Coordination Number Of Atoms Research Articles

Effects of the substitution of P2O5 by B2O3 on the structure and dielectric properties in (90−x) P2O5–xB2O3–10Fe2O3 glasses

90%[xB2O3 (1−x) P2O5] 10%Fe2O3, glass systems where (x=0mol%, 5mol%, 10mol%, 15mol%, 20mol%) was prepared via a melt quenching technique. The structure of glass is investigated at room temperature by, Raman and EPR spectroscopy. Raman studies have been performed on these glasses to examine the distribution of different borate and phosphate structural groups.We have noted an increase from 3 to 4 in the coordination number of the boron atoms from 3 to 4, i.e., the conversion of the BO3 triangular structural units into BO4 tetrahedra.The samples have been investigated by means of electron paramagnetic resonance (EPR). The results obtained from the gef=4.28 EPR line are typical of the occurrence of iron (III) occupying substitutional sites.Moreover, the dielectric sizes such as ε′(ω), ε″(ω), imaginary parts of the electrical modulus, M*(ω) and the loss tanδ, their variation with frequency at room temperature show a decrease in relaxation intensity with an increase in the concentration of (B2O3). On the present work, we have found a weak extinction index with our new glass.

Effect of Bi3+ ions on physical, thermal, spectroscopic and optical properties of Nd3+ doped sodium diborate glasses

Neodymium doped sodium diborate glasses containing bismuth oxide were prepared by melt quenching method. An effort has been made to understand whether the stiffness of tightly bound diborate groups affect upon the addition of heavy metal oxide. The increase in density and decrease in molar volume with increase of Bi2O3 content indicates the opening of diborate glass structure to achieve better packing and bonding. The mixed bonding such as Bi–O–B may occur during glass formation leads to decrease in its glass transition temperature. The increase in oxygen packing density values also indicates the existence of tight packing of the oxide network. The optical properties are measured using UV–visible spectroscopy. The increase in refractive index is attributed to the increasing number of highly polarizing Bi3+ ions with higher atomic weight and coordination number. The decrease in the optical band gap energy with increase in Bi2O3 content is ascribed to shifting of absorption edge to a longer wavelength region. The IR spectra reveal that the glass network consists of tightly bound diborate and BiO6 octahedral units.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: X. Dynamic equilibrium effects in the 29Si NMR spectra of silacycloalkanes

Dynamic equilibria related to change of the coordination number of the silicon atom in bis[N-(dimethylamino) imidato-N′,O]silacycloalkanes in solution were studied by theoretical calculations and experimental measurement of the 29Si NMR chemical shifts. Silacyclopentane derivatives were found to exist in solution as mixtures of species with six- and five-coordinate silicon atoms, and silacyclohexane derivatives, as mixtures of five- and four-coordinate silicon compounds.

EXAFS studies of nanostructured Finemet-type alloys

The nanostructure of nanocrystalline Fe66Cr8Cu1Nb3Si13B9 alloys has been studied using extended X-ray absorption fine structure (EXAFS) and analyzed by the cumulant method. Application of the cumulant method enables us to determine the nearest-neighbor interatomic distance and the coordination number for chromium and copper atoms, and thus we are able to obtain detailed knowledge about the role of both Cr and Cu in the structure of nanocrystals at various stages of crystallization. A detailed analysis of the distribution of alloying elements in the grains and grain boundaries reveals the accumulation of Cr in the surrounding of Fe(Si) nanocrystals. The presence of Cr in the surroundings of Fe(Si) can influence the content's arrangement of the nanograins, allowing diffusion of Si atoms in the grain boundaries. Simulated X-ray absorption spectra of the model produced by FEFF9.05 and the proposed Cu clustering of 19 atoms with average cluster size of about 0.4 nm show a good resemblance to the experimental data of the Cu k-edge.

KAg11(VO4)4 as a candidate p-type transparent conducting oxide

For a material to be a good p-type transparent conducting oxide (TCO), it must simultaneously satisfy several design principles regarding its bulk and defect phase thermochemistry, its optical absorption spectrum, and its electric transport properties. Recently, we predicted Ag3VO4 to be p-type but with low conductivity and an optical band gap not large enough for transparency. To improve on the transport and optical properties of Ag3VO4, we searched an extended material space including quaternary compounds based on Ag, V, O, and an additional atom for a new candidate p-type TCO. From this set of quaternary materials, we selected KAg11(VO4)4, a known oxide with a crystal structure related to that of Ag3VO4. Notably, one could expect a possible enhancement of the concentration of hole producing Ag-vacancy defects in KAg11(VO4)4 due to its different local geometries of Ag atoms (2- and 3-fold coordinated) with respect to the 4-fold coordinated Ag atoms in Ag3VO4. By performing first-principles calculations, we found that KAg11(VO4)4 is an intrinsic p-type conductor and can be synthesized under conditions similar to those predicted for the synthesis of Ag3VO4. However, we predict that the intrinsic hole content in KAg11(VO4)4 is similar to that in Ag3VO4 even though KAg11(VO4)4 contains 2- and 3-fold coordinated Ag, hole producing sites with a lower defect formation energy than the 4-fold coordinated one. Our calculation demonstrates that the advantage from lower coordination number of the Ag atom in KAg11(VO4)4 can be offset by the change in the range of Ag chemical potential in which synthesis is allowed due to the oxide phases that Ag forms with K and that energetically compete with KAg11(VO4)4.

Inside Back Cover: On the Structures of 55-Atom Transition-Metal Clusters and Their Relationship to the Crystalline Bulk (Angew. Chem. Int. Ed. 23/2013)

Homonuclear 55-atom …︁ …︁ transition-metal clusters of the 3d and 4d elements show a correlation between the cluster structure type (atomic symbol, bottom right corner) and the bulk lattice morphology (top left corner). This link is found by gas-phase electron diffraction and DFT calculations. In their Communication on page 6102 ff., D. Schooss et al. also describe how the cluster structure types differ in maximum atomic coordination numbers in analogy to the coordination numbers in the bulk lattices.

On the Structures of 55‐Atom Transition‐Metal Clusters and Their Relationship to the Crystalline Bulk

Correlation of cluster and bulk structure: Electron-diffraction measurements of homonuclear 55-atom transition-metal cluster anions covering essentially all 3d and 4d elements show only four main structure families. Elements with the same bulk lattice morphology generally have a common cluster structure type. The cluster structure types differ in maximum atomic coordination numbers in analogy to the coordination numbers in the corresponding bulk lattices.

Dynamic asymmetry of self-diffusion in liquid ZnCl2 under pressure: An ab initio molecular-dynamics study

The static and dynamic properties of liquid ZnCl2 under pressure are investigated by ab initio molecular-dynamics simulations. The pressure range covers ambient to approximately 80 GPa. The ZnCl4 tetrahedra, which are rather stable at ambient pressure, are shown to deform and collapse with increasing pressure while maintaining an almost constant nearest-neighbor distance between Zn and Cl atoms. The average coordination number of Cl atoms around Zn atoms increases monotonically with pressure, from four at ambient pressure to seven at approximately 80 GPa. Although the self-diffusion coefficients of Zn and Cl atoms, d(Zn) and d(Cl), are almost the same at ambient pressure, the difference between them increases with pressure. At around 10 GPa, d(Zn) is about two times larger than d(Cl). Under further compression, this dynamic asymmetry becomes smaller. The microscopic mechanism of the appearance of the dynamic asymmetry is discussed in relation to the pressure dependence of the local structure.

In situ XAFS Investigation on Zincblende ZnS up to 31.7 GPa

In situ high-pressure x-ray absorption fine structure (EXAFS) measurements on the Zn K-edge in zincblende ZnS are performed up to 31.7 GPa at room temperature using a synchrotron radiation x-ray re-focused by a polycapillary half-lens in the classic energy-scanning transmission mode. The present XAFS data illustrate that ZnS undergoes a phase transition from zincblende F4̄3m to rocksalt Fm3̄m at 16.9 GPa, accompanied by the increase of the first shell coordination number of the absorption zinc atoms and the unit-cell volume collapse of ∼14%. The isothermal equation of state of the F4̄3m ZnS is well presented by the third-order Birch—Murnaghan equation of state with V0 = 158.6 ± 0.8 Å3, B0 = 87 ± 5 GPa and B′0 = 4 (fixed). The high-pressure behavior of ZnS from the XAFS spectra is consistent with the previous high-pressure XRD results. In addition, the present experimental method demonstrates that the large divergent x-ray micro-beam induced by a polycapillary half-lens can suppress effectively the glitches from single crystal diamond anvil and improve significantly the quality of the XAFS data, which will shed light on the high-pressure XAFS applications.

Structure–activity relationships for propane oxidative dehydrogenation by anatase-supported vanadium oxide monomers and dimers

To understand the importance of the effect of molecular structure on reactivity, we have studied the activity of anatase TiO2 (001) supported VOx catalytic sites for propane oxidative dehydrogenation (ODH). First, possible structures of monomeric and dimeric VOx species on anatase (001) after VO4H3 grafting and water elimination were determined. We then studied the conversion reaction of propane to propanol by the supported VOx to elucidate the structure–reactivity relationship. The coordination number of the vanadium atom was the key structural parameter in predicting the catalytic activity. This key structural difference alone resulted in an increase of up to 800 times in the reaction rate of CH bond activation (rate-determining for propane ODH) for the various vanadium oxide species at 600K. These results demonstrate the remarkable sensitivity of the catalytic site activity to its geometric structure and its implications for achieving optimal catalyst performance.

Geometric Transition and Electronic Properties of Titanium-Doped Aluminum Clusters: AlnTi (n = 2–24)

Equilibrium geometries of AlnTi (n = 2-24) clusters were studied using density-functional theory with generalized gradient approximation. The resulting geometries showed that the titanium atom remains on the surface of clusters for n < 20 but is endohedrally doped from n = 20. This structural transition confirms the previous experiment results obtained by studying their abilities for argon physisorption (Lang, S. M.; Claes, P.; Neukermans, S.; Janssens, E. J. Am. Soc. Mass Spectrom.2011, 22, 1508). The average bond lengths, coordination numbers, relative stabilities, electronic properties, and other relevant properties were discussed. It was found that the doped titanium atoms strengthen the stabilities of the pure aluminum clusters. The coordination numbers of titanium atoms along with the average Al-Ti bond lengths undergo dramatic increases during the structural transition. The intra-atomic hybridization exists in both Ti and Al atoms, and charge transfer from Al atoms to Ti atom were found in these complexes, which should reflect the strength of Al-Ti interactions. Electronic structure analysis based on the partial density of states reveals stronger Al-Ti interactions for the endohedrally doped structures.

Topology of 2-Periodic Coordination Networks: Toward Expert Systems in Crystal Design

We have performed comprehensive topological analysis of 2-periodic coordination networks in 10 371 metal–organic compounds. Both local and overall topologies of complex groups were determined, classified, and stored in the electronic databases. Two plane nets, square lattice (sql) and honeycomb (hcb), were found to compose two-thirds of all the coordination networks. Strong correlations were found between local topological characteristics (coordination numbers of atoms or complex groups, coordination figures, formalized coordination modes of ligands and coordination formula) and the overall topology that in many cases allowed us to predict possible topological motifs from the data on chemical composition with high probability. The possibility to develop an expert system that could envisage local and overall topology of periodic coordination networks is discussed, and an example is given of how such a system can work.

Implications of the Crystal Structure of the Ammonia Solvate [Au(NH3)2]Cl·4NH3

Crystals of diammine gold(I) chloride ammonia (1/4), [Au(NH(3))(2)]Cl·4NH(3), have been grown from solutions of AuCl in liquid ammonia. The X-ray diffraction analysis (at 123 K) has shown that the crystals feature an extensive network of hydrogen bonds between the [H(3)N-Au-NH(3)](+) cations (with C(i) symmetry) and the Cl(-) anions, including also the ammonia molecules. There is no evidence for an emerging increase of the coordination number of the gold atom by adopting another ammonia molecule or by approaching a chloride anion. Moreover, the geometry of two distant and angular N-H···Au contacts is not a strong support of hydrogen bonds recently amply discussed in the literature.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VII. Intramolecular coordination effect in the 29Si NMR spectra of siletanes

The effects of intramolecular N→Si coordination and electronic and conformational factors on the chemical shift of 29Si nucleus in silacyclobutane (siletane) derivatives were studied by quantum-chemical methods. Intramolecular coordination induces upfield shift of the 29Si resonance on the average by 50 ppm when the coordination number of the silicon atom increases by unity. The state of conformational equilibrium of siletane derivatives critically affects the accuracy of δSi predictions.

Diallylaminoarenes: a new class of versatile ligands for silver(i) that consistently lead to one-dimensional polymeric metallosupramolecular assemblies

Ligands comprised of a diallylamino group attached to a phenyl, 2-pyridyl, 2-pyrimidyl, 2-thiazolyl and 2-quinolyl ring react with silver(I) salts (perchlorate and/or triflate) to faithfully assemble into 1-D coordination polymers, regardless of variation in the number of additional nitrogen donors, the counterion and the coordination number (three to five) of the silver atoms.

Grain boundaries at the surface of consolidated MgO nanocrystals and acid–base functionality

The increase of the surface basicity-acidity of MgO material by factors of 1.8-3.0 due to consolidation of its nanocrystals was demonstrated by the indicator titration. It was shown that the parallel increase of surface acidity and basicity is attributed to the formation of grain boundaries (GB) after MgO aerogel densification. A simple model predicting the increase of surface acidity-basicity of MgO that correlates with the results of direct measurements was proposed. The model is based on the study of the fine atomic structure at GB surface areas in consolidated MgO nanocrystals in the framework of Density Functional Theory. It is found that the displacements of coordinatively unsaturated surface ions near the GB are significant at the distances ~3-4 atomic layers from the geometrical contact plane between nanocrystals. The detailed analysis of atomic positions inside GB demonstrated the coordination deficiency of surface atoms at the GB areas leading to the formation of stretched bonds and to creation of low coordinated surface ions due to splitting of coordination numbers of surface atoms belonging to GB areas. Density of states for electrons shows the existence of additional states in the band gap close to the bottom of the conduction band. The adsorption energy of CO2 molecules atop oxygen atoms exposed at surface GB areas is of the same order of magnitude as that reported for oxygen atoms at crystallographic edges and corners of MgO crystals. It provides additional options for bonding of molecules at the surface of nanocrystalline MgO increasing the adsorption capacity and catalytic activity.

Density-functional global optimization of (La2O3)n clusters

Structures of stoichiometric (La(2)O(3))(n) (n = 1-6) clusters have been systematically studied by theoretical calculations. Global minimum structures for these clusters are determined by genetic algorithm based global optimizations at density functional level. The ground state structure for La(6)O(9) was found to be highly symmetric with point group O(h) and the centered oxygen atom has the coordination number as large as six, which is the same as the highest coordination number of oxygen atoms in bulk La(2)O(3). Analysis of the binding energies shows that La(6)O(9) has a high stability among the studied clusters. The energies of the highest occupied∕lowest unoccupied molecular orbitals, vertical ionization energy, and vertical electron affinity of each cluster are provided. Electronic structure of La(6)O(9) is discussed by analysis of the frontier molecular orbitals and unpaired spin density distributions of charged clusters.

Finite-Size Effects in O and CO Adsorption for the Late Transition Metals

Gold is known to become significantly more catalytically active as its particle size is reduced, and other catalysts are also known to exhibit finite-size effects. To understand the trends related to finite-size effects, we have used density functional theory to study adsorption of representative adsorbates, CO and O, on the late transition metals Co, Ni, Cu, Ir, Pd, Ag, Rh, Pt and Au. We studied adsorption energies and geometries on 13-atom clusters and compared them to the fcc(111) and fcc(211) crystal facets. In all cases, adsorbates were found to bind significantly more strongly to the 13-atom clusters than to the extended surfaces. The binding strength of both adsorbates were found to correlate very strongly with the average coordination number of the metal atoms to which the adsorbate binds, indicating that the finite-size effects in bonding are not specific to gold.

Strain in Atomistic Models of Nanocrystalline Clusters

Strain, as an easy and clearly defined concept in continuum mechanics, has no direct counterpart in atomistic models. Existing methods, relying on the concept of atomic coordination number, do not provide a complete description of volumetric and deviatoric strains across metallic nanocrystalline microstructures. To overcome those limitations a new method is proposed: the Voronoi Cell deformation (VCD) fully accounts for the local geometry and provides a description of the strain field independent of the atomic coordination. As a typical case of study, a large atomic cluster of 200 Al grains (ca 2 million atoms) and overall size of 33 cubic nanometres was considered.

Emil Zolotoyabko, Basic Concepts of Crystallography

Emil Zolotoyabko's book is an excellent textbook for courses on both geometric/structural and physical crystallography. There are approximately 300 pages with nearly 200 illustrative color figures and an index. The recommended list for general reading comprises references to 17 relevant and well known books as well as to the Powder Diffraction File of the International Center for Diffraction Data. The title of the book and its subtitle are well chosen to represent the whole content of the book. Somewhat unusual for a crystallography text book at the chosen level, the introductory chapter puts geometric/structural and physical crystallography in the wider context of symmetries in nature. The heuristic value and generality of symmetry arguments are illustrated by mentioning that the conservation laws of linear momentum, energy, and angular momentum all follow directly from the homogeneity of space and time. The degeneracy of quantum mechanical states, their lifting by the breaking of symmetries of a confining potential, and selection rules also illustrate the universality of symmetry considerations. This referee feels that this kind of an introductory treatment and the immediately following assertion that “our task within this book is much more modest: to describe the symmetry of crystals and to show how and to what extent symmetry considerations can be used for determining the physical properties of crystals” are bound to impress any reader so that there will probably be many serious students of Emil Zolotoyabko's text book. The next six chapters cover (in 50 pages) crystallographic basics such as direct and reciprocal space as concepts, crystallographic calculations, and the stereographic projection. Reciprocal space is also explained from the physical applications point of view. A nice didactic device is the declaration of four of these chapters as providing the “Language of crystallography” (by making this phrase part of the chapter headings). This will hopefully entice readers to review first all of the “language needed” for the comprehension of the latter parts of the book. The following eleven chapters deal briefly with atomic packings and coordination numbers, i.e. very basic crystal chemistry concepts, as well as comprehensively with the symmetries of crystals (in 115 pages) all the way up their representations in the International Tables. After the initial dealing with individual point symmetries and the introduction of point symmetry groups and Bravais lattices, space groups are derived. Point group mm2 and the Bravais lattices of the orthorhombic crystal system are chosen as examples for the derivation of all of the<?brk?> 22 space groups that their combination entails. This is a very good exercise for the serious student. Seven chapters are left to cover the basics of crystal physics. These chapters utilize the tensor formalism extensively and provide examples of physical property descriptions up to the forth tensor rank. The examples are ferroelectricity, dielectric permittivity, birefringe effects, piezoelectricity, stress/strain and the elastic moduli. Neumann's and Curie's symmetry principles are utilized to “build bridges” between the crystal physics part of the book and the preceding discussions on crystal symmetries. The differences between the propagation of acoustic and electromagnetic waves in crystals are also highlighted. The final chapter is on twinning from both the symmetry and physical properties points of view. All in all, a very nice textbook indeed.