Synthesis and characterization of Th2N2(NH) isomorphous to Th2N3

Using a new, low-temperature, fluoride-based process, thorium nitride imide of the chemical formula Th(2)N(2)(NH) was synthesized from thorium dioxide via an ammonium thorium fluoride intermediate. The resulting product phase was characterized by powder X-ray diffraction (XRD) analysis and was found to be crystallographically similar to Th(2)N(3). Its unit cell was hexagonal with a space group of P3m1 and lattice parameters of a = b = 3.886(1) and c = 6.185(2) ?. The presence of -NH in the nitride phase was verified by Fourier transform infrared spectroscopy (FTIR). Total energy calculations performed using all-electron scalar relativistic density functional theory (DFT) showed that the hydrogen atom in the Th(2)N(2)(NH) prefers to bond with nitrogen atoms occupying 1a Wyckoff positions of the unit cell. Lattice fringe disruptions observed in nanoparticle areas of the nitride species by high-resolution transmission electron microscopic (HRTEM) images also displayed some evidence for the presence of -NH group. As ThO(2) was identified as an impurity, possible reaction mechanisms involving its formation are discussed.     

A comparative relativistic DFT and ab initio study on the structure and thermodynamics of the oxofluorides of uranium(IV), (V) and (VI)

All the possible uranium(VI, V, IV) oxides, fluorides and oxofluorides were studied theoretically by using density functional theory (DFT) in the generalised gradient approximation (GGA), and three different relativistic methods (all-electron scalar four component Dyall RESC method (AE), relativistic small-core ECPs, and zeroth order regular approximation ZORA). In order to test different correlation methods, for the two former relativistic methods hybrid DFT, and, for the AE method, MP2 molecular orbital calculations were performed as well. Single-point AE-CCSD(T) energies were calculated on MP2 geometries as well. Energies of the uranium(VI) and (V) oxofluorides dissociation, uranium(VI) fluoride hydrolysis and oxofluoride disproportionation were calculated and compared against the available experimental thermochemical data. AE-CCSD(T) energies were the closest to the experiment. For GGA DFT methods, all the relativistic methods used yield similar results. For thermochemistry, the best quantitative agreement with the experimental and CCSD(T) values for both U=O and U-F bond strengths was obtained with hybrid DFT methods, provided that a reliable basis set was used. Both the GGA DFT and MP2 MO methods show overbinding of these bonds; moreover, this overbinding was found to be not uniform but strongly dependent on the coordination environment of the uranium atom in each case. U=O vibrational frequencies given by hybrid DFT, however, are systematically overestimated, and are better reproduced by GGA DFT; MP2 values usually fall in-between. Reaction enthalpies, U=O frequencies and complex geometries given by the PBE, MPBE, BPBE, BLYP and OLYP GGA functionals are quite similar, with OLYP performing slightly better than the others but still not as good as hybrid DFT. The geometries of the molecules are found to be influenced by the following factors: the inverse transinfluence (ITI) of the oxygen ligand and, for U(V), and U(IV), the Jahn-Teller distortion.     

Self-interaction artifacts on structural features of uranyl monohydroxide from Kohn�CSham calculations

Invoking a DFT?+?U approach, we explored self-interaction artifacts in results from Kohn?CSham (KS) density functional calculations on the geometry and the vibrational frequencies of uranyl monohydroxide and the corresponding tetra-aqua complex. Exchange?Ccorrelation functionals based on the local density approximation (LDA) and the generalized-gradient approximation (GGA) predict equilibrium geometries for [UO₂(OH)]⁺ that deviate from the results of hybrid DFT calculations and high-level wavefunction-based methods such as CCSD(T). LDA?+?U and GGA?+?U functionals with corrections for the insufficient localization of the U 5f shell yield better agreement, in particular for the angle U-O_h-H. At the LDA level, a linear coordination of the OH ligand results; with the +U correction, the angle U-O_h-H is reduced by ~35°, in good agreement with CCSD(T) results. At the GGA level, the bending angle is changed by ~20°. This relatively strong self-interaction artifact is traced back to a spurious ?? interaction between U 5f and O(p) orbitals which is less pronounced in the presence of further (aqua) ligands.     

Valence-band electronic structure of iron phthalocyanine: an experimental and theoretical photoelectron spectroscopy study

The electronic structure of iron phthalocyanine (FePc) in the valence region was examined within a joint theoretical-experimental collaboration. Particular emphasis was placed on the determination of the energy position of the Fe 3d levels in proximity of the highest occupied molecular orbital (HOMO). Photoelectron spectroscopy (PES) measurements were performed on FePc in gas phase at several photon energies in the interval between 21 and 150 eV. Significant variations of the relative intensities were observed, indicating a different elemental and atomic orbital composition of the highest lying spectral features. The electronic structure of a single FePc molecule was first computed by quantum chemical calculations by means of density functional theory (DFT). The hybrid Becke 3-parameter, Lee, Yang and Parr (B3LYP) functional and the semilocal 1996 functional of Perdew, Burke and Ernzerhof (PBE) of the generalized gradient approximation (GGA-)type, exchange-correlation functionals were used. The DFT/B3LYP calculations find that the HOMO is a doubly occupied π-type orbital formed by the carbon 2p electrons, and the HOMO-1 is a mixing of carbon 2p and iron 3d electrons. In contrast, the DFT/PBE calculations find an iron 3d contribution in the HOMO. The experimental photoelectron spectra of the valence band taken at different energies were simulated by means of the Gelius model, taking into account the atomic subshell photoionization cross sections. Moreover, calculations of the electronic structure of FePc using the GGA+U method were performed, where the strong correlations of the Fe 3d electronic states were incorporated through the Hubbard model. Through a comparison with our quantum chemical calculations we find that the best agreement with the experimental results is obtained for a U(eff) value of 5 eV.     

Adsorption energies of molecular oxygen on Au clusters

The adsorption properties of O(2) molecules on anionic, cationic, and neutral Au(n) clusters (n=1-6) are studied using the density functional theory (DFT) with the generalized gradient approximation (GGA), and with the hybrid functional. The results show that the GGA calculations with the PW91 functional systemically overestimate the adsorption energy by 0.2-0.4 eV than the DFT ones with the hybrid functional, resulting in the failure of GGA with the PW91 functional for predicting the adsorption behavior of molecular oxygen on Au clusters. Our DFT calculations with the hybrid functional give the same adsorption behavior of molecular oxygen on Au cluster anions and cations as the experimental measurements. For the neutral Au clusters, the hybrid DFT predicts that only Au(3) and Au(5) clusters can adsorb one O(2) molecule.     

A DFT study on the electronic structure for iridium nitride under high pressure

We present density functionary theory (DFT) calculations on the structural parameters and electronic structure for iridium nitride by using the generalized gradient approximation (GGA) and the Perdew–Burke–Ernserhof (PBE) exchange-correlation functional. The lattice parameters and bulk modulus (B ₀) for the ground state are obtained, and the energy band structure and electron densities of states (DOS) of IrN₂ are presented. It is found that IrN₂ has a very close indirect energy gap. There is a strong covalent bond between the two nearest N atoms. This gives rise to a very high elastic modulus of IrN₂ and reveals the quasimolecular nature of the N₂ in IrN₂ crystal. Lattice parameters, bulk modulus, and the electronic structure of IrN₂ under high pressure have also been investigated based on DFT. The compressibility along three cell vectors is very close to each other. The band gap increases a little with the pressure even when the pressure is up to 100 Gpa.     

N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构. 由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷, 本文也采用DFT+U(Hubbard 系数)方法对模型的电子结构进行了计算. DFT的计算结果表明N掺杂后, N 2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低, 而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响. DFT+U的计算却给出截然不同的结果, N掺杂并没有导致带隙能的降低, 而只是在带隙中引入一个孤立的杂质能级, 反而F掺杂以及氧空位的引入带来明显的带隙能降低. DFT+U的计算结果与一些实验测量结果能够较好地符合.     

N／F掺杂和N-F双掺杂锐钛矿相TiO2（101）表面电子结构的第一性原理计算

采用密度泛函理论(DFT)平面波赝势方法计算了N/F掺杂和N-F双掺杂锐钛矿相TiO2(101)表面的电子结构.由于DFT方法存在对过渡金属氧化物带隙能的计算结果总是与实际值严重偏离的缺陷,本文也采用DFT+U(Hubbard系数)方法对模型的电子结构进行了计算.DFT的计算结果表明N掺杂后,N2p轨道与O 2p和Ti 3d价带轨道的混合会导致TiO2带隙能的降低,而F掺杂以及氧空位的引入对材料的电子结构没有明显的影响.DFT+U的计算却给出截然不间的结果,N掺杂并没有导致带隙能的降低,而只是在带隙中引入一个孤立的杂质能级,反而F掺杂以及氧空位的引入带来明显的带隙能降低.DFT+U的计算结果与一些实验测量结果能够较好地符合.     

Ab initio total energy calculations of copper nitride: the effect of lattice parameters and Cu content in the electronic properties

We have studied the structural and electronic properties of bulk copper nitride by performing first principles total energy calculations using the full-potential linearized augmented plane wave (FP-LAPW) method. In our study we have considered two types of cells: the ideal cubic anti-ReO₃ structure corresponding to Cu₃N, and a unit cell with an extra Cu atom at the center of the cube. In the first case, our calculated lattice parameter a=3.82 Å is in excellent agreement with the experimental value a=3.807 Å. The structure is semiconductor with a small indirect band-gap. The increasing of the lattice parameter results in larger band-gaps. An addition of an extra Cu atom at the center of the cell results in a slightly larger lattice parameter a=3.88 Å, and the structure becomes fully metallic. Our calculated value is similar to the experimental lattice parameter corresponding to a metallic copper nitride film.     

六角多铁性HoMnO3的电子和能带结构

基于密度泛函理论(DFT)结合投影缀加平面波(PAW)方法, 运用广义梯度近似(GGA), 在考虑电子基态自旋阻挫非共线的磁性结构基础上, 研究了具有六角钙钛矿结构HoMnO₃材料的磁性、电子和能带结构, 并解释了相关实验结果. 结果表明: 当考虑Mn³⁺离子的电子自旋在平面内呈阻挫的三角非共线反铁磁(NAFM)排列时, 六角HoMnO₃的总能降低、能隙变大、磁矩增大、各原子的位置更接近于实验值, 电子态密度(DOS)分布具有与X光吸收谱测量更为一致的结果. 对非共线磁性结构计算得到的电子态密度和能带结构的分析发现, 实验中观察到的1.7和2.3 eV两个光学吸收峰都源于Mn³⁺离子3d与平面内O(3, 4) 2p形成的杂化态与Mn [3d_3z²-r²]之间的电子跃迁, 而Ho 5d空轨道与平面上的O(3, 4) 2p轨道之间在z方向的强烈杂化驱动HoMnO₃产生垂直于平面方向的铁电极化.     

Equiatomic binary phases of Copper-Rare Earth Elements. An overview of monocuprides from first-principles calculations

Equiatomic binary phases of copper with rare earth (RE) elements exhibit either primitive cubic ( ) or orthorhombic (Pnma) structures and in some cases both. By using density functional theory (DFT), we calculated the enthalpies of formation along the series of RE elements combined equimolarly with copper. For RE from Sc to Lu, the calculated enthalpies of formation fall in the range −49.8 kJ/mol for LuCu to −9.1 kJ/mol for the least thermodynamically stable CeCu. Except NdCu, all the other cubic or orthorhombic compounds exhibit lattice stability. Either forms of NdCu indicated lattice instability. Along the Sc-group, the hypothetical primitive cubic and orthorhombic forms of LuCu are found thermodynamically and mechanically stable. The overall trend of the formation enthalpies as a function of the Meyer Periodic Number is consistent with the energy trend of the 4 f-orbital filling as moving from Sc to Lu monocuprides. In addition, the calculated Gibbs free energies indicate that the thermodynamic stability is largely due to the entropic contributions. All standard DFT calculations were also repeated with DFT+U to better describe the correlation between the 5d–4f and 3d shells of RECu compounds. It has been found that DFT+U slightly affects the enthalpies of formation of RECu binaries. Moreover, DFT+U shifts up the f-band energies of RECu with light RE elements (such as La, Ce and Pr) and in contrast lowers them in the case of RECu with heavy RE elements from Nd to Lu.     

Divergent Stabilities of Tetravalent Cerium,Uranium, and Neptunium Imidophosphorane Complexes**

The study of the redox chemistry of mid-actinides (U−Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=P^tBu(pyrr)₂]⁻; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium ( 1-M , 2-M , M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M^4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U^5+/4+, U^6+/5+, and to an unprecedented, well-behaved Np^5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.     

Oxidation behaviour of uranium and neptunium in stabilised zirconia

Yttria stabilised zirconia (YSZ) based (Zr,Y,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions with 6 and 20 mol% actinide were prepared with Y/Zr ratios ranging from 0.2 to 2.0 to investigate uranium and neptunium oxidation behaviour depending on the oxygen vacancies in the defect fluorite lattice. Sintering at 1600 °C in Ar/H₂ yields a cubic, fluorite-type structure with U(IV) and Np(IV). Annealing (Zr,Y,U)O_2−x with Y/Zr=0.2 at 800 °C in air results in a tetragonal phase, whereas (Zr,Y,U)O_2−x with higher Y/Zr ratios and (Zr,Y,Np)O_2−x retain the cubic structure. XANES and O/M measurements indicate mixed U(V)-U(VI) and Np(IV)-Np(V) oxidation states after oxidation. Based on X-ray diffraction, O/M and EXAFS measurements, different oxidation mechanisms are identified for U- and Np-doped stabilised zirconia. In contrast to U, excess oxygen vacancies are needed to oxidise Np in (Zr,Y,Np)O_2−x as the oxidation process competes with Zr for oxygen vacancies. As a consequence, U(VI) and Np(V) can only be obtained in stabilised zirconia with Y/Zr=1 but not in YSZ with Y/Zr=0.2.     

Phase Relationships in the U – Np – O System

U – Np – O phase equilibria was experimentally studied by X-ray diffraction in reducing and oxidizing conditions. Both (U1yNpy)O2 stoichiometric and hyper-stoichiometric compounds were characterized after a heating treatment and resulting lattice parameters were compared to that of previous studies. Synthesized stoichiometric compounds are monophasic and their lattice parameters as a function of Np content are in nice agreement with the Vegard's law between UO2 and NpO2 end-member. Resulting compounds after an oxidizing heat treatment of the stoichiometric samples confirm previous reported U – Np – O relationship. For low Np content, a biphasic hyper-stoichiometric compound is observed: a (U1-y’Npy’)O2+x phase (space group fm-3m) and a (U1-y”,Npy”)3O8 phase (space group C2 mm). For higher Np content (with starting material y = 0.7), only one monophasic hyper-stoichiometric phase (U1-y’Npy’)O2+x appears. Lattice parameters of the (U1-y”,Npy”)3O8 phase differ from the ones of pure U3O8, indicating some Np solubility in this compound. Although Np might remain tetravalent in the (U1y”,Npy”)3O8 phase, this result is in conflict with the assumption of Yamashita et al. (1994).     

Ab initio calculation of structural,lattice dynamical,and thermal properties of cubic silicon carbide

We present first-principles calculations of the structural, lattice dynamical, and thermal properties as well as Raman results for cubic silicon carbide (3C SiC). The plane-wave pseudopotential approach to density functional theory (DFT ) in the local density approximation has been used to calculate the equilibrium properties of 3C SiC, i.e., the ground-state energy, the band structure, the valence electron density, the lattice constant, the bulk modulus, its pressure derivative, and the ionicity factor of the chemical bonds. The linear-response theory within DFT has been used to obtain the phonon frequencies, the eigenvectors, and the mean-square atomic displacements. Furthermore, we calculated the mode Grueisen parameters, the internal-strain parameter, the elastic constants, the Born effective charge, and the high-frequency dielectric constant. The specific heat at constant volume and at constant pressure, the thermal expansion coefficient, the temperature dependence of the lattice constant, and that of the isothermal and adiabatic bulk modulus have been derived within the quasi-harmonic approximation. Finally, the second-order Raman spectrum of 3C SiC has been calculated using phenomenological polarizability coefficients and ab initio frequencies and eigenvectors. © 1995 John Wiley & Sons, Inc.     

Plasma-assisted synthesis and properties of Na(3)N

Dark-blue sodium nitride, Na(3)N, was prepared by the reaction of metallic sodium or liquid Na-K alloy with plasma-activated nitrogen at low pressure. The compound crystallizes in the cubic anti-ReO(3)-type structure (space group Pm3m with a = 4.73301(6) Angstrom and Z = 1) according to powder and single-crystal X-ray diffraction data. Na(3)N decomposes above 104 degrees C into the elements, with DeltaH(f) estimated at +64(2) kJ/mol.