Structure and energetics of Li/Na, Li/K, and K/Na bimetallic hexamers

A systematic study of neutral mixed clusters, Li_6?x Na _x , Li_6?x K _x and K_6?x Na _x (x = 0–6), was performed within the framework of density functional theory. The aim of this work is to explore the geometry variation and the energy change of homonuclear hexamers (Li₆ and K₆) induced by impurities. It is found that the geometry of bimetallic hexamers varies with their compositions. The geometries of resulting clusters show evolution from D_4h symmetry for Li₆ to D_3h symmetry for Na₆ and K₆. The stability of bimetallic hexamers has been also explained in terms of binding energy, excess energy, the second difference in energy, and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps. It is found that replacing each Li–Li bond with Li–Na or Li–K bonds decreases the cluster stability, while replacing each K–K bond by K–Na leads to stability enhancement. Examining the cluster stability, excess energy and second difference in energy reveal that among studied bimetallic hexamers, Li₂Na₄ is the most stable mixed hexamer.     

Ab-initio calculation of structural, electronic, and optical characterizations of the intermetallic trialuminides ScAl3 compound

We present first-principles study of the electronic and the optical properties for the intermetallic trialuminides ScAl₃ compound using the full-potential linear augmented plane wave method within density-functional theory. We have employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for calculating the electronic band structure and optical properties. The electronic specific heat coefficient (γ), which is a function of density of states, can be calculated from the density of states at Fermi energy N(E_F). The N(E_F) of the phase L1₂ is found to be lower than that of D0₂₂ structure which confirms the stability of L1₂ structure. We found that the dispersion of the band structure of D0₂₂ is denser than L1₂ phase. The linear optical properties were calculated. The evaluations are based on calculations of the energy band structure.     

Thermodynamic insights into n-alkanes phase change materials for thermal energy storage

n-Alkanes have been widely used as phase change materials (PCMs) for thermal energy storage applications because of their exceptional phase transition performance, high chemical stability, long term cyclic stability and non-toxicity. However, the thermodynamic properties, especially heat capacity, of n-alkanes have rarely been comprehensively investigated in a wide temperature range, which would be insufficient for design and utilization of n-alkanes-based thermal energy storage techniques. In this study, the thermal properties of n-alkanes (C₁₈H₃₈-C₂₂H₄₆), such as thermal stability, thermal conductivity, phase transition temperature and enthalpy were systematically studied by different thermal analysis and calorimetry methods, and compared with previous results. Thermodynamic property of these n-alkanes was studied in a wide temperature range from 1.9 K to 370 K using a combined relaxation (Physical Property Measurement System, PPMS), differential scanning and adiabatic calorimetry method, and the corresponding thermodynamic functions, such as entropy and enthalpy, were calculated based on the heat capacity curve fitting. Most importantly, the heat capacities and related thermodynamic functions of n-heneicosane and n-docosane were reported for the first time in this work, as far as we know. This research work would provide accurate and reliable thermodynamic properties for further study of n-alkanes-based PCMs for thermal energy storage applications.     

Effect of high hydrostatic pressure on structural stability of Ti3GeC2: A first-principles investigation

An investigation into the structural stability, electronic and elastic properties of Ti₃GeC₂ under high hydrostatic pressure was conducted using first-principles calculations based on density functional theory (DFT). From the energy and enthalpy calculations, and the variations of elastic constants with pressure, we conclude that α‐Ti₃GeC₂ is most stable upon compression to 100 GPa, which is not consistent with the nonhydrostatic in situ synchrotron X-ray diffraction studies. The higher structural stability was analyzed in terms of electronic level. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of all polymorphs of Ti₃GeC₂.     

Lithium–silicon Si n Li (n = 2–10) clusters and their anions: structures, thermochemistry, and electron affinities

The molecular structures, electron affinities, and dissociation energies of neutral Si _n Li (n = 2–10) species and their anions have been studied by the B3LYP and the BPW91 methods in conjunction with a DZP++ basis set. The geometries have been fully optimized with each of the proposed methods. The ground state structure of neutral Si _n Li keeps the corresponding Si _n framework unchanged. For anion, the corresponding Si _n (or ${{\rm Si}_{n}^{-}}$ ) framework changes largely when n ≥ 7. To evaluate the stability of the resulting anions we have calculated the adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vert), and the vertical detachment energy (VDE). The dissociating energies of Li from the lowest energy structures of neutral Si _n Li and their anions are calculated to examine relative stabilities.     

First-principles calculations of the stability and hydrogen storage behavior of C14 Laves phase compound TiCrMn

The structural, elastic properties, electronic structure and hydrogen storage behavior of TiCrMn with a hexagonal C14 structure were investigated by the first-principles calculations within the frame work of DFT. The calculated lattice constants were consistent with the experimental values, and obtained cohesive energy and formation enthalpy showed TiCrMn is of the structural stability. These results also indicated that Mn atoms would optionally substitute on the Cr sites of TiCr₂ phase to form the ternary intermetallic TiCrMn. The five independent elastic constants as well as polycrystalline elastic parameters (bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν and anisotropy value A) were calculated, and then the ductility and elastic anisotropy of TiCrMn were discussed in details. Furthermore, the electronic DOS and charge density distribution of TiCrMn were also calculated, which revealed the underlying mechanism of structural stability and chemical bonding. Finally, the binding energy of hydrogen in hydride TiCrMn(H₃) was investigated, confirming the better hydrogen storage behavior of C14 Laves phase TiCrMn.     

Theoretical investigations on R(O)nS-NO (n=0,1,2) systems

In the current article we report the ab initio study on the stability of S-Nitrosothiols (MeSNO, 1) and their oxidised derivatives (MeS(O)NO, 2) and (MeS(O)₂NO, 3). The bond length, bond order, rotational barrier and bond dissociation energy have been calculated and compared with that of sulfenamide (HS-NH₂) and its oxidised derivatives sulfinamide (H(O)S-NH₂) and sulfonamide (H(O)₂S-NH₂). The S-N bond dissociation energy in the oxidised state is very small compared to parent RSNO indicating the weakness of sigma bond. NBO analysis suggests that the negative hyperconjugative interactions are very strong in S-nitrosothiols and their oxidised derivatives, which weaken the sigma bond and facilitate the release of nitric oxide.     

Influence of fluorine substitution on the properties of CdO nanocluster : a DFT approach

Fluorine substituted cadmium oxide (Cd _n O _n?1F) cluster for n = 2–6 of linear, ring and three dimensional structures were studied using B3LYP exchange correlation function with LanL2DZ as basis set. Different isomers were optimized to obtain structural stability and various parameters such as dipole moment, HOMO–LUMO gap, ionization potential, electron affinity, stability factor, binding energy, vibrational studies and optical absorption were studied and reported. The stability of the cluster depends on the binding energy and vibrational intensity.     

Kinetics of phase transition and thermal stability in Se80?x Te20Zn x (x?=?2, 4, 6, 8, and 10) glasses

Se_80?x Te₂₀Zn _x (x?=?2, 4, 6, 8, and 10) glasses have been prepared using conventional melt quenching technique. The kinetics of phase transformations (glass transition and crystallization) have been studied using differential scanning calorimetry (DSC) under non-isothermal condition at five different heating rates in these glasses. The activation energy of glass transition (E _t), activation energy of crystallization (E _c), Avrami exponent (n), dimensionality of growth (m), and frequency factor (K _o) have been investigated for the better understanding of growth mechanism using different theoretical models. The activation energy is found to be highly dependent on Zn concentration. The rate of crystallization is found to be lowest for Se₇₀Te₂₀Zn₁₀ glassy alloy. The thermal stability of these glasses has been investigated using various stability parameters. The values of these parameters were obtained using characteristic temperatures, such as glass transition temperature T _g, onset crystallization temperature T _c, and peak crystallization temperature T _p. In addition to this, enthalpy-released during crystallization has also been determined. The values of stability parameters show that the thermal stability increases with the increase in Zn concentration in the investigated glassy samples.     

Electronic structure and stability of charged beryllium clusters

Electronic structure studies on neutral, singly and doubly ionized Be _n clusters (n≤5) have been carried out in order to investigate the stability and observability of charged clusters. Our studies employ wave function expansion in terms of gaussian type orbitals and have been carried out within local spin density formalism. It is shown that although small doubly ionized clusters are unstable, they are protected from fragmentation by energy barriers. We illustrate this explicitly for trimers by presenting a Born-Oppenheimer surface of Be₃, Be ₃ ⁺ and Be ₃ ⁺⁺ . It is argued that depending on their geometries, the observable doubly charged clusters can be generated through a one or two photons ionization. We also present results on the distribution of “hole charge” in doubly ionized clusters and show that a small cluster exhibits metallic like behaviour in regard to distribution of missing electronic charge.     

Correlation of thermal analysis data and hydrolysis resistance of polycarbonates

The initial activation energy at zero conversionE _o of thermooxidative decomposition has been taken as a measure of thermal stability of polycarbonates (PC) andE _o has been correlated with the relative hydrolysis rater _h as a measure of their hydrolyse resistance. It is suggested that both decomposition processes are initiated by the same mechanism, the attack of hydrolytic agent onto ester C?O bonds. The following values ofE _o have been found: 187 (PC-M)>87 (PC-A)>43 (PC-C) kJ/mol, and they are correlated with values ofr _h being 0.01 (PC-M)<1 (PC-A)<4.4 (PC-C). It has been found, using a computer modeling technique, that bothE _o andr _h depend on the minimized energy of conformations.     

Electronic shell structure and metal clusters: the self-consistent spheroidal jellium model

The electronic properties of small metal particles within the recently proposed self-consistent spheroidal jellium model [1] are further explored and compared to recent experimental data. Physical properties investigated include ionization potentials, electron affinities and the binding energy of neutral monomers to cationic clusters. The formalism is applied within the size-range 2≦N≦41, but could easily be extended beyondN=41. Finally, we discuss briefly the implications for the study of the dynamical response of open-shell clusters. In sharp contrast to earlier studies the functional is now corrected for self-interaction error, in a way first proposed by Pedew and Zunger [2]. This enables us to calculate reliable values for the electron affinities within ajellium-based model. This has the advantage, that we can calculate the affinities for Cu for all particle numbers for which experimental data are available. In all cases investigated we obtain excellent agreement with experiment, with pronounced shell-effects both for the electron affinities and for the binding energies, confirming in this way that the abundances map the relative stability of (Me) _N clusters, with Me being a sp-metal atom (Na, K, Li, Cu, Ag, Au etc.).     

Conformational stability and rotational energy barrier of RC60–C60R dimers: hyperconjugation versus steric effect

The conformational stability of 4, 4?? disubstituted HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers were calculated at ONIOM approach (AM1:B3LYP/6-31+G**) and density functional theory (B3LYP/6-31G**). The new evidences for stability and rotational energy barriers of these dimers were obtained by natural bond orbital, natural steric and molecular orbital analyses. Based on B3LYP/6-31G** calculations, except for RC₆₀?CC₆₀R (R?=?hydrogen, tert-butyl and trimethylsilyl) where gauche is the most stable conformer, trans is a global energy minimum. The greater stability of the gauche conformer of HC₆₀?CC₆₀H over trans is the result of hyperconjugation, which dominates the instability caused by the steric effect. By increasing the size of the substituent of HC₆₀?CC₆₀R dimers, the trans becomes sterically unstable but the hyperconjugation of bulky substituents dominates (the trans is global energy minimum). The hyperconjugation stability of RC₆₀?CC₆₀R dimers dominates until R?=?iso-propyl (higher stability of trans). In the case of bulky tert-butyl and trimethylsilyl substituents, the steric energy of trans is large and overweighs the hyperconjugation effect. This favors gauche as the most stable conformer. The calculated rotational energy barrier for HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers is less than 7.3 and more than 10?kcal/mol, respectively (depending on substitution).     

Size dependence of the electronic and magnetic properties of fullerenes (C60-C240)

We use the Gutzwiller method for studying various characteristic energies of fullerences in the free state (first and second ionization energy, electron affinity and singlet and triplet excitation energies). The stability of spin density wave (SDW) in fullerene has been investigated. The critical value of (U/β) (U intraatomic correlation energy, β transfer energy to stabilize SDW) is equal to 3 in C₆₀ and 4.25 in C₂₄₀ (icosahedrally symmetric C₂₄₀).     

Theoretical studies on the structures, heats of formation, energetic properties and pyrolysis mechanisms of nitrogen-rich difurazano[3,4-b:3′,4′-e]piperazine derivatives and their analogues

The molecular structure, heats of formation, energetic properties, strain energy and thermal stability for a series of substituted difurazano[3,4-b:3′,4′-e]piperazines and their analogues were studied using density functional theory. The results show that it is a useful way to increase the heat of formation values of energetic compounds by incorporating a five- or six-membered aromatic heterocycle to construct a fused ring system. The calculated detonation properties reveal that introducing one heterocycle to construct a fused ring structure greatly enhances their detonation properties. The substitution of the –NF₂, –NO₂ or –NHNO₂ group is very useful for enhancing the detonation performance for the substituted derivatives. According to molecular structure and natural bond orbital analysis, the introduction of the –NO₂, –NF₂ or –NHNO₂ group decreases the stability of the substituted derivative. There is a weak N–NO₂ bond conjugation in the NO₂-substituted derivatives. An analysis of the bond dissociation energies for several relatively weak bonds suggests that all the unsubstituted derivatives have good thermal stability, but the substitution of –NO₂ or –NF₂ remarkably decreases their stability. Considering the detonation performance and thermal stability, eight compounds may be considered as the potential candidates of high-energy density materials with less sensitivity.     

Effects of Al-doping on the stabilization of monoclinic LiMnO2

Electronic structures of monoclinic LiMnO₂ and LiMn_0.9375Al_0.0625O₂ with ferromagnetic (FM) and antiferromagnetic (AF) ordering have been investigated by ab initio calculation within spin-polarized generalized gradient approximation method. An Al-doping induced complicated AF configuration has been calculated to be the ground state, which suggests a robust Al-doping effect on the magnetic and electronic structures of the monoclinic LiMnO₂. The calculated Mulliken population analyses and partial density of states of Mn-3d and O-2p reveal that a single Al dopant stabilizes its six nearest-neighbor Mn ions in their respective octahedral sites, thereby hindering the migration of Mn ions into the interlayer Li sites during the Li intercalation-deintercalation and therefore improving both the structural stability and the electrochemical performance of the material. Additionally, it is found out that the Al-doping can decrease the JT effect and increase the intercalation voltage. The Al-doping-induced negative formation energy indicates that 6.25% Mn ions in monoclinic LiMnO₂ can be substituted stably by Al ions, to which the equilibrium but not metastable phase of monoclinic LiMn_0.9375Al_0.0625O₂ can be attributed.     

Systematic study of (Na n )2 dimer transient state in Na n +Na n collisions (n=8, 9, 19, and 20)

From distance dependent tight-binding molecular dynamics simulations, we systematically study the Na _n +Na _n collision dynamics around the first two closed shells (n=8 and 20). We investigate the stability of sodium cluster dimers (Na _n )₂, for many events with random relative orientation at finite temperature, various impact parameters and incident energies. We find that (Na₈)₂, (Na₉)₂, (Na₁₉)₂ and (Na₂₀)₂ can exist during about 3000 fs in central collisions while they can exist up to about ten thousands fs in peripheral collisions with larger impact parameters in fusion mechanism at c.o.m energy per atomE _cm/n=0.025 eV. We observe that the lower the incident energy, the longer the lifetime of the cluster dimers in both central and peripheral collisions. There is no apparent difference in the dynamical stability of (Na₈)₂ and (Na₉)₂, (Na₁₉)₂ and (Na₂₀)₂ although (Na₈)₂ and (Na₂₀)₂ are respectively slightly colder than (Na₉)₂ and (Na₁₉)₂ for the same incident energy per atom and the same impact parameter.     

Adiabatic polarization energy in a simple dense fluid

In this note we prey300505 sent the results of a calculation of the adiabatic electrostatic polarization energy, P⁺, of Xe⁺ in fluid Ar over the density range 0.1–1.4 g cm^?3. P₊ was expressed in terms of Lekner's screening function which, within the framework of the Kirkwood approximation, is determined by the (neutral) solute-solvent and the solvent-solvent pair correlation functions. The density dependence of P₊ can be quite well approximated (within ≈10%) by the Born charging energy with the effective ionic radius being identified with the effective hard-core diameter for the (neutral) solute-solvent separation. The P₊ data, together with experimental spectroscope results for the density dependence of the ionization potential of Xe and of CH₃I, result in preliminary information concerning the ground state energy of the quasi-free electron in fluid Ar over a wide density range.     

Poly(ether ether ketone)/poly(aryl ether sulphone) blends: thermal degradation behaviour

The thermodegradative behaviour of blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) was studied by dynamic thermogravimetry in order to analyze their thermal stability. The Freeman-Carrol differential approach was used to determine the kinetic parameters i.e. the apparent activation energy (E_a) and order of reaction (n), of the degradation process. The results indicate that the presence of one component influences the thermal stability of the other. Both, temperature for 5% weight loss (T₅) and E_a for blends show a negative deviation from the linear behaviour, which signifies a lowering of thermal stability compared to homopolymers. The decrease in the thermal stability at low concentration of PES in PEEK has been explained on the basis of chemical interactions of the degradation products of PES, which has lower induction temperature for degradation, with PEEK and also on the reduction of viscosity of the medium. But the decrease in thermal stability at low concentration of PEEK in PES is unusual and at present, without the complete elucidation of degradation mechanism in these blends, is difficult to explain.