Au和3d过渡金属元素混合团簇结构、电子结构和磁性的研究

采用基于密度泛函理论的第一性原理方法系统研究了Au与3d过渡元素构成的混合小团簇的结构、稳定性、电子结构及磁性,得到了Au与3d过渡元素构成的混合小团簇的稳定结构.计算结果表明,Au与3d元素可形成大量的低能异构体,特别是有些异构体在结构上极相近,这不同于共价或离子键类型的团簇.与纯过渡金属团簇类似,这类团簇也表现出复杂的磁性.过渡金属元素的磁矩相比体材料而言既有增强的、也有减弱的,与轨道的交换劈裂密切相关.对于基态构型,AuCr₂,Au₂Cr_{2关键词： 密度泛函理论 第一性原理方法 团簇 电子结构}     

A new family of rare earth compounds,the ternary silicides RE2RhSi3 (RE = Y,La, Ce,Nd, Sm,Gd, Tb,Dy, Ho,Er) - crystal structure electrical and magnetic properties

New ternary silicides RE₂RhSi₃ (RE = Y, La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystallize with a hexagonal symmetry structure which is derived from the AlB₂-type. Si and Rh atoms are inside RE₆ distorted trigonal prisms and are ordered in a two dimensional sublattice perpendicular to the c axis. The a and c parameters are approximately twice those of the corresponding disilicides RESi₂. La₂RhSi₃ and Y₂RhSi₃ are not superconducting down to 1.6 K. Nd₂RhSi₃ is ferromagnetic at 15 K, but the other silicides order antiferromagnetically with metamagnetic phase transitions for certain of them.     

Vibrational spectra of 2, 4, 6-halogen and aminohalogen derivatives of 1, 3, 5-triazine

The IR (infrared) absorption spectra of the derivatives of symmetrical triazine C₃N₃X₃, C₃N₃X₂NH₂, and C₃N₃X(NH₂)₂ (X = F, Cl, Br) have been obtained; in addition, RS (Raman scattering) has been obtained from C₃N₃F₃ and C₃N₃Cl₃. In the case of the trihalogen triazines, the vibrations are attributed to symmetry types and characterized as to their modes. The spectra of the aminohalogen derivatives are interpreted by analogy with the spectra of C₃N₃X₃ and C₃N₃(NH₂)₃. The characteristic vibrations of the ring, NH₂ group, and C-X bonds are identified.The authors are grateful to Kh. E. Sterin and A. V. Bobrov (Commission on Spectroscopy, AS USSR) for their assistance in obtaining and measuring the RS spectra.     

First-principles study on the crystal, electronic structure and mechanical properties of hexagonal Al3RE (RE = La, Ce, Pr, Nd, Sm, Gd) intermetallic compounds

The structural properties, elastic properties and electronic structures of hexagonal Al₃RE intermetallic compounds are calculated by using first-principles calculations based on density functional theory. Since there exists strong on-site Coulomb repulsion between the highly localized 4f electrons of RE atoms, we present a combination of the GGA and the LSDA+U approaches in order to obtain the appropriate results. The GGA calculated lattice constants for the hexagonal Al₃RE intermetallic compounds are in good agreement with available experimental values. The results of cohesive energy indicate that these compounds can be stable under absolute zero Kelvin and the stability of Al₃Gd is the strongest in all of the hexagonal Al₃RE compounds. The densities of states for GGA and LSDA+U approaches are also obtained for the Al₃RE intermetallic compounds. The mechanical properties are calculated from the GGA method in this paper. According to the computed single crystal elastic constants, Al₃La, Al₃Sm and Al₃Gd are mechanically unstable, while Al₃Ce, Al₃Pr and Al₃Nd are stable. The polycrystalline elastic modulus and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations, and the calculated ratio of bulk modulus to shear modulus indicates that Al₃La compound is ductile material, but Al₃Ce, Al₃Pr, Al₃Nd, Al₃Sm and Al₃Gd are brittle materials.     

Mg-TM-H (TM=Sc,Ti, V,Y, Zr,Nb)晶体形成能力和电子性能

基于密度泛函理论的第一性原理的Vienna Ab initio Simulation Package (VASP)软件系统研究了Mg₇TMH₁₆ (TM=Sc, Ti, V, Y, Zr, Nb)的晶体结构和电子性能. 分析了结合能和形成焓,计算得到的结合能显示Mg₇TiH₁₆和Mg₇NbH₁₆的稳定性最强,形成焓的变化表明Mg₇TMH₁₆具有比MgH₂更低的分解温度,电子态密度显示Mg₇TMH₁₆表现出明显的金属特性. 关键词： 第一原理 7TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb)')" href="#">Mg₇TMH₁₆ (TM=Sc,Ti,V,Y,Zr,Nb) 结合能 形成焓     

Structural, energetic, and electronic properties of Sin, Gen, and SinGen clusters

Results of a theoretical study of the properties of Si_n, Ge_n, and Si_nGe_n clusters are presented. An approximate density-functional method in combination with genetic algorithms have been used in an unbiased determination of the structures of the lowest total energy. The resulting structural, energetic, and electronic properties are analysed and compared with each other for the different systems.     

Rings, towers, cages of ZnO

Theoretical electronic structure studies on (ZnO)_n (n= 2–18, 21) have been carried out to show that the transition from an elementary ZnO molecule to the bulk wurtzite ZnO proceeds via hollow rings, towers, and cages. Our first principles electronic structure calculations carried out within a gradient corrected density functional framework show that small Zn_nO_n (n=2–7) clusters form single, highly stable rings. Zn₃O₃ and the symmetric cage Zn₁₂O₁₂ are shown to be particularly stable clusters. Among larger clusters, the most stable are oblong cages, Zn₁₅O₁₅, Zn₁₈O₁₈, and Zn₂₁O₂₁, which are reminiscent of nanotubes.     

Structural stability and structural imitation of (M = Mn,Mo, Nb,Ti, V,Al, Cr,Ni and Si)

The structural stability of La₂Co_17-xM_x (M = Mn, Mo, Nb, Ti, V, Al, Cr, Ni and Si) based on the interatomic potential has been studied. The calculated site preference of the third element M is found to be the 6c site, which is in agreement with the experiments. In the calculations, if the crystal cohesive energy of La₂Co₁₆Mn is taken as the highest one in the crystallization of La₂Co_17-xM_x, the lowest content x of the third element M (M = Mn, Mo, Nb, Ti and V) required to stabilize La₂Co_17-xM_x, is near that found in the experiments. The differences of the cell parameters between the calculated and the experimental values are less than 0.4%. The differences of the atomic parameters for Co (or M) between the calculated and the experimental values are about or even smaller than 1%, and that of La is about 3%. Because the energies of La(Co_1-xAl_x)₁₃ are lower than those of La₂(Co_1-xAl_x)₁₇, La₂(Co_1-xAl_x)₁₇ could not be formed in the experiments. In the calculations, with either a range of deformation of the structure or the reconstruction of the initial structure La₂Co₁₇ from LaCo₅, the same results including the cohesive energy curves and the crystallographic parameters can be retrieved after the action of the interatomic potentials. Received 1st November 2002 / Received in final form 17 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: hchang@aphy.iphy.ac.cn     

Crystal structures,elastic, and lattice dynamical properties of BeB2, NaB2, and CaB2 from the first principles

Based on density functional theory, we have studied the structural stability, elastic, mechanical, and lattice dynamical properties of BeB₂, NaB₂, and CaB₂ compounds in AlB₂, OsB₂, and ReB₂ structures, respectively. Generalized gradient approximation has been used for modeling exchange-correlation effects. Our calculations indicate that ReB₂, AlB₂, and OsB₂ structures are energetically the most stable for BeB₂, NaB₂, and CaB₂ compounds, respectively. The results show that these structures are both mechanically and dynamically stable. The bulk modulus, elastic constants, shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities, and anisotropic factors are also calculated and discussed. Furthermore, the phonon dispersion curves and corresponding phonon density of states are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Dipole properties of PH3, PF3, PF5, PCl3, SiCl4, GeCl4, and SnCl4

Experimental and theoretical photoabsorption cross sections combined with constraints provided by the Kuhn–Reiche–Thomas sum rule, the high-energy behaviour of the dipole oscillator strength density, static dipole polarisabilities, and molar refractivity data when available are used to construct dipole oscillator strength distributions for PH₃, PF₃, PF₅, PCl₃, SiCl₄, GeCl₄, and SnCl₄. The distributions are used to predict dipole sum rules S(k), mean excitation energies I(k), and van der Waals C₆ coefficients.     

Calculated structural, electronic and elastic properties of M2GeC (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W)

Using ab initio calculations, we have studied the structural, electronic and elastic properties of M₂GeC, with M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W. Geometrical optimizations of the unit cell are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site and momentum projected densities shows that bonding is due to M d-C p and M d-Ge p hybridizations. The elastic constants are calculated using the static finite strain technique. The shear modulus C ₄₄, which is directly related to the hardness, reaches its maximum when the valence electron concentration is in the range 8.41–8.50. We derived the bulk and shear moduli, Young’s moduli and Poisson’s ratio for ideal polycrystalline M₂GeC aggregates. We estimated the Debye temperature of M₂GeC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic constants of Ti₂GeC, V₂GeC, Cr₂GeC, Zr₂GeC, Nb₂GeC, Mo₂GeC, Hf₂GeC, Ta₂GeC and W₂GeC compounds, and it still awaits experimental confirmation.     

Photoelectron spectra of carbonyls: Acetaldehyde,acetamide, biacetyl,pyruvic acid,methyl pyruvate and pyruvamide

Photoelectron spectra of a number of carbonyl compounds are reported and assigned on the basis of a correlative scheme which makes heavy use of CNDO/s computations. The assignments of n and π_○ type for monocarbonyls, and n₊, n_?, π_⊕ and π_? type for α-dicarbonyls are quite secure in an experimental sense. All other assignments given are tentative.     

Structures and Spectral Characteristics of Silylborane,Silylaluminum Hydride,Silylphosphine, and Silyl Mercaptan

The vibrational frequencies of several silanes H₃SiX (X=BH₂, AlH₂, PH₂ and SH) are determined. The infrared and Raman spectra are plotted. Several scale procedures were use to improve the theoretical spectra. The geometric parameters in the planar, staggered and eclipsed structures of these species are fully optimized and compared with ab initio calculations. Basis set effects on the calculated structures are discussed. A few thermodynamic parameters, net atomic charges, dipole moment and energy are also computed.     

First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M=Mo, Nb, W, Ta; X=S, Se, Te) monolayers

Using first principles calculations, we investigate the structural, vibrational and electronic structures of the monolayer graphene-like transition-metal dichalcogenide (MX₂) sheets. We find the lattice parameters and stabilities of the MX₂ sheets are mainly determined by the chalcogen atoms, while the electronic properties depend on the metal atoms. The NbS₂ and TaS₂ sheets have comparable energetic stabilities to the synthesized MoS₂ and WS₂ ones. The molybdenum and tungsten dichalcogenide (MoX₂ and WX₂) sheets have similar lattice parameters, vibrational modes, and electronic structures. These analogies also exist between the niobium and tantalum dichalcogenide (NbX₂ and TaX₂) sheets. However, the NbX₂ and TaX₂ sheets are metals, while the MoX₂ and WX₂ ones are semiconductors with direct-band gaps. When the Nb and Ta atoms are doped into the MoS₂ and WS₂ sheets, a semiconductor-to-metal transition occurs. Comparing to the bulk compounds, these monolayer sheets have similar structural parameters and properties, but their vibrational and electronic properties are varied and have special characteristics. Our results suggest that the graphene-like MX₂ sheets have potential applications in nano-electronics and nano-devices.     

A first-principles study of B2 NiAl alloyed with rare earth element Pr, Pm, Sm, and Eu

The structural, elastic, and electronic properties of NiAl alloyed with rare earth elements Pr, Pm, Sm, and Eu are investigated by using density functional theory (DFT). The study suggests that Pr, Pm, Sm, and Eu are all tend to be substituted for Al site. Ni₈Al₇Pm possesses the largest ductility. Only the hardness and ductility of Ni₈Al₇Eu are enhanced simultaneously. The covalency strength of Ni-Al bond in Ni₈Al₇Pm is higher than that in Ni₈Al₇Eu. The covalency strength of Al-Al bond and that of Ni-Ni bond in Ni₈Al₇Eu are higher than that in Ni₈Al₇Pm. Ni-Pm bond and Ni-Eu bond are covalent, and the covalency strength of Ni-Pm bond is greater. Al-Pm bond and Al-Eu bond show great covalency strength and ionicity, respectively.     

Molecular field theory analysis of R2Fe17C (R=PR,Nd, Gd,Tb, Dy,Ho, Er)

The temperature dependence of magnetization is analysed for R₂Fe₁₇C via the two-sublattice molecular field theory. The molecular field coefficients n_FF, n_RF and n_RR are obtained, by which T_C was calculated. Using the least-squares method, the fitted-form of H_R(T) varying with temperature for each compound is presented. The results are analysed. In addition, the parameters F=M_Fe²(0)n_FF/T_C was calculated for each R₂Fe₁₇C. By F, some phenomena different from the normal view were explained.     

Theoretical study on the structure for R2Co17 (R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) and R2Co17T (T=Be, C)

The structural stability of the intermetallic compounds R₂Co₁₇ and R₂Co₁₇T (T=Be, C) is tested by many means including random atom shifts, global deformations and high temperature disturbances under the control of the pair potentials. The structure type and crystal constants of R₂Co₁₇ and R₂Co₁₇Be are close to experimental results. The addition of Be and C in the interstice of R₂Co₁₇ causes a decrease of the cohesive energy, and Be and C only occupies the 9e interstitial site with the Th₂Zn₁₇-type structure or the 6h interstitial site with the Th₂Ni₁₇-type structure. All the above results indicate that the potentials are valid for studying the structural properties of these kinds of anisotropy materials.     

Structure, properties, and formation models of optical ceramics

The structure, properties, and formation mechanisms of Y₃Al₅O₁₂, Y₂O₃, and Lu₂O₃ laser ceramics are investigated. Their microhardness and fracture toughness are determined. It is shown that the change in mechanical properties is related both to the grain size and grain boundary structure. Processes of plastic deformation of crystals by mechanical twinning are considered. Mechanisms of formation and motion of twins in crystals with FCC structure are determined. It is shown that the realization of similar mechanisms in crystals with HCP structure results in the phase transformations. Models of the formation and motion of twin boundaries are proposed which result in pore healing when preparing monolithic samples of highly transparent ceramics.     

On the additivity of atomic and molecular dipole properties and dispersion energies using H,N, O,H2, N2, O2, NO,N2O,NH3 and H2O as models

The zeroth-order theory of intermolecular forces is used to derive additivity relations for rotationally averaged molecular dipole properties and dispersion energy constants by assuming that a molecule is comprised of non-interacting atoms or molecules. Some of the additivity rules are new and others, for example the mixture rule for dipole oscillator strength distributions (DOSDs), Bragg's rule for stopping cross sections and Landolt's rule for molecular refractivities, are well known. The additivity rules are tested by using previously constructed DOSDs and reliable values for the dipole oscillator strength sums S_k , L_k and I_k , and dispersion energy constants C ₆, for H, N, O, H₂, N₂, O₂, NO, N₂O, NH₃ and H₂O as models. It is found that additivity is generally unreliable for estimating molecular properties corresponding to k < -2. Generally for k ≥ -2 and for C ₆, and if the hydrogen molecule is used to represent the hydrogen atom in the additivity rules, the additivity relations yield results that are reliable to within ?20 per cent and the estimates improve substantially as k increases. The effects of molecule formation on DOSDs is examined by comparing the various molecular DOSDs with the sum of the DOSDs for the atoms making up the molecules. Molecule formation results in a net decrease in the amount of dipole oscillator strength for low excitation energies and a compensating net increase for higher energies in a region extending from the absorption threshold to about 100 eV. This is shown to imply that estimates of the stopping average energy I ₀, obtained by using bona fide atomic I ₀ values, are lower bounds to the correct molecular I ₀ results.     

Electron impact ionization cross-section of C2, C3, Si2, Si3, SiC,SiC2 and Si2C

Total ionization cross-sections for C₂, C₃, Si₂, Si₃, SiC, SiC₂ and Si₂C molecules have been calculated by electron impact. Spherical complex optical potential formalism has been employed for obtaining the inelastic cross-sections for these molecules. Then by applying complex scattering potential-ionization contribution method, total ionization cross-sections are derived. These cross-sections are calculated in the energy range from ionization threshold to 2?keV. There are no measurements available in the literature to the best of our knowledge with which our results can be compared. The results show a linear relationship between maximum ionization cross-section and square root of the ratio of polarizability to ionization potential, depending on its atomicity. This gives a confirmation for the consistency of the data reported here. Present work is a maiden attempt to find electron impact ionization cross-section for these molecules, except for C₂ and C₃.