Structural stability and energetics of C,Si, and Ge microclusters: empirical many-body potential energy function calculation

The structural stability and energetics of carbon, silicon, and germanium microclusters containing 3?7 atoms have been investigated by using a recently developed empirical many-body potential energy function (PEF), which comprises two- and three-body atomic interactions. The PEF satisfies both bulk cohesive energy per atom and bulk stability exactly. It has been found that the most stable C_3?4 microclusters are linear withD _∞h symmetry but C_5?7 microclusters are planar withD _nh symmetry. Silicon and germanium microclusters show similar structural stability. TheX _n (X=Si, Ge;n=3?7) microclusters are found to be most stable in the following forms:X ₃ is triangular withD _3h symmetry,X ₄ is tetragonal withT _d symmetry,X ₅ is square pyramidal withD _4h symmetry,X ₆ is bipyramidal square withO _h symmetry, and finallyX ₇ is square pyramidal having two atoms underneath withD _2h symmetry.     

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).     

Natural states of interacting systems and their use for the calculation of intermolecular forces. II. Natural states in the asymptotic 1/R expansion

The asymptotic natural states i.e. the natural states of the subsystems in the various terms of the asymptotic expansion of the primitive wavefunction of the supersystem are defined. Integro-differential equations are derived that allow the direct calculation of these asymptotic natural states and from them, the terms C₆, C₈ etc. in the asymptotic 1/R expansion of the interaction energy. A pictorial interpretation of these states is given and the rapid convergence of the expansion in natural states is explained. The transferability of certain matrix elements is demonstrated and a virtually exact combination rule for the calculation of the C_k coefficients between different atoms from those for like atoms is obtained- The effects of intraatomic electron correlation on the C_k as well as the justification of a core-valence separation are discussed.     

Ab initio study on the stability,geometry and electronic structure of the cyclopropenyl system

Results of an LCAO-MO-SCF investigation into the stability and geometry of the cyclopropenyl cation (C₃H₃⁺), anion (C₃H₃^?) and radical (C₃H₃^o) are presented. By independently varying the two relevant bond angles, the shape of the potential energy curves in the corresponding two-parameter space for these three species has been obtained. It is found that the cation is most stable in the D_3h nuclear configuration, while the anion has minimum energy in the C_s symmetry. In the specific case of the C₃H₃ radical, which exhibits an orbital, as well as spin-degeneracy, in the D_3h configuration, a strong Jahn-Teller effect is observed, leading to estimates for the non-symmetrical equilibrium configuration at C_2v and for the distortion energy of 12 kcal/mole from the D_3h symmetry. In terms of the population analysis and the contour diagrams, the electronic charge distribution has also been studied for these species in their most stable configurations.     

Metallic mono-layered C4N as a potential anode material for Na-ion batteries: A first-principles investigation

First-principles calculations based on density functional theory were used to study the adsorption behaviors of Na on metallic mono-layered C₄N as electrode materials for Na-ion batteries. The adsorption energy of Na atom was calculated to be 2.05 eV, which is much higher than Na bulk cohesive energy and sufficiently ensure stability and safety. It is worth noting that the Dirac-type band structure of mono-layered C₄N has a ultrahigh capacity of 1945.89 mAh/g for Na-ion batteries anode in theory. Remarkably, 2D C₄N has a low diffusion barrier 0.071 and 0.075 eV for path I and path II, respectively. The average open circuit voltage is 1.383 eV when nine Na ions adsorbed on one side of mono-layered C₄N. All the excellent properties show that the mono-layered C₄N will be a potential development of anode materials for Na-ion batteries.     

Low-temperature specific heat of the cluster compound Au55 (P(C6H5)3)12Cl6

Specific-heat measurements on the cluster compound Au₅₅(P(C₆H₅)₃)₁₂Cl₆ at temperatures 0.06 K ≤T≤3 K and in magnetic fields 0≤B≤6 T are reported. While above 0.6 K the specific heatC is dominated by the inter-cluster vibrational contribution observed previously, an anomalous increase ofC towards lowT is observed below 0.3 K, withC ～T ^?2. This contribution develops into a Schottky-like anomaly forB≥0.4 T, indicating that it might be attributed to local moments which are also observed in ESR measurements. From the height of the anomaly one can infer that approximately one tenth of the Au₅₅ clusters carry a magnetic moment. For 0.6 K≤T≤1 K andB=0 our data indicate the absence of a linear electronic specific-heat contribution expected for bulk Au. This possibly constitutes the first direct observation of the quantum-size effect on electronic energy levels in the specific heat.     

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface (C^o_M) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution (C_M), the exact value depending on the deposition time t_d. Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C_L/C_M, is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C^o_M/C_M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C_L/C_M. The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.     

First principle study of magnetic and electronic properties of single X (X = Al,Si) atom added to small carbon clusters (C<Subscript> <Emphasis Type="Italic">n</Emphasis> </Subscript>X, <Emphasis Type="Italic">n</Emphasis> = 2–10)

In this paper, the magnetic and electronic properties of single aluminum and silicon atom added to small carbon clusters (C_nX; X = Al, Si; n = 2–10) are studied in the framework of generalized-gradient approximation using density functional theory. The calculations were performed for linear, two dimensional and three dimensional clusters based on full-potential local-orbital (FPLO) method. The total energies, HOMO–LUMO energy gap and total magnetic moments of the most stable structures are presented in this work. The calculations show that C_nSi clusters have more stability compared to C_nAl clusters. In addition, our magnetic calculations were shown that the C_nAl isomers are magnetic objects whereas C_nSi clusters are nonmagnetic objects.     

Stability and vibrational spectra of toroidal isomers of C 240

To study the thermodynamic and mechanical stability of toroidal isomers of C ₂₄₀, we use a semi-empirical tight-binding theory and calculate their electronic structure, cohesive energy and vibrational spectra within the harmonic approximation. From these, we deduce their free energy at temperatures up to 1500K. The results are also compared to the isomer with icosahedral symmetry. Finally, we discuss within this approach, their stability and abundance.     

Phase behavior of dodecane–hexadecane mixtures in bulk and confined in SBA-15

Phase behaviors of dodecane–hexadecane (n-C₁₂H₂₆–C₁₆H₃₄, C₁₂–C₁₆) binary mixtures in bulk and confined in SBA-15 (pore diameters 3.8, 9.5, and 17.2 nm) are investigated using differential scanning calorimetry. According to the thermal analysis, the bulk mixtures belong to a system of partial miscibility with two solid solutions and a eutectoid in the range of mole fraction $ x_{{{\text{C}}_{ 1 6} }} $  = 0.1–0.8. Under confinement, phase behavior of C₁₂–C₁₆ mixtures is distinct from the bulk. Inside pores of SBA-15 (3.8 nm), the solid mixtures has only a melting boundary. In the pores larger than 9.5 nm, phase behaviors of the mixtures show some resemblance to the bulk system. The growth of the phase diagram with the pore diameter clearly shows the size effect on the phase behavior of the confined mixtures. In comparison with those of chain length difference of pure components of two carbon atoms or less, C₁₂–C₁₆ mixtures exhibit different phase behaviors not only in the bulk but also in the confined state.     

(p, ϱ, T) for {(1−x)C6H6 + xC6D6} and {(1−x)C6H6 + xC6F6} in the range 298 to 373 K and 0.1 to 400 MPa

The behaviour of (p, ?, T) for C₆H₆, C₆F₆, and five mixtures, and of C₆D₆ and (0.5C₆H₆ + 0.5C₆D₆) has been determined at 298.2, 323.2, 348.2, and 373.2 K, and from 0.1 MPa, or saturation pressure, to the point of onset of solidification or to 400 MPa. The experimental results are tabulated and the isothermal densities are represented by a polynomial equation for the secant bulk modulus in terms of the pressure. The temperature and pressure dependence of the molar excess volume is described.     

Heat capacity, enthalpy, entropy, and gibbs energy of terbium diboride as determined from calorimetric measurements within 5–300 K

The heat capacity at constant pressure C _p (T) of terbium diboride synthesized from elements via an intermediate hydride phase was studied experimentally within 5–300 K. A ferromagnetic phase transition manifests itself in the C _p (T) dependence as a sharp maximum at 142.4 ± 0.1 K. The C _p (T) dependence was used to calculate the tempreature dependences of the enthalpy, entropy, and the Gibbs energy and to determine the parameters of the electronic, lattice, and magnetic contributions to the heat capacity of TbB₂.     

In Situ Nitrogen Functionalization of 2D-Ti3C2Tx-MXenes for High-Performance Zn-Ion Supercapacitor

Zinc (Zn) ion supercapacitors (ZISCs) have attracted considerable attention as a viable energy storage technology because they are cost-effective, safe, and environmentally friendly. However, cathode materials with suitable properties are rare and need to be explored. In this regard, metal carbides (MXenes) are a good choice for capacitive energy storage, but they exhibit low capacitance. The energy storage performance of MXenes can be bossed using functionalization with heteroatom doping, e.g., nitrogen (N), to simultaneously modify ZISCs’ fundamental characteristics and electrochemical properties. Herein, we present an in-situ N-functionalization of Ti₃C₂T_x-MXene via a hydrothermal reaction with urea (denoted as N-Ti₃C₂T_x-MXene). N-functionalization into Ti₃C₂T_x-MXene raised Ti₃C₂T_x-MXene’s interlayer spacing and boosted the Zn-ion storage in 1 M ZnSO₄ electrolyte. The N-Ti₃C₂T_x-MXene electrode delivered an excellent specific capacitance of 582.96 F/g at 1 A/g and retained an outstanding cycle stability of 94.62% after 5000 cycles at 10 A/g, which is 1.8 times higher than pristine Ti₃C₂T_x-MXene at identical conditions. Moreover, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum capacitance of 153.55 F/g at 1 A/g, retained 92% of its initial value after 5000 cycles, and its Coulombic efficiency was ~100%. This strategy considerably reduced Ti₃C₂T_x-MXene nanosheet restacking and aggregation and enhanced electrochemical performance. Further, this research elucidated N-Ti₃C₂T_x-MXene’s charge–storage process and offered a fresh approach to the rational design of novel electrode materials for ZISCs.     

Conformational stability of trivinylborane from vibrational spectra and ab initio calculations

The conformational stability, barriers to internal rotation and vibrational frequencies of trivinylborane have been determined from the vibrational spectra and ab initio calculations. The ab initio calculations have been carried out utilizing the RHF/3-21G, RHF/6-31G*, and MP2/6-31G* basis sets and support the vibrational data that there are two stable conformations in the fluid phases separated by a relatively small energy difference. One of the conformations is a near-planar form which has the three vinyl groups twisted out of the BC₃ plane and belongs to the C₃ point group. The other conformer has a non-planar structure and belongs to the C₁ point group. These and other calculated results are compared to the corresponding quantities obtained from the experiment.     

Molecular-dynamics simulation of the structural stability,energetics, and melting of Cu n(n = 13–135) clusters

Cluster properties of copper have been investigated using the Molecular-Dynamics md technique. The structural stability and energetics of spherical Cu_n (n = 13–135) clusters have been investigated at temperatures T = 1 K and T = 300 K. It has been found that the average interaction energy per atom in the cluster decreases and reaches an asymptotic value as cluster size increases. The melting behaviour of clusters n = 13 and n = 55 have been investigated. It has been found that the melting temperature decreases as cluster size increases, and for clusters with multishell structures melting starts from the outermost shell. In the simulation an emprical potential energy function (PEF) proposed by Erkoç has been used, which contains two-body atomic interactions.     

Flattened fullerenes

Quantum-chemical calculations of giant flattened fullerenes C _n (lentil-shaped) have been carried out. The topology, molecular and electronic structure of these fullerenes have been studied. Such molecules consist of two identical coronenoid fragments of a graphite layer, which are arranged one above the other, and a system of polycondensed five- and six-membered cycles, which form a side surface of the cluster. Polyhedral structures with isolated pentagons of three symmetry types (D _6h ,D _6d , andD _3h ) have been considered. The topology of these structures is described in terms of planar molecular graphs. Electronic structures of eleven flattened lentil-shaped C _n clusters (n = 72–216) have been studied in the π approximation. Most of the considered systems have closed or quasi-closed electron shells (according to Hückel) and rather large energy gaps separating the highest occupied and lowest unoccupied MO, which is indicative of their kinetic stability. Fragments of the potential energy surfaces of the C₇₂ and C₉₆ fullerenes have been studied by the MNDO, AM1, and MNDO/PM3 methods. For the C₉₆ cluster, two local energy minima, which correspond to the lentil-shaped isomers withD _6h andD _6d symmetry, have been determined. As a result of optimization of geometric parameters, it was found that all three methods give close values of heights (H = 6.7 Å) and diameters (D = 9.8 Å) for both isomers. The clusters change to quasi-two-dimensional systems (H«D) with increasing sizes of coronenoid fragments.     

Size-Dependent Mechanical Properties in Ni–Al Intermetallic Nano-particles Studied Using Embedded Atom Method

The energy and elastic constants C ₁₁, C ₁₂, and C ₄₄ as a function of particle size for Ni–Al intermetallic nano-particles are investigated by using embedded-atom method. We find that for nano-particles of large size, the size-dependent change in C ₁₂ is slightly different from those in C ₁₁ and C ₄₄, which is attributed to the internal strains. We also find that for the nanoparticles of size larger than 2.5 nm, the calculated C ₁₁, C ₁₂, and C ₄₄ of 1.52, 0.88 and 0.77 eV/ų are in agreement with the available experimental results.     

First-principles investigation of the binary AB2 type Laves phase in Mg–Al–Ca alloy: Electronic structure and elastic properties

First-principles calculations have been carried out to investigate the electronic structure and mechanical properties of the main binary Laves phase CaMg₂, CaAl₂ and MgAl₂ with C14, C15 and C36 structures in Mg–Al–Ca alloy, respectively. The optimized structural parameters were in very good agreement with the experimental values. The calculated heat of formation and cohesive energy showed that the C15-CaAl₂ Laves phase was of the strongest alloying ability and structural stability. The electronic density of states (DOS) and charge density distribution were given. The elastic parameters C_ij were calculated, then the bulk modulus, shear modulus, Young's modulus, Possion's ratio and anisotropy value were derived. The ductility and plasticity were discussed in comparison with the previous experimental and theoretical data. The results showed that C14-MgAl₂ is of the best ductility and C15-MgAl₂ is of the best plasticity in the investigated binary alloys.     

Vibrational relaxation of ethylene oxide and ethylene oxide-rare-gas mixtures

The collision-induced vibrational energy relaxation of ethylene oxide (C₂H₄O) was studied by means of laser-induced fluorescence. The time-dependent population of the vibrational modes v₃ and v₅/v₁₂ was measured after excitation of CH-stretching vibrations near 3000 cm^?1. Rate constants for the vibrational energy transfer by collisions with C₂H₄O and the rare gases are deduced, and a simplified model for the vibrational relaxation of C₂H₄O is discussed.     

Encapsulation of transition metals in aluminum nitride fullerene: TM@(AlN)12 (TM = Ti, Mn, Fe, Co, and Ni)

The structure and stability of endohedral TM@(AlN)₁₂ (TM = Ti, Mn, Fe, Co, Ni) complexes are studied at the level of density functional theory. It is found that complexes with TM = Mn, Fe, and Ni are energy minimum structures with TM at the cage center in T _h symmetry, while those with TM = Ti and Co have more negative inclusion energies and the off-centered structures with TM placed towards one hexagon face in C ₁ symmetry. The calculations predict that the HOMO and LUMO energy gap of TM@(AlN)₁₂ differs from those of the (AlN)₁₂ cage and a free TM atom. The amount of charge that is transferred from the encapsulated guests to the cage increases with the atomic radius. The electronic and magnetic properties of TM@(AlN)₁₂ are discussed.