Cohesive Energy Calculations Research Articles

Structural and electronic properties of GaP nanowires

Structural stability and electronic properties of bare and hydrogenated GaP nanowires in zinc-blende and wurtzite phases have been investigated using first-principles calculations based on density functional theory. It is determined that relaxation of the hydrogenated GaP nanowires is very small compared to that of their bare ones. The wurtzite structural hydrogenated GaP nanowires are found more stable than the zinc-blende structural ones by cohesive energy calculations. It is obtained that all the bare and hydrogenated GaP nanowires in zinc-blende and wurtzite phases considered in this work have semiconductor characteristics. The confinement effect, by which the energy gaps are increased with the decrease of the diameter, is demonstrated for the hydrogenated GaP nanowires in both zinc-blende and wurtzite phases. In contrast to the hydrogenated wurtzite GaP nanowires, the hydrogenated zinc-blende ones are found direct band gap materials for diameters in the range of ∼9<df<∼21A˚.

The relationships between rheological rules and cohesive energy of amphiphilic polymers with different hydrophobic groups

An amphiphilic polymer in water could form an aggregate due to its hydrophobic groups. The rheological properties of the three amphiphilic polymers’ solutions are different because of their different hydrophobic groups. By using a rheometer and a rotatory viscometer, the rheological properties and apparent viscosities of three kinds of amphiphilic polymers have been studied. The results show that all of the polymers are Newtonian fluids at a low shearing rate and pseudoplastic fluids at a high shearing rate. In addition, the amphiphilic polymer-P(AM/BHAM/NaA) has the best thickening behavior and the lowest value of critical aggregation concentration (CAC), which was also verified by fluorescence spectroscopy. By combining creep-recovery with cohesive energy calculation, the structural performance and interaction energy of the aggregate is the reason for the above mentioned phenomenon. By increasing the strength of hydrophobic groups, i.e., with an oil–water partition coefficient, then the structural viscoelasticity of the groups could be improved, and the associate is easily formed in the solution.

Ab initio calculations of ideal and defective bismuth telluride nanotubes

Stability and electronic structures are analyzed for ideal bilayer and trilayer Bi–Te nanotubes and for trilayer nanotubes with exchanged‐pair, antisite, and vacancy defects with a first‐principle method. Cohesion energy calculations reveal the high stability of those nanotubes, the most stable being the trilayer nanotube with semicube patterns. Crystal of nanotubes is more stable than individual nanotubes. From the electronic structure analysis, nanotubes with single Te antisite, Bi antisite, and Te vacancy defects are found to be metallic. Instead, the Bi vacancy defected nanotube is characterized by a semiconductor behavior with promising thermoelectric properties for nanoscale devices.

Thermal stability and structural evolution of quaternary Ti–Ta–B–N coatings

Nanostructured hard coatings are becoming fundamental in many application areas due to their highly enhanced mechanical properties compared to conventional materials. Among others, novel quaternary systems are attracting increasing attention since they are expected to further improve endurance of such coatings. In this work, combined experimental and ab initio analysis of thermal stability and structural evolution of quaternary Ti–Ta–B‐N coatings with Ta content 0–40at.% prepared by reactive magnetron co‐sputtering is presented. All prepared Ti1–x–yTaxByN coatings in the as-deposited state exhibit amorphous structure, which is supported by ab initio calculations. Structural development of Ti–Ta–B–N coatings induced by vacuum annealing leads to the formation of a nanocomposite system consisting of fcc-TiN, possibly fcc-TaN phase and disordered a-BTiy(Taz)Nx phase. At further increase of annealing temperature above 1100°C, other nitride, boride and diboride crystalline phases begin to form. This process is accompanied by release of nitrogen from all samples, phase transformations and grain coarsening. Calculations of formation and cohesive energies for various phases of possible Ti–Ta–B–N decomposition products as well as molecular dynamics simulations of the Ti1–x–yTaxByN system at high temperatures were performed in order to give deeper insight into the structural evolution and thermal stability of the investigated alloy. Increased value of hardness at higher Ta content and low elastic modulus as well as high elastic recovery of the Ti–Ta–B–N coatings are also reported in this work and show a promising way to improve toughness of nitride coatings.

Solvent-free Sc3N@C80-Ih and Its Precursor Cocrystal with Toluene

Solvent-free Sc3N@C80-Ih was prepared for the first time in high quality microcrystalline form via thermal decomposition of its toluene cocrystal. The production route and structural studies of both materials are presented. The structures were determined by x-ray powder diffraction, and confirmed by cohesion energy calculations. The solvent-free endohedral fullerene forms face centered cubic crystals with lattice parameter a = 15.611 Å and the molecules are orientationally disordered. This is a new member of the high-symmetry isostructural expansion series of fullerenes. The parent cocrystals precipitated from toluene possess 1:1 stoichiometry; their structure is orthorhombic of Cmcm space-group. At room temperature the toluene molecules are located in large cavities in a severely disordered way. A symmetry lowering phase transition is observed on cooling indicating the dynamic nature of the disorder. The decomposition, assisted by toluene diffusion at elevated temperatures, is also analyzed in detail.

Evaluation from First Principles the Structural Stability of Mg Containing Different Amounts of Al Atoms under High Pressure

The structural stability and phase transition of magnesium (Mg) containing different amounts of Al under high pressure was studied by means of first-principles total energy calculations. The cohesive energy calculations showed that the hcp and bcc structures of Mg-4.17 at%Al and Mg-8.33 at%Al were of the strong structural stability. The enthalpy for hcp and bcc structures of Mg was dependent upon the Al content. With increasing Al content from 0 to 8.33 at%, the enthalpy for hcp and bcc structures increased monotonously. Based on the enthalpy differences of the hcp and bcc structures under different pressures, the phase transition pressure under which the hcpbcc structural phase transition may take place for pure Mg, Mg-4.17 at%Al and Mg-8.33 at%Al was 60 GPa, 70 GPa and 85 GPa, respectively, indicating that with the increasing Al content, the phase transition pressure became higher and the hcpbcc transition was more difficult.

Molecular thermodynamic characterization of LCST fluid phase behavior and exploring electrostatic algorithms to compute polymer/solvent solubility parameters in the canonical ensemble

The present study aims to characterize the temperature dependence of polyethylene (PE) solubility in hexane at high pressure using an atomistic-level simulation technique, without the need for expensive and time-consuming experimental methods, to gain significant miscibility insights in polymer processes that are present in a wide range of industrial applications. To this end, various molecular dynamics (MD) simulations based upon the OPLS-AA force field are carried out over a range of 425–500 K at 50 and 200 bar to predict the temperature dependence of Hildebrand's solubility parameter of high-density polyethylene (HDPE) and hexane at these high pressures. The NVT simulation results compare quite favorably with the available experimental and theoretical data. The effect of electrostatic potential energy contribution to cohesive energy is also investigated, and it is shown that the solubility parameter decreases with increasing temperature for both molecular mechanical models with and without electrostatic terms. Subsequently, the Flory–Huggins (FH) binary interaction parameter is predicted from the knowledge of temperature dependence of solubility parameters. It is demonstrated that the computed binary interaction parameter increases upon increasing the temperature, indicating that the miscibility of the PE/hexane system decreases by raising the temperature. This conclusion is in agreement with the widely recognized lower critical solution temperature (LCST) fluid phase behavior. Moreover, it is shown that the increase in system temperature decreases the chemical potential factor of the phase stability condition, indicating that at higher temperatures this effect tends to destabilize the polymer–solvent system. Comparisons of electrostatic forces evaluated based on shift functions and the Particle-Mesh Ewald (PME) methods show slight difference, and it is also found that the grid spacing has no noticeable influence on non-bonded energy terms and total potential, employed in the calculation of cohesive energy, a result that is useful to increase the efficiency of future MD simulations.

Elastic Properties, Mechanical Stability, and State Densities of Aluminnides

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds. The results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu2Zr has the strongest alloying ability, followed by AlZr3 and nally the AlCu3. Further analysis nds out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu2Zr can exhibit a good ductility, followed by AlCu3, whereas AlZr3 can have a poor ductility; however, for sti ness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu3 is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicates that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu3, Cu 3d and Zr 4d states for AlCu2Zr, and Al 3s, Zr 5s and 4d states for AlZr3, respectively; in particular, the electronic structure of the AlZr3 shows the strongest hybridization, leading to the worst ductility.

Accurate Computation of Cohesive Energies for Small to Medium-Sized Gold Clusters.

High-level CCSD(T)-F12-type procedures have been used to assess the performance of a variety of computationally less demanding methods for the calculation of cohesive energies for small to medium-sized gold clusters. For geometry optimization for small gold clusters, the PBE-PBE/cc-pVDZ-PP procedure gives structures that are in close agreement with the benchmark geometries. We have devised a CCSD(T)-F12b-based composite protocol for the accurate calculation of cohesive energies for medium-sized gold clusters. Using these benchmark (nonspin-orbit vibrationless) cohesive energies, we find that fairly good agreement is achieved by the PBE-PBE-D3/cc-pVTZ-PP method. In conjunction with PBE-PBE/cc-pVDZ-PP zero-point vibrational energies and spin-obit corrections obtained with the PBE-PBE-2c/dhf-TZVP-2c method, we have calculated 0 K cohesive energies for Au2-Au20. Extrapolation of these cohesive energies to bulk yields an estimated value of 383.2 kJ mol(-1), which compares reasonably well with the experimental value of 368 kJ mol(-1).

Chemical evolution via beta decay: a case study in strontium-90

Using 90Sr as a representative isotope, we present a framework for understanding beta decay within the solid state. We quantify three key physical and chemical principles, namely momentum-induced recoil during the decay event, defect creation due to physical displacement, and chemical evolution over time. A fourth effect, that of electronic excitation, is also discussed, but this is difficult to quantify and is strongly material dependent. The analysis is presented for the specific cases of SrTiO3 and SrH2. By comparing the recoil energy with available threshold displacement data we show that in many beta-decay situations defects such as Frenkel pairs will not be created during decay as the energy transfer is too low. This observation leads to the concept of chemical evolution over time, which we quantify using density functional theory. Using a combination of Bader analysis, phonon calculations and cohesive energy calculations, we show that beta decay leads to counter-intuitive behavior that has implications for nuclear waste storage and novel materials design.

Structural Patterns Arising during Synthetic Growth of Fullerene-Like Sulfocarbide

Carbon-based fullerene-like (FL) solid compounds are a new class of materials with extraordinary mechanical properties, which can be tuned by the dopant choice and its concentration. In this work, FL sulfocarbide (CSx) was studied by DFT simulations during synthetic growth with CmSn (m, n ≤ 2). The energetic and structural effects of S atoms at C sites in a graphene-like network were addressed by geometry optimizations and cohesive energy calculations. Results showed that for S concentrations lower than 10 at. %, smoothly bent pure hexagonal networks predominate. For higher S concentrations, the higher defect concentration leads to stronger deformation of the graphene-like sheets. It was determined that FL-CSx is well-structured (not amorphous) for S contents between 10 and 20 at. %. In contrast to other FL materials, bond rotation mechanisms are not expected to play any significant role during FL-CSx formation, and cross-linking sites are less frequent and may be assimilated in the planar structure durin...

Kinetic pathways for phase separation: An atomic-scale study in Ni–Al–Cr alloys

The kinetic pathways involved in the formation of γ′(L12 structure)-precipitates during aging of concentrated Ni–Al–Cr alloys at 873K, for three distinct alloy compositions, are studied experimentally by atom probe tomography, and computationally with lattice kinetic Monte Carlo (LKMC) simulations using parameters deduced from first-principles calculations of cohesive energies, and from experimental diffusion data. It is found that the compositional evolution of the γ′-precipitate phase does not follow the predictions of a classical mean-field model for coarsening of precipitates in ternary alloys. LKMC simulations reveal that long-range vacancy–solute binding plays a key role during the early stages of γ′-precipitation. With the aid of Monte Carlo techniques using the parameters employed in the LKMC simulations, we compute the diffusion matrix in the terminal solid-solutions and demonstrate that key features of the observed kinetic pathways are the result of kinetic couplings among the diffusional fluxes. The latter are controlled by the long-range vacancy–solute binding energies. It is concluded that, because it neglects flux couplings, the classical mean-field approach to phase separation for a ternary alloy, despite its many qualitatively correct predictions, fails to describe quantitatively the true kinetic pathways that lead to phase separation in concentrated metallic alloys.

First Principles Calculations of the Electronic Structure of ZrN Allotropes

The atomic structures and electronic properties of different ZrN allotropes, including face-centered cubic ZrN (B1 ZrN), hypothetic wurtzite (w) ZrN, and hypothetic two-dimensional (2D) and three-dimensional (3D) layered hexagonal (h) ZrN, are investigated by systematic first-principles calculations. Although the cohesive energy calculation indicates that the B1 ZrN is more stable than the hypothetic w-ZrN and h-ZrN, we suggest that the monolayer h-ZrN may be stable on some substrates. Charge population analysis shows that the polar, covalent bonding character appears between N atoms and Zr atoms for all ZrN allotropes involved in this paper. A Van Hove singularity (VHS) with a high density of states (DOS) locating at 0.2 eV above the Fermi level appears for monolayer h-ZrN, which results from a saddle point of the partially occupied Zr-d(z2) energy bands due to lack of interlayer interaction. Such a VHS observed in the monolayer h-ZrN indicates that this hypothetic monolayer material might be a potential candidate for new superconducting material by electron doping.

Stress reduction in cubic boron nitride by oxygen addition: Explanation of the mechanism by ab-initio simulations

We report ab-initio calculations of the effect of oxygen addition on the characteristics of c-BN. We focus on the lattice constants, bulk modulus, cohesive energy, stress and on the mechanism of stress reduction by oxygen addition. Calculations of cohesive energies are cross-checked by molecular-dynamics simulations. We identify preferred interstitial positions of extra B, N and O atoms trapped in the c-BN(O) lattice. We show that for a given composition the c-BN(O) configurations with interstitial O atoms are energetically preferred over configurations with interstitial B and N atoms, and that the former configurations exhibit lower compressive stress. The results provide a mechanism of the experimentally observed reduction of the stress introduced into c-BN(O) by interstitial B and (mainly) N atoms, based on exchanging places of these atoms and lattice O atoms.

CFx thin solid films deposited by high power impulse magnetron sputtering: Synthesis and characterization

Fluorine containing amorphous carbon films (CFx, 0.16≤x≤0.35) have been synthesized by reactive high power impulse magnetron sputtering (HiPIMS) in an Ar/CF4 atmosphere. The fluorine content of the films was controlled by varying the CF4 partial pressure from 0mPa to 110mPa at a constant deposition pressure of 400mPa and a substrate temperature of 110°C. The films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by applying elastic recoil detection analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and nanoindentation. First-principles calculations were carried out to predict and explain F-containing carbon thin film synthesis and properties. By geometry optimizations and cohesive energy calculations the relative stability of precursor species including C2, F2 and radicals, resulting from dissociation of CF4, were established. Furthermore, structural defects, arising from the incorporation of F atoms in a graphene-like network, were evaluated. All as-deposited CFx films are amorphous. Results from X-ray photoelectron spectroscopy and Raman spectroscopy indicate a graphitic nature of CFx films with x≤0.23 and a polymeric structure for films with x≥0.26. Nanoindentation reveals hardnesses between ~1GPa and ~16GPa and an elastic recovery of up to 98%.

Low Temperature Structure and Supramolecular Interactions of the C60−Cubane Cocrystal

The rotor−stator phase of the fullerene−cubane cocrystal, C60·C8H8, has an orientational ordering phase transition at around 140 K. On the basis of a simulated annealing analysis of the powder X-ray diffraction data, we determined the previously unknown crystal structure of the orthorhombic oriented phase: the space group is Pnma, the positions and the orientations of the constituent molecules significantly changed relative to the rock salt structure of the ambient temperature phase. The experimental structure is confirmed by cohesive energy calculations based on a pair potential method. Related structures, allowed by symmetry considerations, can be excluded by both the experiments and the calculations. The stability of the oriented phase is predominantly controlled by the nearest neighbor fullerene−fullerene and fullerene−cubane interactions.

First-principles study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallics

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds (AlCu 3, AlCu 2Zr, and AlZr 3). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu 2Zr has the strongest alloying ability, followed by AlZr 3 and finally the AlCu 3. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu 2Zr can exhibit a good ductility, followed by AlCu 3, whereas AlZr 3 can have a poor ductility; however, for stiffness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu 3 is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicate that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu 3, Cu 3d, and Zr 4d states for AlCu 2Zr, and Al 3s, Zr 5s and 4d states for AlZr 3, respectively; in particular, the electronic structure of the AlZr 3 shows the strongest hybridization, leading to the worst ductility.

First principles study on substitution behavior and alloying effects of Nb in Ni3Al

By using the first-principles plane-wave pseudopotential method based on the density functional theory, substitution behavior and alloying effect of Nb in Ni3Al are studied. The analyses of the formation enthalpies,the cohesive energies and the densities of states of systems with different substitution behaviors show that Nb prefers to occupy the Al site. The site preference is determined primarily by the electronic structure, and the calculated result is in good agreement with the experimental result. In order to further study the site preference, the occupied sites are divided into being of relaxed distribution and unrelaxed distribution, which are two different distributions. The calculations of total energy, the formation enthalpy, the cohesive energy and the density of states of system indicate that Nb has a preference for unrelaxed distribution. In order to study the alloying effect of Nb in Ni3Al, the calculations of cohesive energy, density of states of system and electronic charge density indicate that Nb in Ni3Al could increase the stability of the alloy. The study also indicates that the limited solubility of Nb in Ni3Al lie between6.25at % and 12.5at %.

First-principles study on the structural stabilities, electronic and elastic properties for zirconium under pressure

Using the first-principles calculation based on density-functional theory (DFT), we investigate the pressure-induced phase transitions, electronic and elastic properties of Zr at 0 K. The metal is shown to exhibit a crystal structure sequence of hcp → ω → bcc with increasing pressure. And the transitions happen at 0.14 GPa and 27.01 GPa, respectively. This is in good agreement with the experimental observation. The density of state (DOS) reveals the basic reason for the stability sequence of Zr. The shear moduli c′, c 44 and bulk modulus B of bcc Zr all increase with pressure. It is found that bcc Zr satisfies the mechanical stability at pressure beyond 9 GPa. Furthermore, the high-pressure limit of 360 GPa for a stable bcc Zr is deduced for the first time from the cohesive energy calculations. The Mulliken population analysis shows that both s and p electrons transfer to the d orbital with increasing pressure, however, the number of s electrons starts to increase when the pressure exceeds about 100 GPa.

Calculation of crystal cohesive energy of ZrB2 compound

The valence electron structure (VES) of ZrB2 was set up with the bond length difference (BLD) method based on empirical electron theory (EET) of solids and molecules, and there were 43 potential hybridization combinations. Based on the calculation result of the melting point, the 16th hybridization step of Zr atom and fifth hybridization step of B atom are ascertained as the final hybridization combination. Therefore, the covalent electron number and the bonding energy of the strongest bond (the B-B bonds), the theoretical melting point, and the crystal cohesive energy of the ZrB2 compound can be figured out.