First-principles investigation of the phase stability of MgAl2 compounds

First-principles have been calculated to investigate the phase stabilities of the intermetallic phase of MgAl2 compounds by electronic structures and the charge distribution maps.The first-principles e...     

Phase transition and elastic properties of NbN under hydrostatic pressure

First-principles pseudopotential calculations are performed to investigate the phase transition and elastic properties of niobium nitrides(NbN). The lattice parameters ao and c0/a0, elastic constants C ij, bulk modulus B0, and the pressure derivative of bulk modulus B0 ′ are calculated. The results are in good agreement with numerous experimental and theoretical data. The enthalpy calculations predict that NbN undergoes phase transition from NaCl-type to NiAs-type structure at 13.4 GPa with a volume collapse of about 4.0% and from AsNi-type to CW-type structure at 26.5 GPa with a volume collapse of about 7.0%. Among the four types of structures, CW-type is the most stable structure. The elastic properties are analyzed on the basis of the calculated elastic constants. Isotropic wave velocities and anisotropic elasticity of NbN are studied in detail. The longitudinal and shear-wave velocities, V P, V S and V m increase with increasing pressure, respectively. The Debye temperature Θ D increases monotonically with increasing pressure except for NiAs-type structure. Both the longitudinal velocity and the shear-wave velocity increase with pressure for wave vector along all the propagation directions, except for V TA([100]) and V TA[001]([110]) with NaCl structure and V TA[010]([100]) with the other three types of structures.     

A novel technique for probing phase transitions in ferroelectric functional materials: Condensed matter spectroscopy

As relaxor ferroelectric functional materials, their crystal structures depend on temperature, component, electric filed, pressure, and so on, which are important for the applications of sensors, transducers, and actuators. For the case of PbTiO₃-based ferroelectrics Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ and some other crystals, they have been extensively investigated due to the excellent electromechanical and piezoelectric properties. Generally, ferroelectric crystal structure and corresponding phase diagram are detected by temperature-dependent high resolution X-ray diffraction, low frequency dielectric permittivity, and domain structures. In this review, we focus on the novel condensed matter spectroscopy (i.e., spectroscopic ellipsometry, transmittance, photoluminescence spectra as well as Raman spectra), which is nondestructive, noncontact, and sensitive optical techniques for probing symmetries, phase transitions and phase diagrams of ferroelectric crystals. Besides, it can supply some other physical and chemical information for ferroelectric and semiconductor functional materials such as optical band gap, electronic transitions, dielectric functions, optical conductivity, absorption, phonon modes, lattice dynamics as functions of temperature and PT composition.     

First-principles Investigation of the Structural,Electronic and Elastic Properties of Al2Ca and Al4Sr Phases in Mg-Al-Ca（Sr） Alloy

First-principles calculations have been carried out to investigate the structural stabilities, electronic structures and elastic properties of Mg17Al12, Al2Ca and Al4Sr phases. The optimized structural parameters are in good agreement with the experimental and other theoretical values. The calculated formation enthalpies and cohesive energies show that Al2Ca has the strongest alloying ability, and Al4Sr has the highest structural stability. The densities of states （DOS）, Mulliken electronic populations, and electronic charge density difference are obtained to reveal the underlying mechanism of structural stability. The bulk modulus, shear modulus, Young＇s modulus, and Poisson＇s ratio are estimated from the calculated elastic constants. The mechanical properties of these phases are further analyzed and discussed. The Gibbs free energy and Debye temperature are also calculated and discussed.     

Electronic,Thermal Expanding,and Optical Absorption Properties of Transition Metal Dichalcogenides: A First-principles Study

A comprehensive investigation was made on the electronic structure, thermal expansion coefficient and light absorption spectrum of total six transition metal dichalcogenides (TMDs) compounds with formula of MX₂ (M=Mo, W, Cr, X=S, Se). First, an indirect-direct band gap transition from bulk to singlelayer was declared for all the six compounds. Moreover, the detailed lattice constants and thermal expansion coefficients provided in the paper were the key information for designing MX₂-based field effect transistors. Finally, the calculated optical absorption spectra demonstrate that these compounds can effectively utilize solar energy and are good photo catalyst candidates. All these present findings will benefit the design of new generation of novel two-dimensional materials.     

Evidence of decisive effect of crystal-field splitting in spin-state transition

Based on the first-principle calculations for 3D Hofmann-like spin-crossover (SCO) compound [Fe(C4H4N2){Pt(CN)4}], the discrepancy of transition mechanism is clarified with quantitatively distinguishable evidence of second order phase transition. It shows that the stretch around 0.2 ? of Fe-N bond length leads to the continuous structure expansion, as the energy splitting ΔEHL between low-spin and high-spin states reduces from 2.554 2 eV to -0.327 8 eV, and the crystal-field splitting (CFS) is reduced from 1.845 8 eV to 0.420 8 eV meanwhile. A physics image relating the calculations results with CFS in the frame of ligand-field theory is presented, which manifests that CFS is a necessary parameter to be introduced directly in the theory of spin-state transition.     

A density functional theory study of the geometric and electronic structure of MgF2 (110) surface

Ab initio density functional theory (DFT) was employed to study geometric and electronic structure of MgF2 (110) surface. Three different clean surface models have been considered. The results show that the surface terminated with one-layer F has the smallest relaxation and the lowest surface energy, which indicates this model is the most energetically favorable structure of MgF2(110) surface. Furthermore, the electronic properties are also discussed from the point of density of states and charge density. Analysis of electronic structure shows that the band gap of the surface is significantly narrowed with respect to the bulk. The electrons of the surface exhibit strong locality and larger effective mass.     

First-principles calculations on the structure and electronic properties of boron doping zigzag single-walled carbon nanotubes

Calculations have been made for single-walled zigzag(n,0) carbon nanotubes containing substitutional boron impurity atoms using ab initio density functional theory.It is found that the formation energies of these nanotubes depend on the tube diameter,as do the electronic properties,and show periodic fea-ture that results from their different π bonding structures compared to those of perfect zigzag carbon nanotubes.When more boron atoms are incorporated into a single-walled zigzag carbon nanotube,the substit...     

First-principle investigation on electronic structures and elastic properties of Al-doped MoSi2

The electronic structures and elastic properties of Al-doped MoSi₂ were calculated using the plane wave pseudo-potential method based on the density functional theory, in which the generalized-gradient approximation (GGA) was used to describe the exchange-correlation potential. Starting from the elastic constants, bulk modulus, shear modulus, elastic modulus and Poisson ratio of Al-doped MoSi₂ were obtained by using the Hill method. The results indicate that conductivity of Al-doped MoSi₂ is improved to some extent in comparison with that of pure MoSi₂ due to the orbit hybridization of Mo 4d, Al 3p and Si 3p electrons. In addition, calculations show that the elastic modulus and the brittleness of Al-doped MoSi₂ are smaller than those of pure MoSi₂, which implies that it is feasible to toughen MoSi₂ by doping Al. The agreement of the conclusion with experiment shows that the present theory is reasonable.     

Structural stability and magnetism of metastable Ni-Pt intermetallic compounds studied by ab initio calculation

The self-consistent electronic structure calculations were carried out with the accurate frozen-core full-potential projector augmented-wave method on 13 Ni-Pt intermetallic compounds of simple crystalline structures, i.e. A15, D0₁₉, D0₃ and L1₂ Ni₃Pt and NiPt₃, and α-NiAs, B1, B2, L2 _a , and L1₀ NiPt. The calculations reveal that the L1₂ Ni₃Pt, L1₀ NiPt and L1₂ NiPt₃ are energetically more stable than their respective competitive structures, indicating that the three structures may be formed in some appropriate conditions. The obtained results match well with the experimental observation or other theory predictions. It is found that there is hybridization between Ni 3d and Pt 5d states, which may significantly affect the structural stability and magnetism of metastable Ni-Pt intermetallic compounds. Supported by the National Natural Science Foundation of China (Grant Nos. 50531040, 50871058), the Ministry of Science and Technology of China (Grant No. 2006CB605201), and the Administration of Tsinghua University     

Electronic structure and transport coefficients of the thermoelectric materials Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> from first-principles calculations

The electronic structures of bulk Bi₂Te₃ crystals were investigated by the first-principles calculations. The transport coefficients including Seeback coefficient and power factor were then calculated by the Boltzmann theory, and further evaluated as a function of chemical potential assuming a rigid band picture. The results suggest that p-type doping in the Bi₂Te₃ compound may be more favorable than n-type doping. From this analysis results, doping effects on a material will exhibit high ZT. Furthermore, we can also find the right doping concentration to produce more efficient materials, and present the “advantage filling element map” in detail.     

Pressure Effect on Elastic and Lattice Dynamic Properties of Beryllium Selenide from First Principles

The lattice dynamic, elastic, and thermodynamic properties of Be Se were investigated with first principles calculations. The phase transition pressure from the zinc blende(B3) to the nickel arsenide(B8) structure of Be Se was determined. The elastic stability analysis suggests that the B3 structure Be Se is mechanically stable in the applied pressure range of 0-50 GPa. Our lattice dynamic calculations show that the B3 structure is lattice dynamically stable under high pressure. Within the quasiharmonic approximation, the thermodynamic properties including the constant volume heat capacity and constant pressure heat capacity are predicted.     

Thermoelectric Properties and Electronic Structure of Ca3Co2O6

The nanosized Ca3Co2O6 powder was synthesized via sol-gel process.The phase composition was characterized by means of X-ray diffraction.Polycfrystalline swnples of Ca3Co2O6 were prepared by a sintering procedure of nanosized power.The seebeck cofficient and electrical conductivity of the samples were measured from 450K up to 750 K.The results show that the Seebeck coefficient increases with the increasing temperature.The electronic structures were calculated using the self-cwtsistent full-potential linearized augmentedc plane-wave (LAPW) method within the density functional theory.The relationship between thermoelectric property and electronic structures was discussed.     

Review: Recent Progress on the Application of REBCO Superconductor Bulks

综述：REBCO超导块材应用的研究进展

张子立¹,戴银明¹,刘建华¹,王磊¹,王秋良^1,2

（1.中国科学院电工研究所,北京 100191;2.中科院大学,北京 100049）

中文说明：

直径几厘米的单筹REBCO超导块材可以轻松捕获1T以上的磁场,而目前最高的捕获磁场可以达到17.6T。在如此紧凑的结构中提供如此大的磁场,使得REBCO超导块材有着非常广阔的应用空间。REBCO块材最常见应用领域是飞轮储能、磁悬浮、电机以及发电机等,而本文希望对REBCO块材磁体的一些非主流应用进行介绍。本文将介绍REBCO块材磁体在磁透镜、紧凑型NMR、磁控溅射系统、医药磁导航、磁分离系统以及磁镜装置中的应用。通过本文的介绍和分析,希望展示出REBCO块材自己的显著优点和可能的应用场景,尤其是REBCO商业带材无法企及的一些应用领域。

关键词：超导,块材磁体,REBa2Cu3O7-x,强磁场

     

Field-induced magnetic phase transitions in ferrimagnet GdMn2Ge2: H - T magnetic phase diagrams

Magnetic properties of a two-sublattice ferrimagnet with antiferromagnetic exchange interaction inside one of the sublattices are calculated within the framework of the molecular field theory taking into account the anisotropy of unstable sublattice. The magnetization curves of single crystal GdMn₂Ge₂ for the magnetic field parallel and perpendicular to the c-axis at different temperatures are calculated. Field-induced magnetic phase transitions in GdMn₂Ge₂ are discussed. Calculated H - T magnetic phase diagrams are in fair agreement with experimental data. Biography of the first author: GUO Guang-hua, associate professor, born in 1964, received Ph. D. degree in 1999, majoring in condensed matter physics.     

Theoretical studies of structures and spectroscopic properties of [(tpy)(bpy)RuC≡CC_6H_4R]~+(tpy = 2,2′:6′,2″-terpyridine,bpy = 2,2′-bipyridine; R = F,Cl, H,Me and OMe)

A series of polypyridine ruthenium(II)acetylide complexes,[(tpy)(bpy)RuC≡CC6H4R]+(tpy=2,2′:6′,2″-terpyridine,bpy=2,2′-bipyridine;R=F(1),Cl(2),H(3),Me(4)and OMe(5))are investigated theoretically to explore their electronic structures and spectroscopic properties.Their ground/excited state geometries,electronic structures and spectroscopic properties are first calculated using density functional theory(DFT)and time-dependent DFT(TDDFT).The absorption and emission spectra of the complexes in acetonitrile solution are also obtained by using TDDFT(B3LYP)method associated with the CPCM model.The calculations show that the energy levels of HOMOs for 1–5 are sensitive to the substituent on phenylacetylide ligand and increase with the same order of the electron-donating ability of the substituents;however,those of polypyridine-based LUMOs vary slightly.The lowest-energy absorptions and emissions for 1–5 are progressively red-shifted in the order of 1≈2345 when the electron-donating groups are introduced into the phenylacetylide ligand.The phosphorescence of 1 are attributed to{[dxz(Ru)+π(C≡C)]→[π*(tpy)]}(3MLCT/3LLCT)transition,whereas those of 2–5 are originated from{[dxz/dxy(Ru)+π(C≡C)+π(C6H4R)]→[π*(tpy/bpy)]}(3MLCT/3LLCT)transitions.     

Spontaneous Magnetic Transitions and Corresponding Magnetoelastic Properties of Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Ge<Subscript>2</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Gd,Tb and Dy)

The spontaneous magnetic transitions and corresponding magnetoelastic properties of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb and Dy) were investigated by using the X-ray diffraction method and magnetic measurement. The results showed that the compounds experience two magnetic transitions, namely the second-order paramagnetic to antiferromagnetic transition at temperature T_N (T_N=368, 423 and 443 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) and the first-order antiferromagnetic - ferrimagnetic transition at temperature T_t (T_t=96, 80 and 40 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) as the temperature decreases. The temperature dependence of the lattice constant a(T) displays a negative magnetoelastic anomaly at the second-order transition point T_N and, at the first-order transition T_t, a increases abruptly for GdMn₂Ge₂ and TbMn₂Ge₂, Δa/a about 10^-3. Nevertheless, the lattice constant c almost does not change at these transition points indicating that such magnetoelastic anomalies are mainly contributed by the Mn-sublattice. The transitions of the magnetoelastic properties are also evidenced on the temperature dependence of magnetic susceptibility χ. The first-order transition behavior at T_t is explained by the Kittel mode of exchange inversion.     

First-principles lattice stability of Fe, Ru and Os

Lattice constants, total energies and densities of states of transition metals Fe, Ru and Os with BCC, FCC and HCP structures were calculated by the GGA+PBE functional and the ultrasoft pseudo-potential plane wave method, and compared with those of the first-principles projector augmented wave (PAW) method, CALPHAD method and experimental data. The results show that the lattice stability of this work is ΔG ^BCC-HCP>ΔG ^FCC-HCP>0, agreeing well with those of PAW method in the first-principles and CALPHAD method except for BCC-Fe. And the densities of state of HCP-Ru and Os have an obvious character of stable phase, agreeing completely with the results of the total energy calculations. Further analyses of atomic population show that the transition rate of electrons from s to p state for HCP, FCC and BCC crystals increases from Fe to Os, and a stronger cohesion, a higher cohesive energy or a more stable lattice between atoms of heavier metals are formed. Foundation item: Project(20070533118) supported by the Doctoral Discipline Foundation of Ministry of Education of China; Projects(50471058, 50271085) supported by the National Natural Science Foundation of China; Project supported by the Postdoctoral Foundation of Central South University, China     

Structure and Electric Properties of （1-x）（Bi1/2Na1/2） TiO3-xBaTiO3 Systems

The structural, dielectric and piezoelectric properties of （1-x）（Bi1/2Na1/2） TiO3-xBaTiO3 ceramics were investigated for the compositional range, x=0.02, 0.04, 0.06, 0.08, 0.10. The samples were synthesized by a conventional solid-state reaction technique. All compositions show a single perovskite structure, and X-ray powder diffraction patterns can be indexed using a rhombohedral structure. Lattice constants and lattice distortion increase while the amount of BaTiO3 increases. The X-ray diffraction results show the morphotropic phase boundary （MPB） of （1-x）（Bi1/2Na12） TiO3-xBaTiO3 exists in near x=0.06-0.08. Temperature dependence of dielectric constant eT33/ε0 measurement reveals that all compositions experience one structural phase and two ferroelectric phases transition below 400℃： rhombohedral （or rhombohedral plus tetragonal） ferroelectric phase ←→ tetragonal antiferroelectric phase ←→ tetragonal paraelectric phase. Relaxor behaviors exist in the course of ferroelectric to antiferroelectric phase transition. Dielectric and piezoelectric properties are enhanced in the MPB range for （ 1-x）（Bi1/2Na1/2）TiO3-xBaTiO3.     

Effect of Nb on plasticity and oxidation behavior of TiAlNb intermetallic compound by density functional theory

Based on the pseudo potential plane-wave method of density functional theory (DFT), Ti_1−x Nb _x Al ( _x =0, 0.062 5, 0.083 3, 0.125, 0.250) crystals’ geometry structure, elastic constants, electronic structure and Mulliken populations were calculated, and the effects of doping on the geometric structure, electronic structure and bond strength were systematically analyzed. The results show that the influence of Nb on the geometric structure is little in terms of the plasticity, and with the increase of Nb content, the covalent bond strength remarkably reduces, and Ti-Al, Nb-M (M=Ti, Al) and other hybrid bonds enhance; meanwhile, the peak district increases and the pseudo-energy gap first decreases and then increases, the overall band structure narrows, the covalent bond and direction of bonds reduce. The population analysis also shows that the results are consistent with the electronic structure analysis. The density of states of TiAlNb shows that Nb doping can enhance the activity of Al and benefit the form of Al₂O₃ film. All the calculations reveal that the room temperature plasticity and the antioxidation properties of the compounds can be improved with the Nb content of 8.33%–12.5% (mole fraction).