A Theoretical Study of 1‐D and 2‐D Helium Lattices

One‐ and two‐dimensional (1‐D and 2‐D) helium lattices have been studied using ab initio RHF/6–31G** computations. Structural, physical and thermochemical properties have been calculated and analyzed for the 1‐D and 2‐D He_N lattices respectively up to N = 50 and N = 36. Asymptotic properties of the 1‐D He_N lattices are obtained by extrapolating N‐dependence properties to large values of N. Analysis of the results show that the bulk per‐atom interaction (binding) energies increase while the optimized interatomic distances (bond lengths) slightly decrease with the increase in size of the 1‐D He_N lattices and both reach their asymptotic values of 0.352 cm^?1 and 3.18775 Å, respectively. Between the square and hexagonal (packed) structures of the 2‐D He_N lattices, the latter is more favored. Extrapolated values of the calculated properties, including lattice parameter, binding and zero point energies, heat capacity, and entropy have also been calculated for both 1‐D and 2‐D He_N lattices. The surface densities for monolayer films of helium atoms with square and hexagonal configurations have been calculated to be respectively 9.84 × 10¹⁸ and 1.04 × 10¹⁹ helium atoms/cm² which are comparable to the experimental value of 2.4 × 10¹⁹ helium atom/m² well within the typical large and directional error bars of the experiments. Surface effects have been investigated by comparing the packed He_N2‐D lattices with the same value of N but with different geometries (arrangements). This comparison showed that the He_N lattices prefer arrangements with the smallest surface area.     

Occupation of trigonal vacancies and the spatial structure of superfullerides

It is assumed that occupation of small trigonal vacancies in the fcc, bcc, and hcp lattices of C₆₀ clusters leads to formation of superfulleride phases. Specific energies of 59 different phases are calculated. These data for fullendes are in agreement with their observed stability. The fcc cluster A₉C₆₀ and the bcc cluster A₁₂C₆₀ are predicted to be the most stable superfullerides. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 6, pp. 1122–1129, November–December, 1997.     

Computational Study of the Dissolution of Some Atomic and Diatomic Species in the Square and Hexagonal 2‐D Helium Lattices

Dissolution of some industrially relevant atomic and diatomic species (Ar, Ne, H, O, H₂, N₂ and O₂) in the 5 × 5 2‐D hexagonal and square helium lattices, as the model of the liquid helium cryogen, has been studied using ab initio MP2/6‐31++G computations. Structural, electronic and thermochemical properties have been calculated and analyzed for these solution lattices. Results of these calculations show that dissolution of Ar, Ne, H and H₂ species is more favored at higher temperatures. A reverse trend is observed for the dissolution of O, N₂ and O₂ species. A staggered orientation is preferred by all diatomic species in both lattices. Results of this study also show that breakage of the O₂ molecule becomes slightly easier in the 2‐D helium lattices as compared with that of the H₂ molecule. Effect of the cavity geometry and size, and position of the solute in the lattice have also been studied. Analysis of the results shows that the range of the interaction between the solute and solvent atoms is only one helium layer.     

Quasi‐Dual‐Packed‐Kerneled Au49(2,4‐DMBT)27 Nanoclusters and the Influence of Kernel Packing on the Electrochemical Gap

Although face‐centered cubic (fcc), body‐centered cubic (bcc), hexagonal close‐packed (hcp), and other structured gold nanoclusters have been reported, it was unclear whether gold nanoclusters with mix‐packed (fcc and non‐fcc) kernels exist, and the correlation between kernel packing and the properties of gold nanoclusters is unknown. A Au₄₉(2,4‐DMBT)₂₇ nanocluster with a shell electron count of 22 has now been been synthesized and structurally resolved by single‐crystal X‐ray crystallography, which revealed that Au₄₉(2,4‐DMBT)₂₇ contains a unique Au₃₄ kernel consisting of one quasi‐fcc‐structured Au₂₁ and one non‐fcc‐structured Au₁₃ unit (where 2,4‐DMBTH=2,4‐dimethylbenzenethiol). Further experiments revealed that the kernel packing greatly influences the electrochemical gap (EG) and the fcc structure has a larger EG than the investigated non‐fcc structure.     

Synthesis of Ultrathin Face‐Centered‐Cubic Au@Pt and Au@Pd Core–Shell Nanoplates from Hexagonal‐Close‐Packed Au Square Sheets

The synthesis of ultrathin face‐centered‐cubic (fcc) Au@Pt rhombic nanoplates is reported through the epitaxial growth of Pt on hexagonal‐close‐packed (hcp) Au square sheets (AuSSs). The Pt‐layer growth results in a hcp‐to‐fcc phase transformation of the AuSSs under ambient conditions. Interestingly, the obtained fcc Au@Pt rhombic nanoplates demonstrate a unique (101)_f orientation with the same atomic arrangement extending from the Au core to the Pt shell. Importantly, this method can be extended to the epitaxial growth of Pd on hcp AuSSs, resulting in the unprecedented formation of fcc Au@Pd rhombic nanoplates with (101)_f orientation. Additionally, a small amount of fcc (100)_f‐oriented Au@Pt and Au@Pd square nanoplates are obtained with the Au@Pt and Au@Pd rhombic nanoplates, respectively. We believe that these findings will shed new light on the synthesis of novel noble bimetallic nanostructures.     

Structural stability analysis of Wigner crystal with Gaussian and Yukawa‐type positive background

Calculations of the ground‐state energies of Wigner crystals having simple cubic (sc), body‐centered cubic (bcc), face‐centered cubic (fcc), diamond, and perovskite structures and (hence) the analysis of relative stability of Wigner crystals of various different structures are reported. The positive background is represented by a periodic array of Gaussians and Yukawa‐type distribution. The effects on stability of the perturbation due to the underlying lattice have been demonstrated. Among the structures, the bcc lattice still remains the most stable known arrangement and the Yukawa‐type background leads to a lower ground state energy value compared to a Gaussian type. The calculations are done for the range of the density parameter r_s corresponding to low densities for the above two cases. The range of low‐density region favorable for Wigner crystallization is found to be above r_s=20. The role of correlation energy is suitably taken into account. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Prediction of hydrocarbon enthalpies of formation by various thermochemical schemes

The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔH_f′s) for 60 closed‐shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000 , 113, 9377). This set of thermodynamic values includes ΔH_f's for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCA's applicability to a variety of hydrocarbons. The ΔH_f's were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring‐conserved isodesmic reactions were used to calculate the ΔH_f′s. Using an atomization energy approach to determine the ΔH_f′s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol^?1. The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA‐P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol^?1. This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol^?1 obtained via the Gaussian‐4 (G4), Gaussian‐3 (G3), and Gaussian‐3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions). © 2012 Wiley Periodicals, Inc.     

Model potential for beryllium clusters

A many-body potential is developed for beryllium based on the electronic information extracted from the total energy surface of clusters with up to 5 atoms. The cluster sequence of growth generated with the potential is in excellent agreement with the quantum mechanical calculations in the literature. The hcp lattice of bulk beryllium is correctly found to be more stable than fcc and bcc lattices.     

Controlled‐Atmosphere Flame Fusion Single‐Crystal Growth of Non‐Noble fcc,hcp, and bcc Metals Using Copper,Cobalt, and Iron

The growth of noble‐metal single crystals via the flame fusion method was developed in the 1980s. Since then, there have been no major advancements to the technique until the recent development of the controlled‐atmosphere flame fusion (CAFF) method to grow non‐noble Ni single crystals. Herein, we demonstrate the generality of this method with the first preparation of fcc Cu as well as the first hcp and bcc single crystals of Co and Fe, respectively. The high quality of the single crystals was verified using scanning electron microscopy and Laue X‐ray backscattering. Based on Wulff constructions, the equilibrium shapes of the single‐crystal particles were studied, confirming the symmetry of the fcc, hcp, and bcc single‐crystal lattices. The low cost of the CAFF method makes all kinds of high‐quality non‐noble single crystals independent of their lattice accessible for use in electrocatalysis, electrochemistry, surface science, and materials science.     

Near‐Infrared‐Emitting CdxHg1−xSe Nanorods Fabricated by Ion Exchange in an Aqueous Medium

Whereas CdSe nanorods that are grown in organic solution have a hexagonal wurtzite structure, which is the limiting case for exchange, HgSe is more commonly encountered as a cubic zinc blende system. An exchange process was performed at room temperature and at atmospheric pressure in an aqueous environment after phase transfer of the original CdSe nanorods, which reinforced the tendency for the endpoint of HgSe to be cubic. Consequently, we observed that under ambient conditions, the exchange process terminated with an average composition of only Cd_0.9Hg_0.1Se. Following the changes during the process by optical spectroscopy and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), we observed that the Hg²⁺ ions diffused into the rods to a point limited by the formation of stacking faults due to the different lattice structures of the two limiting cases of zinc blende and wurtzite. HAADF‐STEM and energy dispersive spectroscopy analyses also confirmed that the Hg substitution did not occur uniformly throughout the individual nanorods, as Hg‐poor and Hg‐rich regions coexist around the stacking faults. The formation of near‐infrared‐emitting alloyed Cd_xHg_1?xSe nanorods in an aqueous medium highlights the subtle dependence of the ion‐exchange process on the differences in the crystal structures of the two endpoint lattices.     

The Structure–Property Correlations in the Isomerism of Au21(SR)15 Nanoclusters by Density Functional Theory Study

Isomerism of atomically precise noble metal nanoclusters provides an excellent platform to investigate the structure–property correlations of metal nanomaterials. In this study, we performed density functional theory (DFT) and time‐dependent (TD‐DFT) calculations on two Au₂₁(SR)₁₅ nanoclusters, one with a hexagonal closed packed core (denoted as Au₂₁ ^hcp ), and the other one with a face‐centered cubic core (denoted as Au₂₁ ^fcc ). The structural and electronic analysis on the typical Au–Au and Au–S bond distances, bond orders, composition of the frontier orbitals and the origin of optical absorptions shed light on the inherent correlations between these two clusters.     

Chemical Bonding of Transition‐Metal Co13 Clusters with Graphene

We carried out density functional calculations to study the adsorption of Co₁₃ clusters on graphene. Several free isomers were deposited at different positions with respect to the hexagonal lattice nodes, allowing us to study even the hcp 2d isomer, which was recently obtained as the most stable one. Surprisingly, the Co₁₃ clusters attached to graphene prefer icosahedron‐like structures in which the low‐lying isomer is much distorted; in such structures, they are linked with more bonds than those reported in previous works. For any isomer, the most stable position binds to graphene by the Co atoms that can lose electrons. We find that the charge transfer between graphene and the clusters is small enough to conclude that the Co–graphene binding is not ionic‐like but chemical. Besides, the same order of stability among the different isomers on doped graphene is kept. These findings could also be of interest for magnetic clusters on graphenic nanostructures such as ribbons and nanotubes.     

Li15Al3Si6 (Li14.6Al3.4Si6), a compound displaying a heterographite‐like anionic framework

The title compound, lithium aluminium silicide (15/3/6), crystallizes in the hexagonal centrosymmetric space group P6₃/m. The three‐dimensional structure of this ternary compound may be depicted as two interpenetrating lattices, namely a graphite‐like Li₃Al₃Si₆ layer and a distorted diamond‐like lithium lattice. As is commonly found for LiAl alloys, the Li and Al atoms are found to share some crystallographic sites. The diamond‐like lattice is built up of Li cations, and the graphite‐like anionic layer is composed of Si, Al and Li atoms in which Si and Al are covalently bonded [Si—Al = 2.4672 (4) Å].     

Modulating fcc and hcp Ruthenium on the Surface of Palladium–Copper Alloy through Tunable Lattice Mismatch

Herein, we report an epitaxial‐growth‐mediated method to grow face‐centered cubic (fcc) Ru, which is thermodynamically unfavorable in the bulk form, on the surface of Pd–Cu alloy. Induced by the galvanic replacement between Ru and Pd–Cu alloy, a shape transformation from a Pd–Cu@Ru core–shell to a yolk–shell structure was observed during the epitaxial growth. The successful coating of the unconventional crystallographic structure is critically dependent on the moderate lattice mismatch between the fcc Ru overlayer and PdCu₃ alloy substrate. Further, both fcc and hexagonal close packed (hcp) Ru can be selectively grown through varying the lattice spacing of the Pd–Cu substrate. The presented findings provide a new synthetic pathway to control the crystallographic structure of metal nanomaterials.     

Synthesis,Crystal Structure and Thermochemical Properties of a One‐Dimensional Chain Complex [Cd(succ)PIP]n

A new complex, [Cd(succ)PIP]_n (PIP=2‐phenyl‐imidazo[4,5‐f]1,10‐phenanthroline, H₂‐succ=succinate), was synthesized and characterized by X‐ray crystallography, elemental analysis, and TG‐DTG. The results show that the complex crystallizes in an orthorhombic space group Pcca; a=14.065(2) Å, b=9.901(8) Å, c=28.933(2) Å and Z=8. The structure of the complex is one‐dimensional chain [Cd(succ)PIP]_n, and each Cd²⁺ is five‐coordinated by two chelating nitrogen atoms from one PIP ligand, three oxygen atoms from three different succ dianionic ligands to form a distorted trigonal‐bipyramida geometry. The constant‐volume combustion energy of the complex, Δ_cU, was determined by an intelligent micro‐rotating‐bomb calorimeter (IMRBC‐type I) at 298.15 K. Then the standard molar enthalpy of combustion, Δ_cH_m^?, and the standard molar enthalpy of formation, Δ_fH_m^? have been calculated.     

Energetics and Structures of Charged Helium Clusters: Comparing Stabilities of Dimer and Trimer Cationic Cores

We present accurate ab initio calculations of the most stable structures of He_n⁺ clusters in order to determine the more likely ionic core arrangements existing after reaching structural equilibrium of the clusters. Two potential energy surfaces are presented: one for the He₂⁺ and the other with the He₃⁺ linear ion, both interacting with one He atom. The two computed potentials are in turn employed within a classical structure optimization where the overall interaction forces are obtained within the sum‐of‐ potentials approximation described in the main text. Because of the presence of many‐body effects within the ionic core, we find that the arrangements with He₃⁺ as a core turn out to be energetically preferred, leading to the formation of He₃⁺(He)_n?3 stable aggregates. Nanoscopic considerations about the relative stability of clusters with the two different cores are shown to give us new information on the dynamical processes observed in the impact ionization experiments of pure helium clusters and the importance of pre‐equilibrium evaporation of the ionic dimers in the ionized clusters.     

Metal Vacancy Ordering in an Antiperovskite Resulting in Two Modifications of Fe2SeO

Small, red Fe₂SeO single crystals in two modifications were obtained from a CsCl flux. The metastable α‐phase is pseudo‐tetragonal (Cmce, a=16.4492(8) Å, b=11.1392(4) Å, c=11.1392(4) Å), whereas the β‐phase is trigonal (P3₁, a=9.8349(4) Å, c=6.9591(4) Å)) and thermodynamically stable within a narrow temperature range. Both crystal structures were solved from twinned specimens. The enantiomers of the β‐phase appear as racemic mixtures. Selenium and oxygen form two individual interpenetrating primitive cubic lattices, giving a bcc packing. A quasi‐octahedrally coordinated iron atom is found close to the center of each surface of the selenium sublattice. The difference between the α‐ and β‐phases is the distribution of iron at 2/3 of the surfaces. α‐ and β‐Fe₂SeO are comparable with metal‐vacancy‐ordered antiperovskites. Each Fe/O lattice can also be described in terms of vertex‐sharing OFe₄ tetrahedra, with a crystal structure similar to that of an antisilicate. Iron is divalent and has a high‐spin d⁶ (S=2) configuration. The β‐phase exhibits magnetoelectric coupling.     

The Minimal Occupancy Level of the Clathrate Hydrate Host Lattice and the Intercalation Heat of the Guest Molecules in the Chlorine Hydrates

The minimal occupancy level (θ_min) of the clathrate lattice of gas molecules is defined as the number of guest molecules in the host clathrate lattice, which can stabilize the thermodynamically unstable empty cage by covering the energy demand of the transformation of hexagonal ice into empty clathrate lattice (ΔH_trans). The θ_min values for chlorine hydrate were determined from the n = f(p)_T=const. relationship and the average molar intercalation heat of chlorine in the type I clathrate lattice was also calculated for both type of cavities.     

Structural Evolution of Boron Clusters on Ag(111) Surfaces – From Atomic Chains to Triangular Sheets with Hexagonal Holes

Unlike graphene and other 2D materials, borophene is 2D polymorphic with diverse hexagonal holes (HHs)-triangles ratios and the concentrations of HHs are highly substrate dependent. Here, we systematically explored the evolution of boron cluster on Ag(111) surface, B_N@Ag(111) (N=1∼36), to understand the nucleation of 2D boron sheet on metal surface. Our calculation showed that, with the size increasing, the structures of most stable B_N clusters undergo an evolution from compact triangular lattice, such as double-chains or triple-chains, to the ones with mixed triangular-hexagonal lattices. The first single-HH appears at N=12 and the first double-HH appears at N=27. The stability of large B_N clusters with mixed structures is derived from the charge transfer between triangular lattice and the HHs, as well as between the substrates and the B_N clusters. Our results provide a deep understanding on the formation of small boron clusters in the initial nucleation stage of borophene growth.     

Site Preference in Multimetallic Nanoclusters: Incorporation of Alkali Metal Ions or Copper Atoms into the Alkynyl‐Protected Body‐Centered Cubic Cluster [Au7Ag8(C≡CtBu)12]+

The synthesis, structure, substitution chemistry, and optical properties of the gold‐centered cubic monocationic cluster [Au@Ag₈@Au₆(C≡C^tBu)₁₂]⁺ are reported. The metal framework of this cluster can be described as a fragment of a body‐centered cubic (bcc) lattice with the silver and gold atoms occupying the vertices and the body center of the cube, respectively. The incorporation of alkali metal atoms gave rise to [M_nAg_8?nAu₇(C≡C^tBu)₁₂]⁺ clusters (n=1 for M=Na, K, Rb, Cs and n=2 for M=K, Rb), with the alkali metal ion(s) presumably occupying the vertex site(s), whereas the incorporation of copper atoms produced [Cu_nAg₈Au_7?n(C≡C^tBu)₁₂]⁺ clusters (n=1–6), with the Cu atom(s) presumably occupying the capping site(s). The parent cluster exhibited strong emission in the near‐IR region (λ_max=818 nm) with a quantum yield of 2 % upon excitation at λ=482 nm. Its photoluminescence was quenched upon substitution with a Na⁺ ion. DFT calculations confirmed the superatom characteristics of the title compound and the sodium‐substituted derivatives.