Tables for the computation of the trapping energy in various sites of fcc matrices. Effect of eccentricity with anisotropic guest molecules

The lattice sums of inverse power potentials, known for long in various types of crystal, are extended here with the goal to be useful for matrix calculations. For this purpose, the interest is restricted to fcc crystals (such as that of the heavy rare gases), but various possible sites - monosubstitutional, octahedral, tetrahedral and bihedral interstitial - are investigated and more recent potential forms (Rⁿ exponential) considered, and the results given in the form of tables. The facts that a guest molecule can be anisotropic and is not necessarily confined at the very centre of its trapping site is investigated and a formula established to compute the energy in this case. The results are discussed and sample applications are appended to examplify the use of the tables and formulas.     

H原子在完美δ-Pu金属体相中的扩散行为

采用密度泛函理论的广义梯度近似方法计算了H原子在δ-Pu金属体相中可能稳定存在间隙位置的嵌入能. 计算结果表明, 八面体间隙是H原子最稳定存在位置, 无自旋极化和自旋极化水平的嵌入能分别为-3.12和-2.22 eV. 四面体间隙位的嵌入能相对稍大, 是次稳定存在位置. 通过能量分析推测了单个H原子在完美δ-Pu金属中可能的扩散路径和扩散能垒. 最可能扩散路径为相邻不同间隙位的交替扩散, 八面体间隙到四面体间隙的扩散能垒为1.06 eV, 而四面体间隙到八面体间隙的扩散能垒为0.38 eV. 另外沿平行晶轴方向四面体间隙到四面体间隙交替直线扩散的能垒为1.83 eV, 八面体间隙到八面体间隙交替扩散路径的能垒最高, 为2.52 eV.     

A new method applicable to study solid compounds with multiple polyhedral structures

A new direct summation method, named as polyhedron method, is proposed to calculate Madelung energy. This method calculates sums of electrostatic interactions over sets of neutral polyhedron unit pairs rather than conventional ion pairs; this gives Madelung constant in a matrix. With robustly rapid convergence, polyhedron method is generally applicable for complex compounds containing multiple polyhedral building‐blocks and numerical polyhedral connection modes. The matrical analysis suggests face‐sharing between octahedral pairs and edge‐sharing between tetrahedral pairs can be electrostatically stable, against Pauling's third rule. Further, the matrical calculation of Madelung energies offers a unique advantage to evaluate enormous configurations of cation distributions in a given lattice in a high‐throughput manner. That is applicable to study solid solution composites, polymorphism, and defect structures, including but not limited to intermediate phase of delithiated cathode compounds, charge order or antisite defects, and extensively magnetic order. © 2016 Wiley Periodicals, Inc.     

A dynamic lattice searching method for fast optimization of Lennard-Jones clusters

A highly efficient unbiased global optimization method called dynamic lattice searching (DLS) was proposed. The method starts with a randomly generated local minimum, and finds better solution by a circulation of construction and searching of the dynamic lattice (DL) until the better solution approaches the best solution. The DL is constructed adaptively based on the starting local minimum by searching the possible location sites for an added atom, and the DL searching is implemented by iteratively moving the atom located at the occupied lattice site with the highest energy to the vacant lattice site with the lowest energy. Because the DL can greatly reduce the searching space and the number of the time-consuming local minimization procedures, the proposed DLS method runs at a very high efficiency, especially for the clusters of larger size. The performance of the DLS is investigated in the optimization of Lennard-Jones (LJ) clusters up to 309 atoms, and the structure of the LJ(500) is also predicted. Furthermore, the idea of dynamic lattice can be easily adopted in the optimization of other molecular or atomic clusters. It may be a promising approach to be universally used for structural optimizations in the chemistry field.     

Kationenverteilungen in einigen Zinnspinell-Mischkristallen

In inverse tin spinels (Mg,M)₂SnO₄ (with M = Zn, Co, or Ni), the distribution of bivalent metal atoms was determined from samples which had been quenched from 1473 or 1073 K or cooled slowly. In Mg₂SnO₄, zinc preferentially occupies tetrahedral sites with ΔG°-values of about 18 kJ/g-atom. Cobalt and magnesium are equally distributed over octahedral and tetrahedral sites, whereas nickel occupies octahedral sites only. Lattice constants of (Mg,Zn)₂SnO₄ solid solutions deviate largely from Vegard's law, but depend linearly on the zinc concentrations in tetrahedral and octahedral sites, according to: a = c₀ + c₁[Zn(tetr)] + c₂[Zn(oct)]. Zinc in tetrahedral sites causes a lattice contraction: in octahedral sites, however, it causes a lattice expansion.     

Calculations of van der Waals interaction energies for Al, Cr, Fe, and Ir acetylacetonate crystals

Experimental data have been analyzed and interpreted for four volatile acetylacetonates of trivalent metals Al, Cr, Fe, and Ir. The crystal lattice energies were calculated by the atom-atom potential method. The lattice energies obtained by using the Buckingham potential are in better agreement with the sublimation heats of these metal complexes than those calculated from the Lennard-Jones potential. The experimental dependences of vapor pressure for the complexes are in satisfactory agreement with the values obtained from the calculated lattice energies and entropies of crystal-gas transitions.     

钛阳极氧化膜的光诱导钴离子注入

发现光诱导下钴离子可以注入钛阳极氧化膜, 利用光电流谱技术和X-射线光电子能谱技术研究这一过程。结果表明: 在浓度≥0.10 mol·cm~(-3)的Co·(NO_3)_2溶液中, 光照使界面上的钴离子部份受激而具有较高的能量, 这些钴离子能够克服膜/溶液界面势垒和TiO_2晶格激活能进入氧化膜的四面体或八面体空位, 并可能与氧配位形成呈蓝色的钴四配位化合物或粉红色的钴六配位化合物。     

Zn2TTHA等双核金属配合物的NMR研究

报道了Ｚｎ、Ｍｇ、Ｐｂ和Ｈｇ与ＴＴＨＡ形成的２：１双核配合物的动态ＮＭＲ研究，在所研究的温度范围内观测到Ｚｎ２ＴＴＨＡ分子内重排过程，并通过模拟谱求出分子内重排过程的速率常数，提出Ｚｎ２ＴＴＨＡ在溶液中的构型由２个对称的八面体单元组成，而不是由２个四面体构成的；用二位交换的Ｂｌｏｃｈ方程估计Ｍｇ、Ｐｂ和Ｈｇ双核配合物的活化能Ｅａ值；发现分子内重排活化能大小的顺序与中心金属离子的离子势呈线性关系。     

Lennard-Jones团簇最低能量构型的预测

针对Lennard-Jones(LJ)团簇的结构优化问题,在前人工作的基础上,提出了一个新的无偏优化算法,即DLS-TPIO(dynamic lattice searching method with two-phase local searchand interior operation)算法.对LJ2-650,LJ660,LJ665-680这666个实例进行了优化计算.为其中每个实例所找到的构型其势能均达到了剑桥团簇数据库中公布的最好记录.对LJ533与LJ536这两个算例,所达到的势能则优于先前的最好记录.在DLS-TPIO算法中,采用了内部操作,两阶段局部搜索方法以及动态格点搜索方法.在优化的前一阶段,内部操作将若干能量较高的表面原子移入团簇的内部,从而降低团簇的能量,并使其构型逐渐地变为有序.与此同时,两阶段局部搜索方法指导搜索进入更有希望的构型区域.这种做法显著地提高了算法的成功率.在优化的后一阶段,借用动态格点搜索方法对团簇表面原子的位置作进一步优化,以再一次降低团簇的能量.另外,为识别二十面体构型的中心原子,本文给出了一个简单的新方法.相比于文献中一些著名的无偏优化算法,DLS-TPIO算法具...     

Structure and magnetic ordering of the anomalous layered (2D) ferrimagnet [NEt4]2Mn(II)3(CN)8 and 3D bridged-layered antiferromagnet [NEt4]Mn(II)3(CN)7 Prussian blue analogues

The reaction of Mn(II) and [NEt(4)]CN leads to the isolation of solvated [NEt(4)]Mn(3)(CN)(7) (1) and [NEt(4)](2) Mn(3)(CN)(8) (2), which have hexagonal unit cells [1: R3m, a = 8.0738(1), c = 29.086(1)??; 2: P3m1, a = 7.9992(3), c = 14.014(1)??] rather than the face centered cubic lattice that is typical of Prussian blue structured materials. The formula units of both 1 and 2 are composed of one low- and two high-spin Mn(II) ions. Each low-spin, octahedral [Mn(II)(CN)(6)](4-) bonds to six high-spin tetrahedral Mn(II) ions through the N?atoms, and each of the tetrahedral Mn(II) ions are bound to three low-spin octahedral [Mn(II)(CN)(6)](4-) moieties. For 2, the fourth cyanide on the tetrahedral Mn(II) site is C?bound and is terminal. In contrast, it is orientationally disordered and bridges two tetrahedral Mn(II) centers for 1 forming an extended 3D network structure. The layers of octahedra are separated by 14.01?? (c?axis) for 2, and 9.70?? (c/3) for 1. The [NEt(4)](+) cations and solvent are disordered and reside between the layers. Both 1 and 2 possess antiferromagnetic superexchange coupling between each low-spin (S = 1/2) octahedral Mn(II) site and two high-spin (S = 5/2) tetrahedral Mn(II) sites within a layer. Analogue 2 orders as a ferrimagnet at 27(±1)?K with a coercive field and remanent magnetization of 1140?Oe and 22,800?emuOe?mol(-1), respectively, and the magnetization approaches saturation of 49,800?emuOe?mol(-1) at 90,000?Oe. In contrast, the bonding via bridging cyanides between the ferrimagnetic layers leads to antiferromagnetic coupling, and 3D structured 1 has a different magnetic behavior to 2. Thus, 1 is a Prussian blue analogue with an antiferromagnetic ground state [T(c) = 27?K from d(χT)/dT].     

Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions

Accurate models of alkali and halide ions in aqueous solution are necessary for computer simulations of a broad variety of systems. Previous efforts to develop ion force fields have generally focused on reproducing experimental measurements of aqueous solution properties such as hydration free energies and ion-water distribution functions. This dependency limits transferability of the resulting parameters because of the variety and known limitations of water models. We present a solvent-independent approach to calibrating ion parameters based exclusively on crystal lattice properties. Our procedure relies on minimization of lattice sums to calculate lattice energies and interionic distances instead of equilibrium ensemble simulations of dense fluids. The gain in computational efficiency enables simultaneous optimization of all parameters for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that enforce consistency with periodic table trends. We demonstrate the method by presenting lattice-derived parameters for the primitive model and the Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting parameters successfully reproduce the lattice properties used to derive them and are free from the influence of any water model. To assess the transferability of the Lennard-Jones parameters to aqueous systems, we used them to estimate hydration free energies and found that the results were in quantitative agreement with experimentally measured values. These lattice-derived parameters are applicable in simulations where coupling of ion parameters to a particular solvent model is undesirable. The simplicity and low computational demands of the calibration procedure make it suitable for parametrization of crystallizable ions in a variety of force fields.     

BiMo 基复合氧化物催化剂晶格氧性质与丙烷选择氧化性能

研究了不同组成、结构的BiMo基复合氧化物催化剂的丙烷选择氧化至丙烯醛的性能.X射线衍射(XRD)、X光电子能谱(XPS)、原位傅里叶变换激光拉曼光谱(FT-LRS)、电子顺磁共振(ESR)等多种表征结果表明,BiMo基复合氧化物催化剂上丙烷经由中间物丙烯选择氧化至丙烯醛,催化剂的晶格氧为选择性活性氧物种.丙烷直接氧化下丙烷至丙烯醛的选择性和收率与催化剂的Mo=O物种的氧化-还原性质密切关联,而Mo=O物种的性质又取决于Mo离子的配位环境,Mo=O物种的选择性转化丙烷经由丙烯至丙烯醛活性随畸变MoO6八面体、共顶点八面体、共边八面体、MoO4四面体配位环境递增.组成、结构优化调变的催化剂上丙烷选择氧化至丙烯醛选择性和收率可达45%和13.5%,催化剂中具有选择氧化活性的晶格氧物种数可达258 μmol/g.     

A modified TIP3P water potential for simulation with Ewald summation

The charges and Lennard-Jones parameters of the TIP3P water potential have been modified to improve its performance under the common condition for molecular dynamics simulations of using Ewald summation in lieu of relatively short nonbonded truncation schemes. These parameters were optimized under the condition that the hydrogen atoms do not have Lennard-Jones parameters, thus making the model independent of the combining rules used for the calculation of nonbonded, heteroatomic interaction energies, and limiting the number of Lennard-Jones calculations required. Under these conditions, this model provides accurate density (rho = 0.997 g/ml) and heat of vaporization (DeltaH(vap) = 10.53 kcal/mol) at 25 degrees C and 1 atm, but also provides improved structure in the second peak of the O-O radial distribution function and improved values for the dielectric constant (epsilon(0) = 89) and the diffusion coefficient (D = 4.0 x 10(-5) cm(2)/s) relative to the original parametrization. Like the original parameterization, however, this model does not show a temperature density maximum. Several similar models are considered with the additional constraint of trying to match the performance of the optimized potentials for liquid simulation atom force field to that obtained when using the simulation conditions under which it was originally designed, but no model was entirely satisfactory in reproducing the relative difference in free energies of hydration between the model compounds, phenol and benzene. Finally, a model that incorporates a long-range correction for truncated Lennard-Jones interactions is presented, which provides a very accurate dielectric constant (epsilon(0) = 76), however, the improvement in this estimate is on the same order as the uncertainty in the calculation.     

Cation distribution in co-doped ZnAl2O4 nanoparticles studied by X-ray photoelectron spectroscopy and 27Al solid-state NMR spectroscopy

Co(x)Zn(1-x)Al(2)O(4) (x = 0.01-0.6) nanoparticles were synthesized by the citrate sol-gel method and were characterized by X-ray powder diffraction and transmission electron microscopy to identify the crystalline phase and determine the particle size. X-ray photoelectron spectroscopy and (27)Al solid-state NMR spectroscopy were used to study the distribution of the cations in the tetrahedral and octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles as a function of particle size and composition. The results show that all of the as-synthesized samples exhibit spinel-type single phase; the crystallite size of the samples is about 20-50 nm and increases with increasing annealing temperature and decreases with Co-enrichment. Zn(2+) ions are located in large proportions in the tetrahedral sites and in small proportions in the octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles. The fraction of octahedral Zn(2+) increases with increasing Co concentration and decreases with increasing particle size. Besides the tetrahedral and octahedral coordinations, the presence of the second octahedrally coordinated Al(3+) ions is observed in the nanoparticles. The change of the inversion parameter (2 times the fraction of Al(3+) ions in tetrahedral sites) with Co concentration and particle size is consistent with that of the Zn fraction in octahedral sites. Analysis of the absorption properties indicates that Co(2+) ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The inversion degree of Co(2+) decreases with increasing particle size.     

Lattice sum calculations for 1/rp interactions via multipole expansions and Euler summation

A method is developed here for doing multiple calculations of lattice sums when the lattice structure is kept fixed, while the molecular orientations or the molecules within the unit cells are altered. The approach involves a two‐step process. In the first step, a multipole expansion is factored in such a way as to separate the geometry from the multipole moments. This factorization produces a formula for generating geometry constants that uniquely define the lattice structure. A direct calculation of these geometry constants, for all but the very smallest of crystals, is computationally impractical. In the second step, an Euler summation method is introduced that allows for efficient calculation of the geometry constants. This method has a worst case computational complexity of O(( log N)²/N), where N is the number of unit cells. If the lattice sum is rapidly converging, then the computational complexity can be significantly less than N. Once the geometry constants have been calculated, calculating a lattice sum for a given molecule becomes computationally very fast. Millions of different molecular orientations or molecules can quickly be evaluated for the given lattice structure. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 208–215, 2001     

Periodic DFT study of the structural and electronic properties of bulk CoAl2O4 spinel

In this study, structural and electronic properties of CoAl2O4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co2+ and Al3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al3+ at tetrahedral sites (and Co2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl2O4 can be interpreted as dominated by the local behavior of Co2+ at octahedral sites.     

Using structural data for estimating the stability of water networks in clathrate and semiclathrate hydrates

Methods are discussed for estimating the energy of formation of a clathrate network from ice Ih based on the structural data for clathrate and semiclathrate hydrates. The energies of real networks in clathrate hydrates and of ideal undistorted networks (8 structures) are calculated from the crystal structure data using the Zimmerman-Pimentel quasiharmonic potential. For semiclathrate hydrates, two versions of estimation are suggested-Estimations for 7 hydrates of amines and quaternary ammonium base salts showed that formation of semiclathrate hydrates requires much energy for network deformation. Two methods are described for estimating the network energies according to topological information. The first technique involves calculating the number of cycles with different numbers of sides per unit cell; this method is applicable to networks involving strained cycles with 4, 7, and 8 sides. The second method represents the lattice energy via the partial energies of polyhedron units forming the lattice; it is applicable to the most widespread class of networks constructed from Allen’s polyhedra. The energies of the tetragonal and orthorhombic networks belonging to this class are estimated. A relative stability series involving 9 structures of empty undistorted networks is formed. Translated fromZhurnal Struktumoi Khimii, Vol. 40, No. 2, pp. 287–295, March–April, 1999.     

Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides

Cobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox‐active building block Co?O. The charge of oxygen anion tends to shift toward the octahedral‐occupied Co instead of tetrahedral‐occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox‐active metal center in CoO₆ octahedra.     

A comparative study of nitrogen physisorption on different C70 crystal structures using an ab initio based potential

Quantum mechanical calculations are performed using the recently developed hybrid method for interaction energies to determine atom site Lennard-Jones potential parameters for the interactions of molecular nitrogen with C(70) molecules. This ab initio based potential is used in grand canonical Monte Carlo simulations to predict surface adsorption properties of N(2) on five known C(70) structures: rhombohedral, fcc, ideal hcp, deformed hcp, and monoclinic crystals. Because of the presence of five-membered carbon rings and the surface curvature of C(70) molecule, the Lennard-Jones potential parameters for nitrogen-carbon interactions obtained from ab initio based calculations are found to be different from that with planar graphite. The simulation results obtained from these two sets of force fields are compared and shown to differ, particularly at low coverage, where the nitrogen-carbon interactions are more important than the nitrogen-nitrogen interactions. The surface area, monolayer capacity, and isosteric heat of adsorption are calculated for various C(70) crystals and found to change appreciably as a result of the shear-induced phase transformation from hcp to rhombohedral lattice.     

K2SrNb6Cl18, dipotassium strontium hexaniobium octadecachloride

The structure of dipotassium strontium hexaniobium octadeca­chloride is based on [Nb₆Clⁱ₁₂Cl^a₆]^4? units (i and a denote `inner' and `outer' ligands, respectively), which have crystallographically imposed symmetry, linked together by K⁺ and Sr²⁺ cations. The K⁺ cation occupies a tetrahedral site in the face‐centered cubic lattice of cluster units, and is bonded to 12 Cl ligands. The Sr atom is located in an octahedral site and is bonded to six outer Cl ligands.