Atomic Bond Lengths Research Articles

Tight binding modelling of electronic band structure in conventional InGaN/GaN and dilute GaAsN/GaAs nitride heterostructures

AbstractWe utilize a semi‐empirical sp3s * tight binding model to investigate the strain and composition effects on electronic structure of conventional InGaN/GaN and dilute GaAsN/GaAs cubic nitride heterostructures. The model includes the second nearest neighbor (2NN) atomic interactions, spin‐orbit splitting and nonlinear composition variation of atomic energies and bond lengths of ternaries. Results show that band gaps of conventional InGaN increase with composition for 0 < x < 1. However, the Γ band gap of dilute GaAsN decreases with composition for x < 0.25 and then starts to increase for 0.26 < x < 1. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

L-(+)-Bornesitol

The structure of the title compound, C(7)H(14)O(6), one natural myo-inositol derivative has been determinaded. Average atom distances, bond lengths and dihedral angles are similar to myo-inositol.

Atomic Structures and Electrical Properties of ZnO Grain Boundaries

Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain‐boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well‐defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain‐boundary orientation relationship and grain‐boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage (I–V) characteristics across ZnO grain boundaries. High‐resolution transmission electron microscopy (HRTEM) observations and lattice‐statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of Pr‐doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three‐plus state and would not produce acceptor states in the boundary. First‐principles calculations revealed that Pr doping instead promotes the formations of acceptor‐like native defects, such as Zn vacancies. We believe that such acceptor‐like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co‐codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain‐boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co‐codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single‐grain‐boundary varistors can be designed by controlling grain‐boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single‐grain‐boundary varistors.

Electrical structures and optical properties of doped earth element (Y，La) in γ-Si3N4

The electronic structures and optical properties, the atomic population, bond lengths, band structures and density of states (DOS) of undoped and doped rare earth elements (Y, La) in γ-Si3N4 have been calculated by means of plane wave pseudo-potential method (PWP) with generalized gradient approximation (GGA). The gap after doping will decrease and enables the formation of new semiconductor，which affords to help finding new semiconductor. We researched optical properties after doping of rare earth elements (Y, La), and found that the static dielectric constant of doped γ-Si3N4 is much higher than that of the undoped material，which may serve as new dielectric and refractive material，and may have special applications in certain optical devices.

Effects of geometry of nano-structured materials on their thermal expansion: tellurium nanocylinders as model system

The temperature-dependence of the atomic bond-lengths and -angles of the unit cell of crystalline tellurium nanocylinders (a few m in length and some 100 nm in diameter) have been investigated by X-ray diffraction in the range from 370 K to 100 K. With decrease in temperature, significant deviations from the established temperature-dependence of the bulk tellurium lattice parameters are observed. The observations in the temperature expansion are attributed to the nano-size and symmetry of the tellurium nanocylinders. The study demonstrates that a qualitative change in the material's properties is achieved when approaching the nano-scale. This opens the road for the possibility of deliberate tailoring of materials properties by proper choice of size and symmetry for thermo-mechanical applications.

First-principles study: Effect of relaxation on the geometrical structure of La 0.5Co 4Sb 12

In this study, the internal parameters of La-filled skutterudite La 0.5Co 4Sb 12 were relaxed following the damped Newton dynamic schemes. The relaxation results show that the distances and the bond lengths of atoms around the filling atom slightly change. The NN (nearest neighbor) Sb atom shows outward relaxation. Contrarily, the NNN (next nearest neighbor) Co atom and the TNN (third nearest neighbor) Sb atom show inward relaxations. The relaxation of internal parameters resulting from La filling has significant effects on the bonding charge density resulting in the change of charge distribution and bond length as well as bond angle. Based on the internal parameters after relaxation, the electronic structures of La 0.5Co 4Sb 12 were calculated by utilizing the FLAPW (Full Potential Linear Augmented Plane Wave) based on the DFT (Density Functional Theory) of the first-principles in detail. As a comparison, the band structure of CoSb 3 was calculated. The effects of La filling on the electronic structure and electrical conduction of La 0.5Co 4Sb 12 were discussed.

Structure, dynamics and thermodynamics of a metal chiral surface: Cu(532)

The structure, vibrational dynamics and thermodynamics of a chiral surface, Cu(532), havebeen calculated using a local approach and the harmonic approximation, withinteratomic potentials based on the embedded atom method. Relaxation of theatomic positions to the optimum configuration results in a complex relaxationpattern with strong contractions in the bond length of atoms near the kink and thestep sites and an equivalently large expansion near the least under-coordinatedsurface atoms. The low coordination of the atoms on the surface substantiallyaffects the vibrational dynamics and thermodynamics of this system. The localvibrational density of states shows a deviation from the bulk behaviour that persistsdown to the tenth layer, resulting in a substantial contribution of the vibrationalentropy to the excess free energy amounting to about 90 meV per unit cell at 300 K.

Synthesis of the Phenoxonium Cation of an α-Tocopherol Model Compound Crystallized with Non-Nucleophilic [B(C6F5)4]- and (CB11H6Br6)- Anions

The phenoxonium cation of a vitamin E model compound has been crystallized using the non-nucleophilic carborane and tetrakis(pentafluorophenyl)borate counteranions. The crystal structures confirm the assignment of the unusually stable phenoxonium cation and indicate that there is a substantial shortening of the carbon-oxygen bond lengths of the para-carbon atoms in the phenolic ring and a substantial increase of the carbon-oxygen bond length at the quaternary carbon. The crystallographic data are in excellent agreement with structural predictions from molecular orbital calculations.

Atomic scale characterization of shear bands in an amorphous metal

The shear bands in a model amorphous CuZr metallic glass are examined using atomistic simulations. The localized shear zones are characterized at atomic scales by the nearest-neighbor atomic bond rotation angle, the nearest-neighbor atomic bond length, the atomic volume, and the potential energy. It is shown that the shear band is indeed a region with a liquidlike structure and different thermomechanical properties. Shear band formation could be a result from both local heating and free volume generation. The variation of shear band properties with temperature and strain rate is also briefly reported.

Influence of crystal structure and formation energies of impurities (Mg, Zn and Ca) in zinc blende GaN

First-principles calculations on neutral metal impurities (Mg, Zn and Ca) in zinc blende GaN were studied. Formation energies were calculated for substitution on the gallium site, the nitrogen site and incorporation in the octahedral interstitial site and the tetrahedral interstitial sites. The calculated results show that the major defects studied have a high formation energy in excess of 5 eV, and the gallium substitutional site is favorable for incorporation. Mg Ga, has particularly low formation energy 1.19 eV and can be expected to incorporate readily into GaN. The local crystal structural changes around the impurity in the lattice were studied after metal atoms occupying the gallium substitutional site. It shows that the lattice constant becomes bigger and the tetrahedral angle between impurities and its nearest N atom becomes smaller mainly due to the extended M—N bond length and big size of impurities atoms, which results in a local lattice distortion. The Zn-N (2.04 Å) bond strength is the smallest among the three impurities which raises the formation energy. Ca Ga is unfavorable due to a large size mismatch in spite of a large bond strength (2.25 Å). The calculated results identify the two key factors determining impurities incorporation in zinc blende GaN: the atomic size of impurities comparing to that of host atoms and the bond strength between the impurities and its neighbors. The results are in well agreement with other calculated and experimental results.

High-energy X-ray measurements of structural anisotropy and excess free volume in a homogenously deformed Zr-based metallic glass

We have used high-energy X-ray scattering to measure the structural anisotropy and excess free volume in a homogeneously deformed Zr-based metallic glass alloy. The scattering results show that bond length anisotropy is present in the samples following isothermal tensile creep deformation. The average atomic bond length in the direction parallel to the tensile loading axis is larger than that in the direction normal to the loading axis. The magnitude of the bond length anisotropy is found to be dependent on the gradient of macroscopic plastic strain along the gauge length. Furthermore, the scattering results show that the excess free volume also increases with increasing macroscopic plastic strain. Results from differential scanning calorimetry analysis of free volume variations along the gauge length of the creep samples are consistent with results from the X-ray scattering experiments.

Base Pair Stability of 8-Chloro- and 8-Iodo-2‘-deoxyguanosine Opposite 2‘-Deoxycytidine: Implications Regarding the Bioactivity of 8-Oxo-2‘-deoxyguanosine

8-Oxo-2'-deoxyguanosine (OdG) is an abundant and promutagenic damaged nucleotide that has been linked to aging and disease. To gain insight into the alternate base pairings of OdG, 8-chloro- and 8-iodo-2'-deoxyguanosine were incorporated into oligonucleotides and, along with 2'-deoxyguanosine and 8-bromo-2'-deoxyguanosine, were tested for their stability in base pairs opposite dC. We found a strong correlation between increased atomic radius and bond length at C8 and decreased base pair stability. These findings along with NMR studies on the base conformation of the corresponding nucleosides support the theory that the steric bulk of the 8-oxygen plays a role in OdG mutation and disease.

Structure–Property Correlations in Y–Ca–Mg–Sialon Glasses: Physical and Mechanical Properties

The physical and mechanical properties of 12 glasses from the Y–(Mg,Ca)–Si–Al–O–N and (Mg,Ca)–Si–Al–O–N systems were investigated. The effect of the substitution of magnesium for calcium through two series of glasses, one consisting of oxides glasses and the other of glasses containing 6 at.% of nitrogen (15 e/o N), was considered. The change of the glass transition temperature through the glass series provides evidence for a mixed‐alkaline‐earth effect between magnesium and calcium species. The indentation hardness (H), Young's modulus (E), and indentation fracture toughness (KC) were found to increase significantly with either the magnesium or the nitrogen content, and nitrogen also seems to enhance the effect of magnesium on the properties. The network structure was analyzed both by29Si and27Al Magic Angle Spinning Nuclear Magnetic Resonance and by neutron scattering experiment, which allows for the estimation of some atomic bond lengths in such complex glasses. Nitrogen was found to have a significant structural effect on the magnesium environment and on the glass polymerization degree, and hence on the glass properties.

Characterization of brookite and a new corundum-like titania phase synthesized under hydrothermal conditions

Brookite rich samples were synthesized under hydrothermal conditions by using TiCl 3 as precursor. They also contained a new titanium oxide phase that has the same crystalline structure as Ti 2O 3, and is stable after annealing in air. Samples were characterized with X-ray powder diffraction, transmission electron microscopy and thermogravimetry. Crystalline phases were refined by using the Rietveld method, from which phase concentrations and atomic bond lengths were obtained as a function of sample annealing temperature. Samples contained brookite, anatase, rutile and the new corundum-like phase: Brookite's concentration was larger than 50 wt%, while the one of the corundum-like phase reached 20(6) wt%. The local symmetry and the atomic bond lengths of these two phases depended on the crystallite size; for both, there is a correlation between the evolution of the atomic bond lengths with temperature and their transformation into another phase. The hydrothermal conditions stabilized brookite, anatase, and the corundum-like phase at high temperature: This last phase was stable in air, even at 900°C.

Sulfate Ions in Titania Polymorphs

Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method.

Thermal conductivity decomposition and analysis using molecular dynamics simulations: Part II. Complex silica structures

Using molecular dynamics simulations, the thermal conductivity of silica-based crystals is found to be a result of two independent thermal transport mechanisms associated with atomic structure. The first mechanism is temperature independent, produces a thermal conductivity on the order of 1 W/m K, and is related to short length scale behavior. It is governed by the silicon coordination, which is unique to a given structure. The second mechanism is temperature dependent and is related to long length scale behavior. At a temperature of 300 K, the associated thermal conductivity ranges from 9 W/m K for the c-direction of quartz to 0.4 W/m K for zeolite-A. This mechanism is controlled by the atomic bond lengths and angles. Complex unit cells, notably cage structures, can distort the SiO 4 tetrahedra, leading to a shortening of the phonon mean free path and a spatial localization of energy. The results suggest that an alternative to the available minimum thermal conductivity model for amorphous materials is needed for the crystalline state.

First-principles study of equilibrium properties and electronic structure of Ti3Si0.75Al0.25C2solid solution

Based on first-principles pseudopotential total energycalculations, the effects of Al substitution in Ti3SiC2,yielding chemical formula Ti3Si0.75Al0.25C2,were studied by examining the equilibrium properties and electronic structure.The lattice configurations, cohesive energy, atomic bonding properties andbulk modulus, as well as the electronic band structure, were investigated.Firstly, we obtained the equilibrium crystal parameters that includedlattice constants, internal atomic coordinates and bond lengths. Then,the degrees of Ti–Si, Ti–C and Ti–Al covalent bondings in Ti3Si0.75Al0.25C2were illustrated and compared with the corresponding values in Ti3SiC2 andTi3AlC2compounds. Furthermore, the magnitude and anisotropy ofelectrical conductivity were discussed by investigating the densityof states and band structure. The bulk modulus of Ti3Si0.75Al0.25C2yielded a value close to that of Ti3AlC2,which indicated the high bulk modulus had been preserved. Inaddition, we predicted better oxidation resistance of Ti3Si0.75Al0.25C2than that of Ti3SiC2,and this was recently verified by an isothermal oxidation experiment of Al-cemented Ti3SiC2-basedceramic.

Protein subsets with correlated atomic bond lengths

The protein folding problem is one of the great challenges of computational biology. To better understand the nature of known protein structures, an analysis of atomic bond lengths in structures in the Protein Data Bank was performed. A bimodal population distribution was found for many bond distances. Additionally, an association between amino acid atomic bond lengths was revealed. These results suggest new parameters for protein modeling and structure refinement programs.

Thermal stability and structural deformation of rutile SnO 2 nanoparticles

Nanoparticles of rutile SnO 2 were synthesized by precipitation at room temperature. Samples were characterized with X-ray diffraction, transmission electron microscopy, thermoanalysis and nitrogen physisorption by BET method. The rutile crystalline structure was refined by Rietveld method. Crystallites had spherical morphology with crystallite sizes growing with the annealing temperature. The spherical crystallites aggregate to form grains composed of a number of crystallites defining the specific surface area and porosity. The crystallites contained hydroxyls in their structure and on their surface generating considerable amount of tin vacancy sites in the structure. These hydroxyls modify the Sn–O bonds, increase the lattice parameters and produce asymmetry in the representative rutile tin–oxygen octahedron. As the dehydroxylation was done with the annealing temperature, the atomic bond length between the oxygen atoms shared by adjacent octahedra decreased, contracting the lattice and increasing the symmetry.

Nanocomposite Hard Coatings for Wear Protection

AbstractNanocomposite thin films successfully promote hardness, oxidation resistance, improved wear behavior, and other properties relevant for wear-reducing coatings. Such coatings are composed of nanocrystalline grains of transition-metal nitrides or carbides surrounded by an amorphous hard matrix. The properties of nanocomposite coatings, especially hardness, are directly linked to nanostructure. The codeposition of the amorphous and nanocrystalline phases of different compositions results in different morphologies, which in turn affect the coating's properties. A maximum hardness ranging from 30 GPa to reported values above 60 GPa has been observed for most nanocomposite coatings. To obtain enhanced hardness, the domain size of the nanocrystalline phase must be below 10 nm, while the thickness of the amorphous layer separating the nanocrystals must be maintained at only a few atomic bond lengths. The prime reason for the hardness enhancement is the absence of dislocation activity.