Density-functional theory study of the effect of pressure on the elastic properties of CaB6

Under high pressure, the long believed single-phase material CaB6 was latterly discovered to have a new phase tI56. Based on the density-functional theory, the pressure effects on the structural and elastic properties of CaB6 are obtained. The calculated bulk, shear, and Young's moduli of the recently synthesized high pressure phase tI56-CaB6 are larger than those of the low pressure phase. Moreover, the high pressure phase of CaB6 has ductile behaviors, and its ductility increases with the increase of pressure. On the contrary, the calculated results indicate that the low pressure phase of CaB6 is brittle. The calculated Debye temperature indicates that the thermal conductivity of CaB6 is not very good. Furthermore, based on the Christoffel equation, the slowness surface of the acoustic waves is obtained.     

LiF-CrF3二元系的热力学模拟

采用相图计算(CALPHAD)技术对LiF-CrF3体系进行了相图的优化计算.对液相分别采用置换熔体模型与缔合物模型进行描述,中间化合物Li3CrF6则采用准化学计量比化合物模型描述.模型参数的优化选取实验相平衡数据及第一性原理预测的数据,优化结果表明,缔合物模型比置换熔体模型更能准确地描述液相的实验相平衡数据.     

First-principles study of helium clustering at initial stage in ThO_2

The clustering behavior of helium atoms in thorium dioxide has been investigated by first-principles calculations. The results show that He atoms tend to form a cluster around an octahedral interstitial site(OIS). As the concentration of He atoms in ThO_2 increases, the strain induced by the He atoms increases and the octahedral interstitial site is not large enough to accommodate a large cluster, such as a He hexamer. We considered three different Schottky defect(SD) configurations(SD_1, SD_2, and SD_3). When He atoms are located in the SD sites, the strain induced by the He atoms is released and the incorporation and binding energies decrease. The He trimer is the most stable cluster in SD_1. Large He clusters, such as a He hexamer, are also stable in the SDs.     

Elastic,Piezoelectric and Acoustic Properties of Wurtzite MnO from Density Functional Calculation

The wurtzite MnO has been obtained lately and is expected to have large potential in varies applications.Since elastic properties are the bases of various applications, we calculate these properties of wurtzite MnO based on the density-functional theory and compare it with other two phases of MnO (rocksalt and zinc-blende MnO). The Young's modulus of wurtzite and zinc-blende MnO are 65.6 GPa and 73.4 GPa, respectively, which are much lower than those of rocksalt MnO (177.6 GPa). More importantly, both the Poisson ratio and the bulk modulus to shear modulus ratio indicate that wurtzite MnO should have much better ductile properties than rocksalt MnO. The calculated piezoelectric constants of wurtzite MnO are comparable to those of ZnO. This suggests wurtzite MnO is a good piezoelectric material.Furthermore, the slowness surfaces of acoustic waves of them are given from Christoffel equation.     

Edge effects on the characteristics of uranium diffusion on graphene and graphene nanoribbons

The first principles density-functional theoretical calculations of U adatom adsorption and diffusion on a planar graphene and quasi-one-dimensional graphene nanoribbons(GNRs) are performed. An energetic preference is found for U adatom diffusing to the hollow sites of both graphene and GNRs surface. A number of U distinctive diffusion paths either perpendicular or parallel to the ribbon growth direction are examined. The edge effects are evidenced by the calculated energy barriers of U adatom diffusion on armchair and zigzag nanoribbons surfaces. The calculation results indicate that the diffusion of U adatom from the inner site toward the edge site is a feasible process, particularly in zigzagGNR. It is viable to control the initial morphology of nuclear carbon material to retard the diffusion and concentration of nuclides.     

A First-Principles Study on the Vibrational and Electronic Properties of Zr-C MXenes

Within the framework of density functional theory calculations,the structural,vibrational,and electronic properties of Zr_(n )C_(n-1)(n=2,3,and 4)and their functionalized MXenes have been investigated.We find that the most stable configurations for Zr-C MXene are the ones that the terminal groups F,O,and OH locate on the common hollow site of the superficial Zr layer and its adjacent C layer.F and OH-terminated Zr_3C_(2 )and Zr_4C_(3 )have small imaginary acoustic phonon branches aroundΓpoint while the others have no negative phonon modes.The pristine MXenes(Zr_2C,Zr_3C_(2 )and Zr_4C_3)are all metallic with large DOS contributed by the Zr atom at the Fermi energy.When functionalized by F,O and OH,new hybridization states appear and the DOS at the Fermi level are reduced.Moreover,we find that their metallic characteristic increases with an increase in n.For(Zr_(n )C_(n-1))O_2,Zr_2CO_(2 )is a semiconductor,Zr_3C_2O_(2 )is a semimetal,and Zr_4C_3O_(2 )becomes a metal.