Values Of Cohesive Energy Research Articles

Predicting trends in rate parameters for self-diffusion on FCC metal surfaces

The hopping self diffusion coefficient of an adatom on the (1 0 0), (1 1 0) and (1 1 1) surfaces of nine FCC metals have been investigated using Monte Carlo variational transition state theory and the Lennard-Jones (L-J) interactions. The metals that have been studied are Ag, Al, Au, Cu, Ir, Ni, Pd, Pt and Rh. The potential parameters for the L-J interactions have been determined from the known experimental values of cohesive energies and lattice constants. The ratio, R, of the cohesive energy to the activation energy for diffusive hopping on the (1 1 1) and (1 0 0) surfaces are found to be 30 and 6, respectively. For diffusive hopping on the (1 1 0) surface, R depends on the direction of diffusion: R is 5 and 2.8 along the [1 1 0] and [0 0 1] directions, respectively. The pre-exponential factor, D 0, for these metals is found to vary within a factor of three from the corresponding average value, ( D 0) av for a given surface and diffusion channel. Also, the pre-exponential factors corresponding to diffusion on the (1 1 1), (1 0 0), (1 1 0) [1 1 0] and (1 1 0)[0 0 1] surfaces are found to satisfy an empirical expression in terms of R, lattice constant, and the distance between the two nearest binding sites. The results on the activation energies and pre-exponential factors are compared with other experimental and theoretical data.

Effect of composition and forming parameter on evaporated CdSeTe films deposited at room temperature

Amorphous films of the ternary compound CdSeTe, of composition in the range 0–10 at.% Cd and with a thickness of about 200 nm, have been prepared by thermal evaporation. The optical band gap was determined and found to be in the range 0.5–1.0 eV and arose from indirect transitions. A sharp decrease in the value of optical band gap ( E opt) is observed. The electrical properties of Cd thin films have been studied extensively. In general dc measurements have indicated Ohmic conductivity at low electric fields resulting from the thermal excitation of carriers from centers in the band gap. When the temperature is low enough so that carriers cannot be excited into one of the allowed bands, the dominant conduction may take place via hopping, whereby carriers hop from occupied to unoccupied sites located within a band of localized states situated within the band gap. The electrical and optical data were consistent and realized from the binding energy represented by the cohesive energy values. The generalized (8− n) rule was used to estimate the average co-ordination number. The obtained results were treated in the frame of the chemical bond approach proposed by Bicerano and Ovshinsky. The phase separation phenomena and the morphology were also studied for the prepared films.

Optical and electrical properties of vacuum evaporated In doped Se amorphous thin films

The dc current–voltage characteristics of thin films of the metal chalcogenide glassy semiconductor Se 100- X In X with 3⩽ X⩽15 using a square four-point probe of silver paste were obtained. Results showed a nearly ohmic behaviour. The resistance obeyed an Arhenius-type dependence on the ambient temperature. The optical band gap was determined and found to be in the range 1.5–1.75 eV and arose from indirect transitions. The electrical activation energy lies in the range 0.79–1.02 eV. The electrical and optical data were consistent and realized by binding energy represented by the cohesive energy values. The generalised “8− n” rule was used to estimate the average co-ordination number. Obtained results were treated in the frame of the chemical bond approach proposed by Bicerano and Ovshinsky (Non-Cryst. solids, 1985; 74:75). Also, it was observed that the prepared films are almost stable against γ-dose up to 10 Mrad.

Relevance of Charge-Density Measurements for High-Precision Calculations

High-precision photon diffraction data in beryllium, diamond, silicon, copper and germanium are reanalysed with the aim of assessing the accuracy of experimental X-ray and γ-ray structure factors by comparing derived values for cohesive energy with the thermochemical ones. Special attention is devoted to the diffraction data analysis for a meaningful and accurate comparison between theory and experiment. A good overall agreement of local density approximation calculations is found when the electron density exhibits spherical symmetry around each atomic site. On the contrary, the analysis of experimental data in 3d transition metals, namely vanadium, chromium, iron, cobalt and nickel, points out failures of the theory in reproducing the asphericity of the electron distribution.

Effect of copper addition on electrical and optical properties of As 10Se 90 thin films according to chemical bond

The dc current-voltage characteristics of thin films of the metal chalcogenide glassy semiconductor As(10%)Se(90%)Cu x with 0 ⩽ x ⩽ 1 using a coplanar gold electrode were obtained. Results showed a nearly ohmic behaviour with the resistance having an Arrhenius-type dependence on the ambient temperature. The optical band gap was in the range (1.47–1.53 eV) and arose from indirect transitions; increasing the Cu content resulted in decrease of the optical gap and electrical activation energy, which was in the range 0.4–0.6 eV. The electrical and optical data were consistent and realized by binding energy represented by the cohesive energy values. The generalised ‘8-n’ rule was used to estimate the average coordination number. Obtained results were treated in the frame of chemical bond approach proposed by Bicerano and Ovshinsky

Adsorption of Cationic Surfactants on Rutile

The adsorption and zeta potential of two series of cationic surfactants, i.e. alkyltrimethylammonium bromide and alkylpyridinium chloride, on rutile have been studied. The two series varied only in their hydrocarbon chain length. The effect of chain length on the lateral two-dimensional aggregation phenomena of surfactant molecules on the rutile surface has been explained by evaluating various thermodynamic parameters such as the effective number of CH2 groups removed from the aqueous environment on to the solid surface (dn/d In C), the mean cohesive energy per CH2 group (Φ) and the average mean aggregation number of the hemimicelle (nhm). The value of the mean cohesive energy in the case of alkyltrimethylammonium bromide (0.599 kcal/mol) is in very good agreement with literature values. It supports hemimicellar formation.

Equilibrium structure and bonding of small silicon clusters studied using an orbital-free kinetic-energy functional.

We use an orbital-free kinetic-energy functional to study the equilibrium structure and bonding of small ${\mathrm{Si}}_{\mathit{n}}$ clusters up to n=6. The results are compared with those obtained using the tight-binding density-functional scheme, Langevin molecular dynamics, and local-density approximation. Bond lengths and angles are reproduced very well, but values of cohesive energy are less accurate.

Cohesive Energy of NiO: A Quantum Monte Carlo Approach

The variational quantum Monte Carlo method has been applied to the first-principles calculation of the cohesive energy of NiO with the fcc type-II antiferromagnetic structure. The trial wave functions used for the calculations for O and Ni atoms and solid NiO have been chosen to be of the form of a Jastrow exponential factor multiplying a Slater determinant. The pseudopotentials introduced to avoid the large fluctuations of energy in the core region have been evaluated with the aid of a local approximation whose utility and accuracy were assessed through comparison with the nonlocal method in the calculation for isolated atom. By appropriate choice for the form of the trial functions, a promising result has been obtained for the calculated value of cohesive energy, which is consistent with experiment.

Cohesive properties of 4d-transition-metal carbides and nitrides in the NaCl-type structure.

We present a study of the cohesive energies (${\mathit{E}}_{\mathrm{coh}}$) of all 4d-transition-metal carbides and nitrides in the NaCl-type structure using ab initio linear-muffin-tin-orbitals type total-energy calculations and an extensive analysis of thermodynamic data. Many of the carbides considered here are metastable and their cohesive energy is not known from direct measurements. In these cases, we estimate ${\mathit{E}}_{\mathrm{coh}}$ using thermodynamic model calculations and analyses of phase diagrams. This information allows us to perform a detailed comparison of theoretical (${\mathit{E}}_{\mathrm{coh}}^{\mathrm{th}}$) and thermodynamic (${\mathit{E}}_{\mathrm{coh}}^{\mathit{e}}$) cohesive energy values. The difference ${\mathit{E}}_{\mathrm{coh}}^{\mathrm{th}}$-${\mathit{E}}_{\mathrm{coh}}^{\mathit{e}}$ is positive for all carbides and nitrides, and we discuss the various sources of error in the theoretical approach. Trends in theoretical ${\mathit{E}}_{\mathrm{coh}}$ values are remarkably improved when spectroscopic data are used to correct calculated atomic total energies. We also show that most of the discrepancy between theoretical and thermodynamic results cancels in the difference \ensuremath{\Delta}${\mathit{E}}_{\mathrm{coh}}$ between a carbide MC and a nitride MN of the same transition-metal M, and excellent agreement with experiments is obtained for the systems where thermodynamic information is available, viz., for M=Zr, Nb, and Mo. This fact allows us to estimate the unknown cohesive energies and enthalpies of formation for YN, TcN and for the metastable NaCl structures of RuN, RhN, and PdN, from a combination of ab initio results and assessed thermodynamic information for the corresponding carbides. Our method of theory-based predictions of thermodynamic quantities is compared with the semiempirical method of Miedema. We also discuss the role of extrapolation methods related to ab initio results in the estimation of thermodynamic information that is used in the modeling of alloy phase diagrams.

Thermodynamic and ballistic aspects of ion mixing

Ballistic and thermal spike contributions to ion mixing are discussed using the results of recent marker experiments which determine the dominant moving species in ion mixing of several metallic bilayer systems. A greater flux of atoms from the high cohesive energy side to the low cohesive energy side is the result of the thermal spike contribution to ion mixing. A greater flux from the top layer to the bottom layer in ion mixing of bilayers consisting of elements of similar and high cohesive energy values is the result of the ballistic contribution to ion mixing. The relative importance of ballistic and thermal spike contributions is influenced by the magnitude of cohesive energy. A model based on a fractal geometry approach to spike formation is used to understand the competition between ballistic and thermal spike contributions to ion mixing.

The Electronic Structure and Cohesive Energy of HfO2, ZrO2, TiO2, and SnO2 Crystals

AbstractEnergy band structure calculations for tin, titanium, zirconium, and hafnium dioxides are made according to the LMTO method. The values of cohesive energies as well as of the total and partial densities of states are calculated. The results are used for the analysis of the change of the forbidden gap width in the series of dioxides. Relative values of the cohesive energy for TiO2 and SnO2 are explained as a result of repulsion between the oxygen atoms forming pairs, and – in the case of SnO2 – of the absence of hybridization between O2p‐ and d‐states as well.

Unified study of lattice mechanics of alkali cyanides

An extended three-body force shell model (ETSM) for alkali cyanides has been developed by including the effects of coupling between the translational and rotational motions of cyanide (CN -) molecules in the framework of original TSM devised by Singh and Sanyal [Physica 132B (1985) 201]. This ETSM has been applied to describe the static, dynamic, anharmonic elastic and photoelastic properties of the alkali cyanide crystals. This model has satisfactorily reproduced their experimental phonon dispersion curves and cohesive energy values. However, the agreements between the experimental and our theoretical values on the third order elastic constants are not so good. These agreements have been found to improve significantly with the exclusion of three-body interaction (TBI) effects, which play a vital role in the prediction of harmonic and anharmonic elastic properties of alkali halides. This might be so due to the overestimated effects of TBI from the Cauchy violations ( C 12– C 44) which are much larger for alkali cyanides than those for alkali halides. It has been concluded that the ETSM, which has reproduced the lattice statics and dynamics of alkali cyanides so well, is incapable to explain their anharmonic elastic and thermal properties. However, this situation is expected to improve by using a more appropriate method for estimating TBI contributions and incorporating the effects of covalent and anharmonic interactions.

Origins of the universal binding-energy relation.

The universality of the relation between binding energy and interatomic separation occurs for metallic and covalent bonds in a wide range of situations, spanning diatomic-molecule energetics, chemisorption, bimetallic adhesion, cohesion in solids, and even interactions in nuclear matter. This has intrigued physicists for some time, and here we provide some insights into its origin. We considered the electron density distribution as the variable linking the total energy and interparticle separation. In the spirit of effective-medium theory, a host electron density as seen by each atom was computed. We found that in every case (cohesion, chemisorption, and diatomic molecules), the host electron density was, to a good approximation, a simple exponential function of interparticle separation. This arises primarily because of the essentially exponential decay of the electron density into vacancy sites, into interstitial regions, into the vacuum from surfaces, or into the vacuum from isolated atoms. This suggested a scaling of the electron density which provides a universal relationship between the scaled interatomic separation and the scaled electron density.This density scaling is key. We applied the scaling of the electron density to impurity-binding-energy--host-electron-density curves computed via the effective-medium approximation. A universal energy--electron-density relationship resulted. This could be combined with the previously noted universal relation between scaled electron density and interparticle separation to yield the universal binding-energy--distance relation. First-principles values of cohesive energies of solids and the energetics of certain diatomic molecules were also correlated with host electron densities, despite the fact that these types of energies are fundamentally different from each other and from impurity binding energies. We found a universal relationship between energies and host electron densities for cohesion and certain diatomic molecules which was the same as the one discovered for impurity binding energies. This, together with the universal relationship between electron density and interatomic separation, helps one to understand how a single energy-distance relation could describe chemisorption and cohesion as well as diatomic energetics.

Lattice mechanics of divalent-metal carbonates.

We have investigated the lattice mechanics of ${\mathrm{MnCO}}_{3}$, ${\mathrm{FeCO}}_{3}$, and ${\mathrm{NiCO}}_{3}$, for the first time, using a rigid-molecular-ion model which has been found to be adequately suitable for such descriptions for the calcite-structure crystals [Phys. Rev. B 35, 4462 (1987)]. The values of cohesive energy calculated by us for these crystals have shown reasonably good agreement with the corresponding thermochemical data. The trend followed by their phonon dispersion curves (PDC's) seems to be realistic as they have close resemblance with the measured PDC's of their counterparts (${\mathrm{NaNO}}_{3}$ and ${\mathrm{CaCO}}_{3}$) belonging to the calcite family.

Calculated thermal properties of metals.

The thermal properties of the 14 nonmagnetic cubic metals through the 4d transition series are derived from first-principles electronic-structure calculations coupled with a Debye treatment of the vibrating lattice. Debye temperatures and Gr\"uneisen constants are derived from an analysis of the compressional characteristics of rigid-lattice binding curves and are used to define the contribution of the lattice vibrations to the free energy. A minimization of the resulting free energy with respect to volume yields temperature-dependent lattice separations and coefficients of thermal expansion. Theoretical values of cohesive energies, equilibrium lattice separations, bulk moduli, Debye temperatures, Gr\"uneisen constants, and coefficients of thermal expansion are derived directly from computed electronic-structure results. Good agreement with experiment is found for all computed quantities.

Total Energy Calculation in α-SiO2 and β-Si3N4

AbstractThe self-consistent electronic structures and total energies for the crystalline α-SiO2 and β-Si3N4 are studied by means of the first-principles orthogonalized linear combination of atomic orbitals method. The calculated band structures are compared with the earlier non-self consistent results. The total energies in the local density approximation are evaluated as a function of lattice parameters. Reasonable values of equilibrium lattice constants and cohesive energies are obtained but the bulk modulus for α-SiO2 is overestimated.

Static and Thermophysical Properties of Mixed Crystal Systems

AbstractAn analysis of the interionic potential is performed to study the static and thermal properties of KClKBr and KBrKI mixed crystals for the entire range of composition. The interionic potential consists of the long‐range Coulomb and three body interaction, the short‐range van der Waals attraction, and the overlap repulsion, effective up to the second neighbour ions. Instead of using the BM‐type short‐range potential the Hafemeister and Zarht (HZ)‐type repulsive potential is considered between the nearest as well as between the next nearest neighbours. The three body charge transfer parameter f(r) and repulsive hardness parameters are evaluated using respectively the Cochran formula and overlap integrals. The van der Waals interaction coefficients are determined using the Slater‐Kirkwood variational approach. The calculated values of cohesive energy, bulk modulus, and the pressure derivatives of bulk modulus for the mixed crystals under study agree fairly well with available experimental data. The results achieved by previous investigators are also shown for the sake of comparison with the present and experimental investigations.

Interionic Forces and Elastic Properties of Na2S

AbstractAn interionic potential model is proposed for crystals with antifluorite structure which consists of long‐range Coulomb and three‐body interactions (TBI), and short‐range dispersive and repulsive interactions up to second‐neighbour ions. This potential is used to calculate the cohesive energy, second‐order elastic constants, and their pressure derivatives of Na2S. The calculated values of elastic constants and cohesive energy agree well with their measured data which shows that the TBI contribute significantly to these properties.

Study of static and thermophysical properties of alkaline earth fluoride crystals

An analysis for cohesive energy, bulk modulus and its pressure derivatives in BaF 2, SrF 2 and CaF 2 crystals has been carried out by means of interionic potential model. Interionic potential consist of long range coulomb and three-body interactions along with short-range attractive van der Waals and repulsive Hafemeister-Zarht type interactions. The three body force parameter ƒ(r) and vdWI coefficients C and D are determined using the Cochran's formula and Slater-Kirkwood variational (SKV) integrals. The calculated values of cohesive energy, bulk modulus and its pressure derivatives for solids under study are found generally in good agreement with available experimental data. The results achieved by other investigations [13, 14] have also been compared with results calculated in the present study.

Static and thermophysical properties of chalcogenide crystals with NaCl structure

In the present communication an analysis of interionic potentials in fourteen chalcogenide crystals has been performed. This interionic potential has been used to predict the values of cohesive energy, isothermal bulk modulus and the pressure derivatives of bulk modulus in the solids under study. The many body interaction (MBI) effects have been taken into account within the framework of Hafemeister Zarht potential. Instead of using BM potential the Hafemeister-Zarht (HZ) type short range overlap potential has been considered between nearest as well as between next nearest neighbour ions. The short-range interactions, effective up to second neighbours are treated by considering the hardness parameter as an ionic property. The hardness parameter ρ ij is evaluated using the data on overlap integrals. The results achieved in the present study are generally in good agreement with available experimental data. Values of cohesive energy, bulk modulus and its pressure derivatives calculated by previous investigators have also been shown for the sake of comparison.