Ca5N4高压新相的第一性原理研究

通过在氮中引入杂质离子,利用高压手段获得具有新奇结构的多氮化合物是目前被广泛应用的研究方法.钙氮材料在催化、光电方面有着广泛的应用.具有较低电离能的钙(Ca)元素很容易和氮原子形成离子键钙氮化物.高压为寻找新型钙氮化合物提供了全新的技术途径.因此,利用高压方法,通过改变配比的方式,寻找具有新奇特性的钙氮高压结构,是一项非常有意义的工作.本文利用基于密度泛函理论的结构搜索方法,在100 GPa条件下,通过预测得到了一个稳定的Ca5N4相.该结构内部氮原子之间以N-N共价单键键合,氮原子和钙原子之间是离子键相互作用,且钙氮之间的电荷转移量为1.26 e/N atom.能带结构计算表明P 21/c-Ca5N4是一个直接带隙为1.447 eV的半导体结构.最后,系统地给出了该结构的拉曼振动光谱,并指认了拉曼振动模式,为实验合成该结构提供了理论指导.     

Theoretical investigation of a series of bis(1H-tetrazol-5-yl)furazan and bis(1H-tetrazol) derivatives as high-energy-density materials

Using density functional theory (DFT), a series of bis(1H-tetrazol-5-yl)furazan and bis(1H-tetrazol) derivatives with different linkages and substituents are investigated theoretically as potential high-energy-density materials (HEDMs). The heat of formation (HOF), detonation properties, natural bond orbital (NBO) and thermal stabilities are calculated and reported. The introduction of a furazan ring, an –N=N– bridge group and an –N₃ substituent is beneficial to increase the HOF of the title compounds. NBO analysis shows that there are electronic delocalisation effects among the bridge groups, furazan and tetrazole rings, and substituted groups. The conjugation effects and electronic transitions are influenced by the different linkages and substituents. The estimated detonation velocities and pressures indicate that the –ONO₂ and –NO₂ groups and the –N=N– linkage play important roles in enhancing the detonation properties. The bond dissociation energy (BDE) calculations reveal that the –NO₂ group is the substituent group which causes the least thermal stability. The bond between the substituent group and the tetrazole ring is the weakest bond in the title molecules. Considering the detonation performance and the thermal stability, 17 compounds may be promising candidates for HEDMs with good performance. Eight of them (A3, A4, C3, C4, D3, F3, G1 and G3) have better detonation properties than HMX.     

Pressure-Stabilized New Phase of CaN_4

We propose a new CaN_4 high pressure structure with the P2_1/m space group.The P2_1/m-CaN_4 structure is constituted by the infinite armchair N-chain.The dynamical stability and mechanical stability are verified by the calculations of phonon dispersion curves and elastic constants.The enthalpy difference calculation shows that the P2_1/m phase is more stable than the reported P4_12_12 phase.The advantaged properties of P2_1/m-CaN_4,such as high nitrogen content(58.3%) and low polymerization pressure(18.3 GPa),allow it to be a potential high energy materlal.Band structure calculation shows that the P2_1/m-CaN_4 structure is a metallic phase.The nonpolar covalent single N-N bond is a sigma bond.The charge transfer between the Ca and N atoms results in an ionic bond interaction.     

Dissociation of H_2 on Mg-coated B_(12)C_6N_6

The dissociation of H_2 molecule is the first step for chemical storage of hydrogen, and the energy barrier of the dissociation is the key factor to determine the kinetics of the regeneration of the storage material. In this paper, we investigate the hydrogen adsorption and dissociation on Mg-coated B_(12)C_6N_6. The B_(12)C_6N_6 is an electron deficient fullerene, and Mg atoms can be strongly bound to this cage by donating their valance electrons to the virtual 2p orbitals of carbon in the cluster. The preferred binding sites for Mg atoms are the B_2C_2 tetragonal rings. The positive charge quantity on the Mg atom is 1.50 when a single Mg atom is coated on a B_2C_2 ring. The stable dissociation products are determined and the dissociation processes are traced. Strong orbital interaction between the hydrogen and the cluster occurs in the process of dissociation, and H_2 molecule can be easily dissociated. We present four dissociation paths, and the lowest energy barrier is only 0.11 eV, which means that the dissociation can take place at ambient temperature.     

Computational studies of the halooxyhalocarbenes XOCX (X = F,Cl)

Ab initio calculations at the B3LYP, MP2, MP4 and CCSD(T) levels of theory were performed to predict the stability of the halooxyhalocarbenes, XOCX (X = F, Cl). The calculations indicate that the nonlinear FOCF molecule is stable with an energy 16 kJ mol^?1 below the energy of possible reacting fragments F₂ and CO. However, a nonlinear equilibrium structure for ClOCCl was located, but it was found to be about 192 kJ mol^?1 higher in energy than the energy of Cl₂ and CO. The charge distribution in these molecules was analysed using the atoms in molecules (AIM) method.     

X-ray photoelectron investigation of charge distribution in copper(II) phthalocyanine complexes

The charged states of atoms in unsubstituted copper(II) phthalocyanine (CuPcH₁₆) and hexade-cafluorinated copper(II) phthalocyanine (CuPcF₁₆) complexes and in thin films of them deposited on silicon substrates by vacuum thermal evaporation are investigated by X-ray photoelectron spectroscopy (XPS). The C(1s), N(1s), Cu(2p) core level energies and the charged states of atoms in the studied complexes are calculated using the DFT method. The performed experimental study and theoretical calculations show that the introduction of electron acceptor substituents into benzene rings mostly affects the atoms of benzene rings and insignificantly affects the charge state of nitrogen atoms in the pyrrole ring.     

Magnetic ground state and electronic structure of CeRu(2)Al(10)

We present a combined theoretical and experimental study of the electronic structure for CeRu(2)Al(10) based on ab initio band structure calculations and x-ray photoemission spectroscopy (XPS) data. Our calculations were performed for the base unit cell and for the hypothetical unit cell which enables antiferromagnetic ordering. The stability of the magnetic phase was investigated within fixed spin moment calculations. When additional 4f correlations are not included in the LSDA C U approach, CeRu(2)Al(10) exhibits an unstable magnetic configuration with the difference in total energy per unit cell between the weakly magnetic state and the non-magnetic one of the order ~0.3 meV. We found that Coulomb correlations among 4f electrons, when they are included in the LSDA C U approach, stabilize the magnetic structure. In the weakly correlated system (small U) an antiferromagnetic (AFM) ground state with the lowest total energy is preferred. The situation is, however, the opposite when the 4f correlations are strong. In this case the ferromagnetic (FM) ground state is preferred. By comparing our calculations with the experimental data we conclude that the 4f correlations in CeRu(2)Al(10) are weak. We also carried out a structural relaxation of atomic positions within the Cmcm unit cell and we found that the Al atoms exhibit noticeable displacement from their positions known from x-ray diffraction (XRD) analysis.     

Scattering of e± from N2 in the energy range 1 eV–10 keV

The differential, integrated elastic, inelastic, total, momentum transfer, viscosity, and ionisation cross sections for electron and positron scattering from the homonuclear diatomic nitrogen molecule over an incident energy range of 1?eV–10?keV are calculated using the additivity rule. Dirac partial wave analysis is employed to calculate the cross sections of the constituent atoms of molecules, using a complex optical model potential (OPM). Comparison of our results of the additivity rule with the available experimental data and other theoretical predictions is presented. Our findings produce reasonable results in intermediate and high energies.     

基于密度泛函理论的5~10元瓜环结构参数与前线轨道的计算

在气相环境中,使用密度泛函理论（DFT）优化由n个苷脲单元组成的瓜环[n]（CB[n]）（n=5~10）,并使用密度泛函（DFT）概念指数和Multiwfn软件包计算和分析CB[n]的结构参数、前线轨道能量和化学稳定性.结果表明：α-N结构最稳定,α=O、γ-γ、γ-H和β-H（2）结构的化学稳定性较差;以CB[6]为界,主要二面角的变化呈现中心对称的形式;随着苷脲单元n的增加,CB[n]的端口直径、空腔直径和圆外径线性增大;前线轨道E_HOMO、E_LUMO值及E_LUMO-E_HOMO值逐渐降低,化学活性逐渐增强、稳定性逐渐减弱;端口O原子是最大的亲电活性位点,LUMO的电子云分布主要与H原子有关,且次甲基H原子对LUMO电子云的贡献最大;次甲基C原子、亚甲基C原子和指向CB[n]端口方向的亚甲基H原子对LUMO的电子云分布具有抑制作用,抑制能力的大小为指向CB[n]端口方向的亚甲基H原子>亚甲基C原子>次甲基C原子.为研究瓜环的超分子组装提供理论依据.     

Conversion of surface carbidic to subsurface carbon on cobalt (0001): a theoretical study

Preliminary HFS-LCAO calculations of a nine atom cobalt cluster reveal a minimal energy difference between surface carbidic and subsurface carbon configurations. The electron withdrawing power, and therefore the poisoning effect on potential CO adsorption, is maximal for subsurface C, but localized to immediately neighboring metal atoms. If the metal lattice is kept fixed, the barrier for moving the carbon atom between the two sites is high (4.70 eV) because of steric repulsion. If the three-fold hollow of the cobalt cluster is stretched slightly by only 1%, the barrier is reduced by nearly 50%. By analogy to effective medium calculations, we may expect thermally active phonon modes to allow as much as a 10% lattice relaxation, which can reduce the surface to subsurface carbon barrier by 90% (0.49 eV). Coadsorption with oxygen favors the subsurface carbon site.     

Synthesis of high nitrogen doping of carbon nanotubes and modeling the stabilization of filled DAATO@CNTs (10,10) for nanoenergetic materials

In this work, we have performed synthesis of nitrogen-doped carbon nanotubes using chemical vapor deposition method. Morphology, structure and composition of the carbon nanotubes (CNTs), as well as concentration and distribution of nitrogen inside CNTs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray dispersive spectroscopy and X-ray photoelectron spectroscopy techniques. A bamboo-like structure of the nitrogen-doped CNTs has been observed. Temperature dependency on the synthesis of nitrogen-doped carbon nanotubes has been investigated and discussed. Diameter and growth rate of these hybrid materials are obviously temperature dependent. Nitrogen concentration inside the CNTs increases with declining synthesis temperature. Nitrogen-doped CNTs with nitrogen content up to 10.4 at% can be achieved at a low temperature of 800 ^oC. Synthesis of the high nitrogen CNTs proposes a feasible way to develop novel nanoenergetic materials. Besides the experimental study, we have carried out Density Functional Theory calculations on five energetic molecules named n-oxides of 3,3′-azo-bis(6-amino-1,2,4,5-tetrazine) (DAATO), where n=1-5 refer to oxygen atoms, encapsulated in CNTs (10,10), in order to investigate the chemical stabilization of filled DAATO_n inside CNTs (10,10). In fact, the predicted adsorption energy values confirmed the chemical stability of the hybrid systems DAATO_n@CNTs (10,10) under normal conditions.     

Molecular Spectral Analysis and Ab Initio Calculations of bis(3-aminopyridinium) Tetrachlorocuprate (II), 3-Ammoniumpyridinium Tetrachlorocuprate (II), and bis(3-aminopyridinium) Hexachlorodicuprate (II)

The aromatic character, distortion, and stabilization as a result of single and double protonation of 3-aminopyridine like three different complex salts were studied by infrared-, ultraviolet spectral analysis, proton nuclear magnetic resonance, and quantum chemical ab initio calculations. Linear-dichroic infrared spectroscopy was applied for identification of the infrared bands. The correlation structure-spectroscopic properties of the model systems are determined: bis(3-aminopyridinium) tetrachlorocuprate (II) salt, where the ring nitrogen atom participates in protonation; 3-ammoniumpyridinium tetrachlorocuprate (II) salt, where both nitrogen atoms are protonated; and a complex with copper (II) bis(3-aminopyridinium) hexachlorodicuprate (II), where the metal ion is coordinated through amino group.     

The superstructures of the clean Pt(100) and Ir(100) surfaces

The clean and reconstructed surfaces of Pt(100) and Ir(100) were investigated by low energy electron diffraction (LEED). It is shown that two superstructures can be observed in the case of platinum. The structure Pt(100)-hex, which is commonly called Pt(100)-(5 × 20), transforms to Pt(100)-hex-R0.7° above 1100 K. It is shown that this stable phase differs from the first one by a slight rotation of the hexagonal surface layer by 0.7°. For Ir(100) only the well known (1 × 5) superstructure is observed without any rotation of the outer layer. The rotation angle of 0.7° for platinum and the stability of the unrelated structure for iridium can be interpreted by simple calculations of the coordination of surface atoms with those of the second layer. The method assumes that the surface layer is of ideal hexagonal structure in the case of platinum and nearly hexagonal in the case of iridium. The results are in good agreement with the experiment.     

RHEED study of the (1 1 0) cleavage surface of CdTe:Cr single crystals

The structure of (1 1 0) plane of Cr-doped CdTe single crystals has been studied by reflection high energy electron diffraction and scanning electron microscopy. Diffraction patterns consist of diffraction spots and Kikuchi lines. However, for very small incident angle, the Debye rings are observed. The constant lattice attributed to these rings is 0.8% less then for pure CdTe. These anomalous properties of the near surface layer are likely to occur due to the concentration of Cr atoms creating compressive surface strains or the effect of crystal cleavage.     

Nanoscale high energetic materials: a polymeric nitrogen chain N(8) confined inside a carbon nanotube

We present a theoretical study of a new hybrid material, nanostructured polymeric nitrogen, where a polymeric nitrogen chain is encapsulated in a carbon nanotube. The electronic and structural properties of the new system are studied by means of ab initio electronic structure and molecular dynamics calculations. Finite temperature simulations demonstrate the stability of this nitrogen phase at ambient pressure and room temperature using carbon nanotube confinement. This nanostructured confinement may open a new path towards stabilizing polynitrogen or polymeric nitrogen at ambient conditions.     

Effect of bias voltage and nitrogen pressure on the structure and properties of vacuum-arc (Mo + Ti6%Si)N coatings

Effect of deposition conditions in reactive nitrogen atmosphere on the growth morphology, phase composition, structure, and mechanical characteristics (microhardness) of vacuum-arc multilayer coatings obtained using evaporation of the (Ti6%Si) and Mo cathodes is studied with the aid of raster electron microscopy, energy-dispersive elemental microanalysis, and microindentation. It is demonstrated that nitrogen atoms are redistributed to the region of the strongest nitride-forming element (Ti) in relatively thin layers (about 7 nm) consisting of substances with substantially different heats of formation (−336 kJ/mol for TiN and −34 kJ/mol for MoN). Such a process leads to lamination with the formation of nitride TiN and metal Mo (weaker nitride-forming element). Nitrogen–metal bonds are saturated in the layers of strong nitrideforming elements Ti(Si) when the nitrogen pressure increases from 6 × 10^–4 to 5 × 10^–3 Torr in the condensation procedure. Thus, the compound is filled with nitrogen to the stoichiometric composition and, then, the second system of layers based on molybdenum is saturated with nitrogen with the formation of the γ-Mo₂N phase. An increase in bias potential U _SP from–100 to–200 V stimulates mixing in thin layers with the formation of the (Ti, Si, Mo)N solid solution and leads to a decrease in microhardness from 37 to 32 GPa.

     

Structures and electronic properties of Mo2nNn（n=1-5）:a density functional study

The lowest-energy structures and the electronic properties of Mo2nNn(n=1-5) clusters have been studied by using the density functional theory(DFT) simulating package DMol 3 in the generalized gradient approximation(GGA).The resulting equilibrium geometries show that the lowest-energy structures are dominated by central cores which correspond to the ground states of Mo n(n = 2,4,6,8,10) clusters and nitrogen atoms which surround these cores.The average binding energy,the adiabatic electron affinity(AEA),the vertical electron affinity(VEA),the adiabatic ionization potential(AIP) and the vertical ionization potential(VIP) of Mo2nNn(n=1-5) clusters have been estimated.The HOMO-LUMO gaps reveal that the clusters have strong chemical activities.An analysis of Mulliken charge distribution shows that charge-transfer moves from Mo atoms to N atoms and increases with cluster size.     

超临界CO2在金属铀表面吸附的理论研究

依据实验数据,假定CO2在金属铀表面吸附氧化初期形成的吸附中间体为UCO2.根据密度泛函理论(DFT)的Becke3lyp方法计算得UCO2(C2v构型)分子的5A1态能量最低,这与用原子分子反应静力学与群论确定UCO2的基电子状态为5A1的结果一致.计算表明,CO2在金属铀表面的吸附是放热反应,其吸附量随着温度的升高而不断减少,其吸附热在1 atm下为51.68 kJ.mol-1,该值大于40 kJ.mol-1,故CO2在金属铀表面的吸附是化学吸附.     

Thermoelectric properties of homogeneously and non-homogeneously doped CdTe15/16M1/16 (M=N,P, As,Sb) and Cd15/16TeM1/16 (M=Na,K, Rb,Cs)

The electrical transport properties of p-doped semiconductors CdTe_15/16M_1/16 (M=N, P, As, Sb) and Cd_15/16TeM_1/16 (M=Na, K, Rb, Cs) with two configurations are investigated through first-principles calculations combined with Boltzmann transport theory under the relaxation time approximation. It is found that N and Cs atoms in the homogeneous structure induce much sharper electron densities of states (DOSs) and flatter energy bands at the valence band edges than the rest of doped elements, resulting in much larger Seebeck coefficients. The calculations reveal that most of the Seebeck coefficients and electrical conductivities are impacted unfavorably by the conglomeration of impurity atoms considered. Though the power factors for homogeneous doping of N and Cs are comparatively smaller, the electronic figures of merit are much larger at 800–1000 K than the rest ones due to much smaller electronic thermal conductivities, therefore probably enhancing the thermoelectric figures of merit. The results show that doping the elements with electronegativities distinct from the host atoms can enhance the Seebeck coefficients and the thermoelectric performances of bulk semiconductors efficiently if the energy levels of doped atoms resonate with those of host atoms and the arrangement of doped atoms is modulated appropriately to avoid deteriorating the sharpness of the DOS (or transport distribution).     

Probing the structural,electronic and magnetic properties of small Au4M (M = Sc–Zn) clusters

Density-functional method PW91 has been selected to investigate the structural, electronic and magnetic properties of Au₄M (M =Sc–Zn) clusters. Geometry optimisations show that the M atoms in the ground-state Au₄M clusters favour the most highly coordinated position. The ground-state Au₄M clusters possess a solid structure for M = Sc and Ti and a planar structure for M = V–Zn. The characteristic frequency of the doped clusters is much greater than that of pure gold cluster. The relative stability and chemical activity are analysed by means of the averaged binding energy and highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap for the lowest energy Au₄M clusters. It is found that the dopant atoms can enhance the thermal stability of the host cluster except for Zn atom. The Au₄Ti, Au₄Mn and Au₄Zn clusters have relatively higher chemical stability. The vertical detachment energy, electron affinity and photoelectron spectrum are calculated and simulated theoretically for all the ground-state structures. The magnetism calculations reveal that the total magnetic moment of Au₄M cluster is mainly localised on the M atom and vary from 0 to 5 μ_B by substituting an Au atom in Au₅ cluster with different transition-metal atoms.