Metal clusters that freeze into high energy geometries

Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.     

溴化钾团簇相变的分子动力学研究

众所周知，团簇的尺寸介于原子或分子与大块物质之间。由于它有较大的表面／体积比而有独特的物理化学性质。团簇在成核，晶体生长，材料科学和纳米器件加工等领域起了至关重要的作用。近年来，计算机模拟已成为物理，化学，生物，天文，地质等领域的重要研究工具。计算机模拟不仅可以     

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.     

Statistical fragmentation of hot atomic metal clusters

Fragmentation processes of highly excited neutral and charged atomic metal clusters are studied in the framework of an equilibrium statistical model. In the particular case of hot (near and above melting) neutral and charged sodium clusters of 100 and 200 atoms, a microcanonical Metropolis sampling is used to compute mass (or charge) correlation functions as a function of the excitation energy. This method allows to take the strong anharmonicities in the internal phonon spectrum realistically into account which are linked to the internal structural changes like melting. It is found that, at high enough excitation energy, the system exhibits a phase transition. This phase transition is specific for fragmenting finite systems. From the shape of the caloric curve one sees that the two phases involved are connected by a van der Waals loop characterizing a first order phase transition. Here we observe an enhanced fission and multifragmentation into two or more charged clusters with more than 10 atoms each. Various fragment correlations are studied.     

Pt–Co nanocluster in hollow carbon nanospheres

Recently, it has been reported that small Pt/Co bimetallic nanoclusters into hollow carbon spheres (HCS) show outstanding catalytic performances in deriving biomass fuels due to the small particle size and the homogeneous alloying. Thus, the knowledge about the thermal evolution and stability of the nanoclusters into the HCS has a great importance. We have simulated the heating process beyond the melting point for the bare and encapsulated Pt/Co clusters into the HCS with the different sizes of 55, 147, and 309. The different thermodynamic and structural properties of the nanoclusters have also been investigated in this work. Our results show that the nanoclusters are more stable into the HCS than the bare clusters. The melting points of the supported clusters are also higher than the unsupported clusters. The confined nanoclusters have also lower excess energy values than the bare clusters which means that the encapsulation of Pt/Co nanoclusters into the HCS is favorable. The structural investigations show that a core–shell structure cannot be observed for the different supported and unsupported clusters and the initial mixed structure of the different nanoclusters remains also at the melting points. To more investigate this claim, the radial chemical distribution function (RCDF) and radial distribution function (RDF) of the bare and encapsulated clusters have also been calculated and discussed. © 2018 Wiley Periodicals, Inc.     

Cooperative dynamics and self-diffusion in superheated crystals

Molecular dynamics simulations have been used to study the atomistic scale dynamics of superheated crystals under different temperature and pressure conditions. The limit of superheating was determined by monitoring a suitable order parameter. The occurrence of homogeneous melting was related to the generation of structural defects characterized by the presence of pairs of particles having defective coordination. At temperatures close to the homogeneous melting point such particles formed extended stringlike clusters. Particles involved in clusters change continuously as a result of local structural rearrangements. These can result in the displacement of particles from one lattice site to another, thus providing a mechanism for self-diffusion.     

A molecular dynamics study of small water clusters comparing two flexible models for water

We report results from a molecular dynamics study of small water clusters, (H₂O) _n=2,3,4,6,8, comparing the recent polarizable, dissociable (PD) model of Halley et al. with the central force Stillinger-Rahman (SR) model. Dynamics calculations in the microcanonical ensemble quantified short-time averaged temperatures and RMS bond length fluctuations, which, in combination with simulated thermal quenching, identified structures and structural and phase changes of the clusters. Both models generally pointed to similar global minimum energy configurations, but local minima configurations, relative energies, and RMS bond length fluctuations varied more significantly. The PD model, with its incorporation of many-body effects, more accurately reproduced the structural features of clusters predicted from ab initio calculations, although it over-estimated the binding energies. Solid-like, pre-melting, and liquid-like states were identified based upon RMS bond length fluctuations for the O-O, O-H and H-H pairs. The SR and PD models yielded very similar trends in melting temperature as a function of cluster size. This suggests that general trends in the melting phenomenon do not depend strongly upon details of the models. The melting temperatures for the dimer and tetramer closely resembled the bulk melting temperature, while those for other sizes were considerably lower.     

Competing thermal activation mechanisms in the meltinglike transition of Na(N) (N = 135-147) clusters

The meltinglike transition in unsupported icosahedral Na(N)() clusters, with N = 135-147, has been studied by isokinetic molecular dynamics simulations based on an orbital-free version of density functional theory. A maximum in the melting temperature, T(m), is obtained for Na141, while the latent heat, deltaE, and entropy of melting, deltaS, are maximal for Na147. These observations are in close agreement with calorimetric experiments on N clusters. The size evolution of deltaS is rationalized by the emergence of important premelting effects associated with the diffusive motion of atomic vacancies at the cluster surface. The precise location of the maximum in T(m) is explained in terms of two different thermally activated structural instability mechanisms which trigger the meltinglike transition in the size ranges N = 135-141 and N = 141-147, respectively.     

Melting and freezing characteristics and structural properties of supported and unsupported gold nanoclusters

Molecular dynamics simulations in conjunction with MEAM potential models have been used to study the melting and freezing behavior and structural properties of both supported and unsupported Au nanoclusters within a size range of 2 to 5 nm. In contrast to results from previous simulations regarding the melting of free Au nanoclusters, we observed a structural transformation from the initial FCC configuration to an icosahedral structure at elevated temperatures followed by a transition to a quasimolten state in the vicinity of the melting point. During the freezing of Au liquid clusters, the quasimolten state reappeared in the vicinity of the freezing point, playing the role of a transitional region between the liquid and solid phases. In essence, the melting and freezing processes involved the same structural changes which may suggest that the formation of icosahedral structures at high temperatures is intrinsic to the thermodynamics of the clusters, rather than reflecting a kinetic phenomenon. When Au nanoclusters were deposited on a silica surface, they transformed into icosahedral structures at high temperatures, slightly deformed due to stress arising from the Au-silica interface. Unlike free Au nanoclusters, an icosahedral solid-liquid coexistence state was found in the vicinity of the melting point, where the cluster consisted of coexisting solid and liquid fractions but retained an icosahedral shape at all times. These results demonstrated that the structural stability in the structures of small Au nanoclusters can be enhanced through interaction with the substrate. Supported Au nanoclusters demonstrated a structural transformation from decahedral to icosahedral motifs during Au island growth, in contrast to the predictions of the minimum-energy growth sequence: icosahedral structures appear first at very small cluster sizes, followed by decahedral structures, and finally FCC structures recovered at very large cluster sizes. The simulations also showed that island shapes are strongly influenced by the substrate, more specifically, the structural characteristic of a Au island is not only a function of size, but also depends on the contact area with the surface, which is controlled by the wetting of the cluster to the substrate.     

金属铜升温熔化过程的分子动力学模拟

采用分子动力学方法模拟了金属铜的升温熔化过程.原子间作用势采用FS (Finnis-Sinclair)势，结构分析采用双体分布函数(PCF)、均方位移(MSD)等方法.计算结果表明,在连续升温过程中，金属铜在1444 K熔化，在该熔化点的扩散系数为4.31×10－9 m2•s－1.上述结论与实验值相当接近，并且比之采用EAM镶嵌原子势所作模拟得到的结果更佳，说明FS势可以用来处理象液铜这样较复杂的无序体系.本文指出了升温速率在金属熔化过程中所起的作用.     

Structural transitions and melting of copper clusters

Molecular dynamics is used to study the melting and structural transitions of small copper clusters. The melting temperature is found to be proportional to the average coordination number. Small icosahedral clusters melt at slightly higher temperatures than the cubic structures. Small cuboctahedral clusters are not stable but transform via a nondiffusive transition to icosahedral structure.     

Vibrations and thermodynamics of clusters of polycyclic aromatic hydrocarbon molecules: the role of internal modes

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.     

自由表面的Ni原子团簇的熔化

采用分子动力学模拟技术研究了不同尺寸的Ni原子团簇的熔化过程.团簇的最初构型为FCC结构.研究结果表明,原子团簇的熔化温度与原子团簇中原子的个数有关,团簇的熔化首先从表面开始,当外层原子成为液态后,整个团簇的熔化从液态层开始,直至核心区域.该熔化过程可以被称为非均质熔化,自由表面充当非均质形核位置.作为对比,对无自由表面的大块固态Ni的熔化过程也进行了模拟,其熔化温度高于实验温度约400 K.表明对无自由表面的大块固态的熔化过程,液相形成无非均质形核位置,熔化的本质过程受均质形核机理控制.     

Structure of starches extracted from near isogenic wheat lines

Differential scanning calorimetry (DSC), acidic hydrolysis and different physico-chemical approaches were used to study thermodynamic and structural characteristics of starches from near-isogenic wheat lines to establish the effect of different combinations of active granule-bound starch synthase isoforms, taking part in amylose biosynthesis, on the structure and thermodynamic properties of starches. Obtained results suggest that the effect of different GBSS I combinations is realized through altered amylose localization within starch granules, reflecting in changes of melting temperature of crystalline lamellae (T _m) and rates of acidic hydrolysis. It has also been demonstrated that changes in T _m values for native wheat starches are determined by amylose content in amylopectin clusters.     

Melting phenomena: effect of composition for 55-atom Ag-Pd bimetallic clusters

Understanding the composition effect on the melting processes of bimetallic clusters is important for their applications. Here, we report the relationship between the melting point and the metal composition for the 55-atom icosahedral Ag-Pd bimetallic clusters by canonical Monte Carlo simulations, using the second-moment approximation of the tight-binding potentials (TB-SMA) for the metal-metal interactions. Abnormal melting phenomena for the systems of interest are found. Our simulation results reveal that the dependence of the melting point on the composition is not a monotonic change, but experiences three different stages. The melting temperatures of the Ag-Pd bimetallic clusters increase monotonically with the concentration of the Ag atoms first. Then, they reach a plateau presenting almost a constant value. Finally, they decrease sharply at a specific composition. The main reason for this change can be explained in terms of the relative stability of the Ag-Pd bimetallic clusters at different compositions. The results suggest that the more stable the cluster, the higher the melting point for the 55-atom icosahedral Ag-Pd bimetallic clusters at different compositions.     

Structural transitions and melting in LJ(74-78) Lennard-Jones clusters from adaptive exchange Monte Carlo simulations

Phase changes in Lennard-Jones (LJ) clusters containing between 74 and 78 atoms are investigated by means of exchange Monte Carlo simulations in the canonical ensemble. The replica temperatures are self-adapted to facilitate the convergence. Although the 74- and 78-atom clusters have icosahedral global minima, the clusters with 75-77 atoms have decahedral ground-state structures and they undergo a structural transition to icosahedral minima before melting. The structural transitions are characterized by quenching and by looking at the Q4 and Q6 orientational bond order parameters. The transition temperatures are estimated to be 0.114, 0.065, and 0.074 reduced units for LJ75, LJ76, and LJ77, respectively. These values, their ordering and the associated latent heats are compared with other estimates based on the harmonic superposition approach.     

Morphology of homogeneous copolymers of ethylene and 1‐octene. III. Structural changes during heating as revealed by time‐resolved SAXS and WAXD

The structural changes of two linear polyethylenes, LPEs, with different molar mass and of two homogeneous copolymers of ethylene and 1‐octene with comparable comonomer content but different molar mass were monitored during heating at 10 °C per minute using synchrotron radiation SAXS. Two sets of samples, cooled at 0.1 °C per minute and quenched in liquid nitrogen, respectively, were studied. All LPEs display surface melting between room temperature and the end melting temperature, whereas complete melting, according to lamellar thickness, only occurs at the highest temperatures where DSC displays a pronounced melting peak. There is recrystallization followed by isothermal lamellar thickening if annealing steps are inserted. The lamellar crystals of slowly cooled homogeneous copolymers melt in the reverse order of their formation, that is, crystals melt according to their thickness. Quenching creates unstable crystals through the cocrystallization of ethylene sequences with different length. These crystals repeatedly melt and co‐recrystallize during heating. The exothermic heat due to recrystallization partially compensates the endothermic heat due to melting resulting in a narrow overall DSC melting peak with its maximum at a higher temperature than the melting peak of slowly cooled copolymers. With increasing temperature, the crystallinity of quenched copolymers overtakes the one of slowly cooled samples due to co‐recrystallization by which an overcrowding of leaving chains at the crystal surfaces is avoided. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1975–1991, 2000     

Origin of the diverse melting behaviors of intermediate-size nanoclusters: theoretical study of AlN (N = 51-58, 64)

Microscopic understanding of thermal behaviors of metal nanoparticles is important for nanoscale catalysis and thermal energy storage applications. However, it is a challenge to obtain a structural interpretation at the atomic level from measured thermodynamic quantities such as heat capacity. Using first-principles molecular dynamics simulations, we reproduce the size-sensitive heat capacities of Al(N) clusters with N around 55, which exhibit several distinctive shapes associated with diverse melting behaviors of the clusters. We reveal a clear correlation of the diverse melting behaviors with cluster core symmetries. For the Al(N) clusters with N = 51-58 and 64, we identify several competing structures with widely different degree of symmetry. The conceptual link between the degree of symmetry (e.g., T(d), D(2d), and C(s)) and solidity of atomic clusters is quantitatively demonstrated through the analysis of the configuration entropy. The size-dependent, diverse melting behaviors of Al clusters originate from the reduced symmetry (T(d) → D(2d) → C(s)) with increasing the cluster size. In particular, the sudden drop of the melting temperature and appearance of the dip at N = 56 are due to the T(d)-to-D(2d) symmetry change, triggered by the surface saturation of the tetrahedral Al(55) with the T(d) symmetry.     

Revisiting the structural instability observed in small particles in the electron microscope

We present experimental results of the structural instability of gold clusters in the electron microscope. Observations of surface melting in a dynamic HREM study of small gold particles (2–5 nm) are provided. We argue that the temperature regime of the particle during observation, remains quite high at about 50–100 K below the particle's melting temperature. At this temperature, a further 15–30 eV would be enough to raise the particle's temperature above the melting point. We conclude that the structural instabilities observed are due to a mechanism based on a recrystallization from a highly excited state.