get electrons becomes enhanced which means the ability of B atoms to lose electrons enhances too and the degree of the covalency between B atoms increases when applied pressure increases.The frequencies of other vibration modes except for Tig mode shift to higher frequencies which is caused by the distortion of B6 octahedron.Meanwhile,the distance between the fifth and sixth optical branches at R point keeps closing but that of Tig mode and T2u mode at I-point keeps far with increasing pressure.Increased pressure contributes to the frequency gap between acoustic and low-frequency optical branches becoming larger first then smaller which reflects the interaction of acoustic and optical branches.The values of mode Gruneisen parameter at applied pressure not only coincide with experimental values but also indicate the thermal conductivity of LaB6 should increase under pressuresThe density function theory has been used to calculate the elastic and mechanical properties under hydrostatic pressure from OGPa to 90GPa.When applied pressure increases,the three independent elastic parametersC11、C12 and C44 increase and C11 has the fastest increase because the greater sensitivity which indicates larger bulk and Young’s modulus in LaB6,The calculated bulk and shear modulus become larger with increased pressure and the ratio between them implies LaB6 behaves brittleness.Moreover the calculated Young’s modulus,Poisson’s ratio and hardness illustrate LaB6 has the largest stiffness and hardness which is related to its geometry and bonding characteristics.In addition,the calculated velocity of longitude elastic wave is larger than that of transvers elastic wave which coincide with present experiments.The anisotropic factors by different calculated method show LaB6 behaves anisotropic in mechanical properties.The calculated results are in good agreement with experimental values,which indicates our calculation method and results are valid and reliable.