Placement of H atoms in the crystal lattice of cubic titanium oxyhydride: Simulation and diffraction experiment

Possible models for the arrangement of hydrogen atoms in the sites of the cubic lattice of titanium oxyhydride TiO_yH_p with vacancies in the metallic and nonmetallic sublattices are considered for the first time. It has been established that interstitial H atoms in oxyhydrides occupy vacant octahedral sites 4(b) of the oxygen sublattice. No displacement of H atoms in tetrahedral sites 8(c) is observed.     

DFT study on the reed diethylaluminum cation-like system: structure and bonding in Et(2)Al(CB(11)H(6)X(6)) (X = Cl, Br)

Electronic and molecular structure has been investigated in the diethylaluminum cation-like system Et(2)Al(CB(11)H(6)X(6)) (1, X = Cl; 2, X = Br) and neutral compounds AlX(3) (X = Cl, Br, Me, C(6)H(5)) with DFT B3LYP and BP86 levels of theory. The calculated geometries of Et(2)Al(CB(11)H(6)X(6)) (1, X = Cl; 2, X = Br) are in excellent agreement with those determined experimentally by X-ray crystallography. The Al-X bond distances 2.442, 2.445 A in 1 and 2.579, 2.589 A in 2 are longer than those expected for single bonds based on covalent radius predictions (Al-Cl = 2.15 A and Al-Br = 2.32 A) and those observed for bridged Al-X-Al bonds (2.21 A in Al(2)Cl(6), 2.33 A in Al(2)Br(6)) and are close to sum of ionic radii of Al(3+) and X(-) (Al-Cl = 2.35 A and Al-Br = 2.50 A). The optimized geometries of the neutral compounds AlX(3) (X = Cl, Br, Me(3), C(6)H(5)) at BP86/TZ2P show Al-Cl = 2.088 A in AlCl(3), Al-Br = 2.234 A in AlBr(3), Al-C = 1.973 A in AlMe(3), Al-C = 2.255 A in Al(C(6)F(5))(3). These bond distances are similar to those expected for single bonds based on covalent radius predictions. The calculated charge distribution indicates that the aluminum atom carries a significant positive charge while the ethyl and carborane groups are negatively charged. The Cl and Br atoms in compounds 1 and 2 are slightly positive while, in neutral compounds AlX(3) (X = Cl, Br, Me(3), C(6)H(5)), X is negatively charged. Energy decomposition analysis of Et(2)Al(delta)(+)(carborane)(delta)(-) shows that the bonding between the fragments is more than half electrostatic. The ionic character of the Al...Cl bonds in compound 1(59.8%) is greater than the Al.Br bonds in the compound 2 (57.9%). This quantifies and gives legitimacy to the designation of these types of compounds as "ion-like". The Al-X bonding in AlX(3) is mainly covalent with percentage ionic character 28.2% in AlCl(3), 31.5% in AlBr(3), 25.6% in AlMe(3), 18.4% in Al(C(6)F(5))(3).     

Sizes and properties of clusters of magnetic ferrite structures

The structures of the coordination spheres or orbits of hexagonal and cubic crystals and their sizes, coordination numbers, and coordinates of atoms and cavities have been studied. The orbits of atoms of all sublattices of octahedral and tetrahedral cavities have been calculated. The close-packed structures (FCC and HCP) of oxygen ions have been considered as basic structures. In both structures, the metal cations are distributed over octahedral and tetrahedral cavities. The developed method is used for calculating the orbits of clusters with the hexagonal crystal structure of magnetoplumbite, ilmenite, and corundum, as well as with the cubic structure of spinel and perovskite.     

Wave functions derived from experiment. V. Investigation of electron densities,electrostatic potentials,and electron localization functions for noncentrosymmetric crystals

The constrained Hartree-Fock method using experimental X-ray diffraction data is extended and applied to the case of noncentrosymmetric molecular crystals. A new way to estimate the errors in derived properties as a derivative with respect to added Gaussian noise is also described. Three molecular crystals are examined: ammonia [NH(3)], urea [CO(NH(2))(2)], and alloxan [(CO)(4)(NH)(2)]. The energetic and electrical properties of these molecules in the crystalline state are presented. In all cases, an enhancement of the dipole moment is observed upon application of the experimental constraint. It is found that the phases of the structure factors are robustly determined by the constrained Hartree-Fock model, even in the presence of simulated noise. Plots of the electron density, electrostatic potential, and the electron localization function for the molecules in the crystal are displayed. In general, relative to the Hartree-Fock model, there is a depletion of charge around hydrogen atoms and lone pair regions, and a build-up of charge within the molecular framework near nuclei, directed along the bonds. The electron localization function plots reveal an increase in the pair density between vicinal hydrogen atoms.     

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs)

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.     

Metal distributions in Al∼1.1Be∼0.6B22 of α-AlB12 structure type

Distributions of metals in Al_～1.1Be_～0.6B₂₂ were investigated by X-ray diffraction using four different crystals. The boron frameworks of the crystals are almost the same as that in α-AlB₁₂. However, the metal distributions are considerably different from that in α-AlB₁₂; Al atoms occupy only Al(1, 2, 3) sites with a great increment in occupancy of Al(3), leaving the other two, Al(4, 5), almost empty; on the other hand, Be atoms partially occupy two to five sites that are vacant in the case of α-AlB₁₂. Significant differences in the Be distributions are also found among the four Al_～1.1Be_～0.6B₂₂ crystals. However, in every case, including α-AlB₁₂, the metallic valence electron numbers, allotted to the B₁₂ and B₂₀ units in proportion to the frequencies in the contacts of the units with the metals, are ～2 and ～5.5, respectively. It is inferred that the variations in the metal distributions, primarily caused by the difference in the atomic sizes of Al and Be, arise, so as to preserve a negative charge balance between B₁₂ and B₂₀; a ratio of about 1:3 should presumably be essential to the stabilization of α-AlB₁₂ structure type compounds.     

Cubic aluminum silicides RE8Ru12Al49Si9(Al(x)Si12-x) (RE = Pr, Sm) from liquid aluminum. Empty (Si,Al)12 cuboctahedral clusters and assignment of the Al/Si distribution with neutron diffraction.

Two new quaternary aluminum silicides, RE8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4; RE = Pr, Sm), have been synthesized from Sm (or Sm2O3), Pr, Ru, and Si in molten aluminum between 800 and 1000 degrees C in sealed fused silica tubes. Both compounds form black shiny crystals that are stable in air and NaOH. The Nd analog is also stable. The compounds crystallize in a new structural type. The structure, determined by single-crystal X-ray diffraction, is cubic, space group Pm3m with Z = 1, and has lattice parameters of a = 11.510(1) A for Sm8Ru12Al49Si9(Al(x)Si12-x) and a = 11.553(2) A for Pr8Ru12Al49Si9(Al(x)Si12-x) (x approximately 4). The structure consists of octahedral units of AlSi6, at the cell center, Si2Ru4Al8 clusters, at each face center, SiAl8 cubes, at the middle of the cell edges, and unique (Al,Si)12 cuboctohedral clusters, at the cell corners. These different structural units are connected to each other either by shared atoms, Al-Al bonds, or Al-Ru bonds. The rare earth metal atoms fill the space between various structural units. The Al/Si distribution was verified by single-crystal neutron diffraction studies conducted on Pr8Ru12Al49Si9(Al(x)Si12-x). Sm8Ru12Al49Si9(Al(x)Si12-x) and Pr8Ru12Al49Si9(Al(x)Si12-x) show ferromagnetic ordering at Tc approximately 10 and approximately 20 K, respectively. A charge of 3+ can be assigned to the rare earth atoms while the Ru atoms are diamagnetic.     

Chemical bonding in isostructural Li-containing ternary chalcogenides

First principle calculations were performed for the first time to study the electronic structure of LiGaTe₂, LiInTe₂, and LiInSe₂ chalcogenides with a chalcopyrite structure. Peculiarities of chemical bonding are discussed and electron density and difference density maps are constructed for crystals and sublattices. Major information about chemical bonding in crystals is conveyed by the difference density. The chemical bond in chalcogenides is a donor-acceptor bond.     

Density distribution in the liquid Hg-sapphire interface

We present the results of a computer simulation study of the liquid density distribution normal to the interface between liquid Hg and the reconstructed (0001) face of sapphire. The simulations are based on an extension of the self-consistent quantum Monte Carlo scheme previously used to study the structure of the liquid metal-vapor interface. The calculated density distribution is in very good agreement with that inferred from the recent experimental data of Tamam et al. (J. Phys. Chem. Lett. 2010, 1, 1041-1045). We conclude that, to account for the difference in structure between the liquid Hg-vapor and liquid-Hg-reconstructed (0001) Al(2)O(3) interfaces, it is not necessary to assume there is charge transfer from the Hg to the Al(2)O(3). Rather, the available experimental data are adequately reproduced when the van der Waals interactions of the Al and O atoms with Hg atoms and the exclusion of electron density from Al(2)O(3) via repulsion of the electrons from the closed shells of the ions in the solid are accounted for.     

Principles Determining the Structure of Transition Metals

For the better part of a century researchers across disciplines have sought to explain the crystallography of the elemental transition metals: hexagonal close packed, body centered cubic, and face centered cubic in a form similar to that used to rationalize the structure of organic molecules and inorganic complexes. Pauling himself tried with limited success to address the origins of transition metal stability. These early investigators were handicapped, however, by incomplete knowledge regarding the structure of metallic electron density. Here, we exploit modern approaches to electron density analysis to first comprehensively describe transition metal electron density. Then, we use topological partitioning and quantum mechanically rigorous treatments of kinetic energy to account for the structure of the density as arising from the interactions between metallic polyhedra. We argue that the crystallography of the early transition metals results from charge transfer from the so called “octahedral” to “tetrahedral cages” while the face centered cubic structure of the late transition metals is a consequence of anti-bonding interactions that increase octahedral hole kinetic energy.     

液体和非晶态NiAl3合金结构的从头算分子动力学模拟

应用从头算分子动力学方法模拟了液体以及淬冷形成的NiAl₃合金体系,得到了它们的对相关函数、结构因子、键对分析信息.结果分析表明,在淬冷条件下得到的体系呈现非晶态性质,且非晶态结构类似于液态NiAl₃合金的结构,可以用液体结构近似描述非晶态性质.还进行了电子结构分析,得到液体NiAl₃合金的电子态密度和电荷分布.在液体镍铝合金中,镍为电子受体,部分电子由铝向镍转移,支持了Candy等人的XPS实验结果.镍铝间强烈作用,形成带有弱共价键性质的金属键.镍在合金中相当分散,这能部分解释由淬冷形成的NiAl₃合金制得的骨架镍催化剂活性增强的原因.     

Bonding study in all-metal clusters containing Al4 units

The nature of the bonding of a series of gas-phase all-metal clusters containing the Al4 unit attached to an alkaline, alkaline earth, or transition metal is investigated at the DFT level using Mulliken, quantum theory of atoms in molecules (QTAIM), and Hirshfeld iterative (Hirshfeld-I) atomic partitionings. The characterization of ionic, covalent, and metallic bonds is done by means of charge polarization and multicenter electron delocalization. This Article uses for the first time Hirshfeld-I multicenter indices as well as Hirshfeld-I based atomic energy calculations. The QTAIM charges are in line with the electronegativity scale, whereas Hirshfeld-I calculations display deviations for transition metal clusters. The Mulliken charges fail to represent the charge polarization in alkaline metal clusters. The large ionic character of Li-Al and Na-Al bonds results in weak covalent bonds. On the contrary, scarcely ionic bonds (Be-Al, Cu-Al and Zn-Al) display stronger covalent bonds. These findings are in line with the topology of the electron density. The metallic character of these clusters is reflected in large 3-, 4- and 5-center electron delocalization, which is found for all the molecular fragments using the three atomic definitions. The previously reported magnetic inactivity (based on means of magnetic ring currents) of the pi system in the Al42- cluster contrasts with its large pi electron delocalization. However, it is shown that the different results not necessary contradict each other.     

Synthesis, crystal structure, and properties of two modifications of MgB(12)C(2)

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.     

The principle of maximum filling and characteristics of sublattices of hydrogen atoms

The most important characteristics of the Voronoi-Dirichlet polyhedra of 4005928 crystallographically different atoms in sublattices only containing hydrogen atoms were determined in the structures of 151044 inorganic, organoelement, and organic compound crystals. In such sublattices, the Voronoi-Dirichlet polyhedra of H atoms most often had 15 faces, and these faces were most often pentagonal. A comparative analysis of H- and C-sublattices was performed to find that the most important geometric and topological characteristics of H-sublattices were caused by the absence of short-range and local long-range order in the mutual arrangement of hydrogen atoms in the structure of crystals.     

A new look at bonding in trialuminides: reinvestigation of TaAl3

Single crystals of TaAl3 were grown at high temperatures from an Al-rich, binary solution. TaAl3 adopts the D0(22) structure type, space group I4/mmm with a=3.8412(5) A, c=8.5402(17) A, and Z=2. The structure type, which is the preferred structure for all group 5 trialuminides and TiAl3 as well as the high-temperature form of HfAl3, is a binary coloring of the face-centered-cubic (fcc) arrangement. The distribution of Ta atoms creates a three-dimensional network of vertex and edge-sharing square pyramids of Al atoms. Temperature-dependent electrical resistivity and magnetic susceptibility measurements are consistent with TaAl3 being a metallic compound with a relatively low density of states at the Fermi surface. Furthermore, tight-binding electronic structure calculations are utilized to describe the bonding in these compounds and to compare their stability with respect to the alternative fcc-related, e.g., the D0(23) (ZrAl3-type) and the L1(2) (AuCu3-type), structures. A modified Wade's rule argument provides insights into the structural preferences.     

Energy band genesis from sublattice states in MgSiN2 and MgGeN2 crystals

The electronic structure of MgSiN₂ and MgGeN₂ orthorhombic crystals and their sublattices was analyzed at the density functional theory level using the sublattice method. The energy-band structure of the orthorhombic crystals is compared to the structure of their hypothetic analogues with the chalcopyrite lattice. The origin of the specific features of the valence band structure from the sublattice states is determined in the studied crystals, which is mainly due to the interaction of atoms in SiN₄ and GeN₄ cation tetrahedra.     

Role of sublattices in the formation of the chemical bond in ion-covalent crystals

Crystals of ZnGeAs₂ and AgGaTe₂ are used as examples to show the results of applying the sublattice method. The latter enables one to visualize the formation of valence band structure, electron density and chemical bond as a result of interactions between the atoms in various sublattices of the crystal.     

The nature of electronic states and chemical bonding in lithium and potassium carbonates

In the context of the density functional theory the calculation of electron energy and space distribution in (Li_xK_1?x )₂CO₃ (x = 0, 0.5, 1) have been made employing the pseudopotential method in the basis of numerical pseudo-orbitals. The electron structure of alkali metal carbonates is shown to be essentially different from alkaline-earth ones, this being connected with the change in the charge state of both the anion as a whole and the atoms composing it. The mechanisms of chemical bond formation were investigated by the sublattice method and it was shown that its peculiarities were due to the presence of nonequivalent oxygen and cation sublattices. In Li₂CO₃, anion atoms are covalently bonded, while in LiKCO₃ and K₂CO₃ the ion constituent presents.     

Bond insertion, complexation, and penetration pathways of vapor-deposited aluminum atoms with HO- and CH(3)O-terminated organic monolayers

The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS-(CH(2))(16)-X (X = -OH and -OCH(3)) chemisorbed at polycrystalline Au[111] surfaces was studied using time-of-flight secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, and infrared reflectance spectroscopy. Whereas quantum chemical theory calculations show that Al insertion into the C-C, C-H, C-O, and O-H bonds is favorable energetically, it is observed that deposited Al inserts only with the OH SAM to form an -O-Al-H product. This reaction appears to cease prior to complete -OH consumption, and is followed by formation of a few overlayers of a nonmetallic type of phase and finally deposition of a metallic film. In contrast, for the OCH(3) SAM, the deposited Al atoms partition along two parallel paths: nucleation and growth of an overlayer metal film, and penetration through the OCH(3) SAM to the monolayer/Au interface region. By considering a previous observation that a CH(3) terminal group favors penetration as the dominant initial process, and using theory calculations of Al-molecule interaction energies, we suggest that the competition between the penetration and overlayer film nucleation channels is regulated by small differences in the Al-SAM terminal group interaction energies. These results demonstrate the highly subtle effects of surface structure and composition on the nucleation and growth of metal films on organic surfaces and point to a new perspective on organometallic and metal-solvent interactions.     

纳米Al2O3颗粒对纯Fe液诱导凝固过程的分子动力学模拟

采用分子动力学(MD)方法模拟了不同半径大小的纳米Al2O3颗粒夹杂在三个温度下(1750、1730和1710K)对纯Fe液的诱导凝固过程,并分析了作为诱导核心的纳米Al2O3颗粒的结构演变及其对Fe原子体系的凝固过程的影响.发现在诱导过程中,纳米Al2O3颗粒的内部保持较好的晶型结构,仅表面原子有结构变形;诱导凝固的Fe原子主要为面心立方(fcc)和密排六方(hcp)原子;纳米Al2O3颗粒的尺寸越大,发生诱导凝固的温度越高;诱导凝固得到的Fe晶体的晶格取向受纳米Al2O3颗粒在Fe液中的漂移程度影响.