The use of the embedded atom model for liquid metals: Liquid potassium

The method for calculations the embedded atom potential for liquid metals based on the diffraction data on the structure close to the melting temperature was applied to potassium. The embedded atom potential parameters were adjusted using the data on the structure of potassium at 343, 473, and 723 K and the thermodynamic properties of potassium at temperatures up to 37240 K. The use of the molecular dynamics method and the embedded atom potential gave close agreement with the experimental data on the structure, density, and potential energy of liquid metal along the p ? 0 isobar at temperatures up to 2200 K and along the shock adiabat up to a pressure of ～85 GPa and 37240 K. The calculated bulk compression modulus at 343 K was close to its actual value, and the self-diffusion coefficients increased under isobaric heating conditions following a power law with an exponent of 1.6478. The melting temperature of body-centered potassium with the embedded atom potential was (319 ± 1) K, which was close to the actual melting temperature. The potential obtained incorrectly described crystalline potassium.     

The embedded atom model of liquid cesium

The embedded atom potential was calculated for cesium over the temperature range 323–1923 K at pressures up to 9.8 GPa from the diffraction data on the structure of the metal close to the temperature of fusion (T _f). The parameters of the embedded atom potential were adjusted using the data on the thermodynamic properties and structure of liquid cesium. The embedded atom potential well predicts the structural and thermodynamic characteristics of the liquid metal as the temperature increases along the liquid-vapor equilibrium line and under strong compression. The calculated potential energy and structure of liquid cesium closely agree with the experimental data at temperatures up to 1373 K. The calculated bulk compression modulus is close to its experimental values at all temperatures except 323 K. The self-diffusion coefficients increase as the temperature grows by a power law with an exponent close to 2 and satisfy the Stokes-Einstein equation. Deviations from experimental data at temperatures above 1400 K are explained by the metal-nonmetal transition that occurs as the density decreases.     

Embedded atom model for liquid metals: Liquid iron

A method for calculating the embedded atom model potential suggested earlier for liquid Ga and Bi uses data on the structure and thermodynamic properties of metals close to their melting points. This method was applied to liquid iron at temperatures and pressures up to 5000 K and 360 GPa. Several iron models with the potential of the embedded atom model were constructed by the method of molecular dynamics at temperatures from 1820 to 5000 K and densities from 8.00 to 12.50 g/cm³. The thermodynamic, structural, diffusion, and viscosity properties of iron were calculated. The self-diffusion coefficients decreased almost linearly as the volume of the system became smaller. The conclusion is drawn that iron in the external region of the Earth’s core behaves as a liquid with self-diffusion coefficients of about ～10^-5 cm²/s and viscosity ～10^-3?10^-2 Pa s. At the boundary between the external and inner core regions, at densities of 11–12 g/cm³, iron has the properties of an amorphous phase and its self-diffusion coefficient becomes too low to be estimated by the method of molecular dynamics. Under the Earth’s inner core conditions, the embedded atom model of iron spontaneously crystallizes.     

Molecular dynamics simulation of shock compression of metals: Iron and iron-sulfur solutions

The embedded atom model potential suggested earlier was improved to correctly describe iron at high pressures and temperatures. Correction was introduced using the shock compression data. The properties of body- and face-centered cubic (BCC and FCC) lattices and liquid iron at compression degrees up to 50% of the normal volume and temperatures up to 10000 K were calculated. At degrees of compression 0.7–0.6 and 0 K, the FCC lattice is thermodynamically stable. The temperature of fusion increases to ≈9700 K at compression to 50% of initial volume (pressure 585 GPa). The pressure of pure iron at 5000 K and density 12.5 g/cm³ is ≈250 GPa and is substantially lower than in the center of the Earth according to the geophysical data (360 GPa). An embedded atom model potential for a 10 at % solution of sulfur in iron which allows the properties of the melt in the center of the Earth to be described correctly is suggested; the viscosity of the melt under these conditions is not high (0.0156 Pa s); these results are close to those obtained in ab initio calculations. The possibility of partial Earth core crystallization is shown.     

Computer simulation of the properties of liquid metals: Gallium,lead, and bismuth

The embedded atom model (EAM) potentials of liquid gallium, lead, and bismuth calculated by the author using the Schommers algorithm were refined and written in a unified analytic form more convenient for applications. Pair contributions to EAM potentials are described by piecewise continuous functions. The form of EAM potentials admits the transition to a high-density state characteristic of shock compression. Series of models of these liquid metals were constructed by the molecular dynamics method at temperatures up to 1500 (Zn), 3000 (Ga, Pb), and 1800 K (Bi). For all the metals, close agreement with experiment was obtained over the whole temperature range for density, structure, bulk compression modulus, and self-diffusion coefficient. The standard deviations of model pair correlation functions (PCF) from the diffraction PCFs of gallium and lead were on the order of 0.01. As distinct from alkali metals, the calculated energy of gallium and lead models was close to actual energy over the whole temperature range, and excess electronic heat conductivity was almost unobservable. With bismuth, agreement with experiment for energy and structural characteristics was noticeably worse, which shows that the embedded atom model is less applicable to bismuth.     

The embedded atom model for liquid metals: Liquid gallium and bismuth

A method for calculating embedded atom potentials in liquid metals is suggested. The method uses diffraction structural data, density, bulk compression modulus, and thermal expansion coefficient close to the melting point. The method was applied to liquid gallium and bismuth at temperatures from their melting to critical points. The critical temperatures of these metals were estimated at 4940 and 4225 K. The other critical parameters were also determined. The self-diffusion coefficients were found to increase almost linearly as the temperature grew. The model allows changes in the structural characteristics of the metals when the temperature increases by several hundred kelvin units to be correctly described.     

A Molecular Dynamics Study of Nickel Crystallization at Strong Supercoolings

The homogeneous crystallization of liquid nickel models containing 2048 particles in the basic cube was studied by molecular dynamics. The potential of the embedded atom method was used. The models were constructed under zero pressure or constant volume conditions. The state of the structure was evaluated from the number of atoms with the coordination number 12. The concentration of such atoms in the stable and metastable liquids increased as the temperature decreased. At the selected potential of the embedded atom model, the equilibrium crystallization temperature at zero pressure was 1415 K. The existence of the lower boundary of liquid nickel supercooling was established. The liquid crystallized under isothermal conditions by the cluster mechanism with the formation of a predominantly closely packed structure below 850 K at zero pressure and below 1075 K at a constant volume (6.588 cm³/mol). The mechanism of nucleation was different from that accepted in classic nucleation theory. Nucleation was accompanied by an increase in the number of atoms with the coordination number 12, the formation of bound groups (12-clusters) from these atoms, and the growth of these groups, as with the crystallization of rubidium under strong supercooling conditions and coagulation of impurities from supersaturated solutions. At the initial stage, bound groups had a very loose structure and contained a large number of atoms with coordination numbers other than 12; the linear size of the largest group rapidly approximated the basic cube size. These atoms played a leading role in crystallization and activated the transfer of atoms in bound groups having different coordination numbers into the coordination state corresponding to a closely packed lattice. An important role in the formation of 12-clusters of the threshold (critical) size played cluster size fluctuations, which were especially strong close to the lower boundary of liquid supercooling.     

Solid–liquid interface growth of Cu50Ni50 under deep undercoolings

Molecular dynamics simulations have been performed to explore interface growth of liquid Cu₅₀Ni₅₀ alloy by using an embedded atom potential, namely due to Zhou. The simulated melting temperature is 1585 K in agreement well with the experimental value of 1600 K. The calculated interface velocity increases with decreasing temperature ranging from 1585 K to 1100 K, where the calculated values are a little higher than the experimental ones at higher temperatures, and in agreement with the experimental ones at lower temperatures; while the calculated interface velocity decreases with decreasing temperature lower than 1100 K. The activation energy of atom is 0.0048 eV, almost close to zero under deep undercoolings, although the crystal growth still proceeds with the speed ranging from 50 m s^?1 to 10 m s^?1. The crystal growth of Cu₅₀Ni₅₀ is not controlled by diffusion mechanism under deep undercoolings.     

Molecular dynamics simulation of the thermodynamic properties of mercury at pressures below 2.5 GPa and temperatures below 10000 K

Models of mercury were constructed by molecular dynamics using the interparticle potential of the embedded atom model (EAM) at temperatures below 10 000 K and pressures below 2.5 GPa. The thermodynamic properties of the models were presented on the isobars of 0.5, 1.0, 1.5, 2.0, and 2.5 GPa. The compressibility factors Z = pV/(RT) were calculated; the coordinates of the inversion points of the Joule–Thomson coefficient below 5600 K were found from the positions of minima on the Z(p, T) isobars. At densities above 8–9 g/cm³, the results of simulation agreed well with experiment; at lower densities there were discrepancies associated with a loss of metal properties by real mercury. The behavior of the models was analyzed in the region of the van der Waals loop. The calculated critical temperature of mercury was found to be significantly overestimated relative to the experiment. Modeling the “meta-mercury” with the EAM potential with excluded embedded potential contribution gave better agreement with the equation of state of mercury at lower densities. The states with Z = 1 can be observed below 1.0 GPa. The calculated temperature of the inversion of the Joule–Thomson coefficient increased monotonically to 5600 K as the pressure increased to 2.5 GPa.     

The simulation of metallic hydrogen-helium solutions under the conditions of internal Jupiter regions

Several series of liquid metallic hydrogen, liquid helium, and hydrogen-13 at % helium solution models were constructed by the method of molecular dynamics at state parameters corresponding to three levels of the Jupiter shell. The one-component classical plasma approximation including electronic contributions to energy and pressure was used for hydrogen. Helium was described by the interparticle potential suggested by Aziz et al., and hydrogen-helium pairs, by the Lennard-Jones potential with adjustment parameters (the models contained from 1968 to 2048 particles in the basic cube). The thermodynamic, structural, and diffusion characteristics of solutions and viscosity at 10 000, 15 000, and 20 300 K and molar volumes V of from 0.35 to 1.3 cm³/mol were determined. The mass fraction of the heavy component was found to be approximately 6% at the higher and 14% at the lower level. Hydrogen-helium solutions exhibited very weak positive deviations from ideality. Their viscosity was close to 0.002 Pa s at V = 0.35 cm³/mol and 0.0004 Pa s at V = 1.3 cm³/mol and very weakly depended on temperature at a constant volume. The velocity of sound was 25–50 km/s; it decreased as the density lowered. The form of the partial pair correlation functions was characteristic of metallic liquids and close to the data on the one-component classical plasma at plasma parameter values Γ = 12–16.     

Crystalline Anionic Germanate Covalent Organic Framework for High CO2 Selectivity and Fast Li Ion Conduction

The metalloid-centered covalent organic framework has attracted great interest from both its structure and application. Heavier elements have seldomly been incorporated in the covalent organic frameworks, even if they exhibit special structural features and properties. Herein, we reported the first crystalline germanate covalent organic framework with hexacoordinated germanate linked by an anthracene linker. The existence of counterion lithium ions in the framework provides a high CO₂ uptake of 88.5 cm³ g⁻¹ at 273 K and a high CO₂/N₂ selectivity of 101. A significantly improved lithium ion conductivity of 0.25 mS cm⁻¹ at room temperature was observed due to the soft germanium center.     

Interface induced crystal structures of dioctyl-terthiophene thin films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.     

Pulse radiolysis study of the reaction of OH radicals with C2H4 over the temperature range 343–1173 K

The reaction of OH and OD radicals with ethylene in the presence of 1 atm argon and 6 Torr water vapor was studied in the temperature range 343–1173 K. The results reveal three kinetically separate temperature regions: (1) 343–563 K, where the disappearance of OH radical is dominated by the addition of OH to the double bond of ethylene; (2) 563–748 K, where concurrent reactions of addition, the reverse reaction of addition and H-atom abstraction is dominant; and (3) 748–1173 K, where H-atom abstraction is likely the main reaction. The rate for hydrogen abstraction is 2.4 × 10^?11 exp[(?2104 ± 125)/T] cm³/molec-s (for OD 2.1 × 10^?11 exp[(?2130 ± 172)/T] cm³/molec-s). There was no obvious pyrolysis of ethylene below 1073 K. The study of OD radical with ethylene shows a small isotope effect.     

Thermal Stability of 2,3,4-, 2,4,5- and 3,4,5-tri- Methoxybenzoates of Light Lanthanides

The physico-chemical properties and thermal stability in air of light lanthanide 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates were compared and the influence of the position of –OCH³ substituent on their thermal stability was investigated. The complexes of these series are crystalline, hydrated or anhydrous salts with colours typical of Ln³⁺ ions. The carboxylate group is a bidentate, chelating ligand. The thermal stability of 2,3,4-, 2,4,5- and 3,4,5-trimethoxybenzoates of rare earth elements was studied in the temperature range 273–1173 K. The positions of methoxy groups in benzene ring influence the thermal properties of the complexes and their decomposition mechanism. The different thermal properties of the complexes are connected with various influence of inductive and mesomeric effects of –OCH₃ substituent on the electron density in benzene ring. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature-dependent vibrational spectroscopic studies of pure and gold nanoparticles dispersed 4-n-Hexyloxy-4’-cyanobiphenyls

Raman spectra of pure and 2 wt.% gold nanoparticles (GNPs) dispersed liquid crystalline compound 4-n-Hexyloxy-4?- cyanobiphenyls (6OCB) has been recorded as a function of temperature from room temperature (solid crystal) to 80°C (isotropic liquid) in the spectral region of 500–2500 cm^?1. The variation of Raman spectral parameters (peak positions and line width) with temperature is used to explain the changes in molecular alignment and its effect on inter-/intra-molecular interactions at crystal-Nematic (K-N) transition. To understand the change in molecular structure during phase transition and on account of dispersion of gold nanoparticles in pure liquid crystal more precisely, two spectral regions 1000–1500 cm^?1 and 1500–2400 cm^?1 have been selected separately. From the detailed study, it is concluded that increased orientational/vibrational freedom of the molecules as well as delocalisation of electron clouds results in the spectral anomalies at K-N transition. The geometrical structure of 6OCB was optimised using density functional theory (DFT) and theoretical Raman spectra have been obtained for comparison with experimental spectra. The tentative assignment of vibrational modes observed in our region of study was calculated based on potential energy distribution (PED) using vibrational energy distribution analysis (VEDA) calculation.     

Lattice temperature and hyperfine interactions of Pb1−xSnxTe (0.21 ≤ x ≤ 0.75)

Thin (<15 μm) samples of lead tin telluride, Pb_1?xSn_xTe (x = 0.21, 0.25, 0.55, and 0.75) have been studied by temperature dependent Mössbauer spectroscopy using the 23.8 keV gamma radiation of ^119mSn. The tin atom occupies a lattice site having cubic symmetry (QS = 0 ± 0.020 mm sec^?1) over the temperature range 78 ≤ T ≤ 240 K, and there is no evidence for a rhombic (low temperature) to cubic (high temperature) phase transition such as that reported for SnTe in this temperature interval. The lattice temperature as probed by the Sn atom is independent of the compositional parameter x and is similar to that reported for SnTe from Mössbauer studies and for Pb_0.63Sn_0.37Te from X-ray powder diffraction data. Radiation damage produced by 2-MeV proton irradiation to a total fluence of ～10¹⁷ cm^?2 at liquid nitrogen temperature does not have any effect on the Mössbauer parameters, possibly because the major damage is annealed at temperatures below 150 K.     

Molecular electronic structure of metal phthalocyanines

The molecular–electronic structure of the metal phthalocyanines (Fe, Co, Ni and Cu) has been determined by the molecular orbital treatment. Coulomb integrals of the metal atom occurring in the secular determinants have been approximated equivalent to the valence state ionization energy (VSIE) of a metal orbital for a particular charge configuration. The calculated π-electron charge densities have been found to be higher on the nitrogen atoms as compared to the other atoms in the molecule. This is in agreement with the e.s.r. studies of the metal phthalocyanines. To test the correctness of the molecular orbital calculations, the π-π* transitions (14,000 cm^?1 ? 30000 cm^?1), d-d* transitions (20000 cm^?1 ? 60000 cm^?1) and charge transfer transitions (15000 cm ^?1 ? 30000 cm^?1) have been calculated in the metal phthalocyanine molecules. The calculated frequencies have been compared with the observed ones and found in fair agreement.     

Hydrogen bonded NHO chains formed by chloranilic acid (CLA) with 4,4′-di-t-butyl-2,2′-bipyridyl (dtBBP) in the solid state

In crystalline state 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (chloranilic acid, CLA) forms with 4,4′-di-t-butyl-2,2′-bipyridyl (dtBBP) the hydrogen bonded chains along the b-axis. From one side of the CLA molecule the proton transfer takes place and the hydrogen bond length is very short (2.615 Å). A continuous infrared absorption is observed for dtBBP·CLA in the wavenumber range between 3100 and 800 cm⁻¹ also indicating the strong hydrogen bonds. The DSC measurements show a weak, close to continuous, phase transition at 414 K. The complex dielectric permittivity for a single crystal sample was measured in the temperature range 100-440 K and at frequencies between 200 Hz and 2 MHz. The dielectric response is a combination of semiconducting properties and a relaxation process most probably connected with the proton dynamics in the hydrogen bonds. The mechanism of the structural phase transition is discussed.     

Laser Desorption Ionization of Melamine and Identification of Fragmentation Channels from Ion Velocity Distribution Measurements

Fragmentation frequently accompanies intact laser desorption ionization of a parent non-volatile compound, desorption and dissociation dynamics has been a subject of intense studies over the past decade. As a preliminary lest system for future laser desorption study of energetic compounds such as explosives and propellents, we studied UV laser desorption ionization of melamine at a laser power density of approximately 4.4 MW/cm². Several gas-phase dissociation channels of the parent and fragment ions formed in UV laser desorption ionization of melamine films can be identified from their velocity distributions. A phenomenological desorption temperature of the order of 20000 K is estimated from fitting the experimental velocity distributions to Maxwellian functions.