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.     

Local atomic structures of palladium nanowire

In this paper, investigation of the structure of palladium nanowire has been performed by using genetic algorithm simulation based on the molecular dynamics. Our calculation employs a well-fitted, tight-binding many-body potential for Pd atoms. Some local atomic structures and defects in nanowires have been reported. The melting behavior of palladium nanowire has also been investigated. An interesting result is that the diffusion of the central atoms results in the beginning of the melting. The moving central atoms build up a monostrand atomic chain during the melting process. The single atomic chain is very stable which can exist in a wide temperature region (800-950 K). The formation of the single atomic chain causes some new defects in the nanowire. And the new defects result in the decrease of the thermal stability of the nanowire. Interestingly, the liquid from the nanowire melting has a supercooled feature because the splitting of the second peak of pair correlation function is observed. The curves of the internal energy and the local cluster are used to monitor the phase transition. The melting of the nanowire is not only due to the single atomic diffusion, but also the diffusion of the local clusters.     

Structural phase stability of Se clusters in zeolites

The effect of confinement on the structural phase of Se clusters in zeolites (LTA and FAU) has been studied in the temperature range of 300 ～ 550K by Raman scattering and differential scanning calorimetry. It has been found in Raman spectra that the structural unit and phase stability of Se clusters are greatly affected by the geometrical restriction in the zeolites. No clear phase transformation was observed by DSC measurements for Se clusters in LTA zeolites, while Se clusters in FAU zeolites exhibit a distinct endothermic peak which is ascribed to a glass transition. The glass transition temperature of amorphous Se (320 K) is remarkably elevated when Se rings/chains are incorporated in FAU zeolites; the higher glass transition temperature for the smaller window diameter of zeolites. We have also found that the activation energy for the glass transition is decreased drastically in the zeolites; the smaller activation energy for the smaller window size. The observations imply that the kinetics of the structural relaxation at the glass transition of Se is significantly influenced by the absence of inter-cluster interaction and that the thermokinetic dimension becomes lower owing to the geometrical restriction in the zeolites.     

Further evidence for phase transitions in biphenyl near 40 K and 16 K

Phase transitions have been detected in biphenyl at about 42 and 17 K by noting changes in the transmitted light intensity when the crystal is placed at extinction between crossed polaroids and the crystal temperature slowly varied. The phase transition at 42 K is gradual (i.e. it occurs over a range of temperature) while the 17 K transition is abrupt. The effect is observed only for light propagating normal to an a*b section which implies that the atomic displacements at the phase changes are largely restricted to this plane. An analogous behaviour is observed for biphenyl-d₁₀ except that the onset of the gradual transition on cooling is at 38 K and the abrupt transition is at 24 K.     

Structural fluctuation and atom-permutation in transition-metal clusters

The atomic structure and thermodynamic properties of transition-metal clusters containingN atoms are investigated forN=6 and 7 using the method of molecular dynamics, where Gupta's potential taking into account many-body interaction is employed. The caloric curve (total energy — temperature curve) and the structural fluctuations are studied. The “fluctuating state” is found forN=6 in the region of the temperature near below the melting point, where clusters undergo structural transition from one isomer to others without making any topological change. The fluctuating state differs from the coexistence state in that the former involves no atomic diffusion, and goes to a structural phase transition of the bulk whenN is increased. On the other hand, the motion of atom-permutation is found in the low-temperature region of the liquid state, being induced by the cooperative motion of two atoms. It is discussed that such a motion easily occurs along the surface and may be considered to be one of the characteristics of small clusters. The fluctuating state is discussed in relation to the structural fluctuation of gold clusters observed experimentally.     

Melting, freezing, sublimation, and phase coexistence in sodium chloride nanocrystals

Calorimetry measurements, performed by multicollision induced dissociation, have been used to probe the melting of a number of (NaCl)nNa+ clusters with n=22-37. The clusters anneal at 225-325 K and melt at 750-850 K. (NaCl)22Na+ and (NaCl)37Na+, which can adopt geometries that are perfect fragments of the bulk lattice melt at around 850 K. The other clusters, which (except for n=31) must have defects, melt at temperatures which are up to 100 K lower than the perfect nanocrystals. The internal energy distributions become bimodal near the melting temperature. This is the signature of slow dynamic phase coexistence where clusters spontaneously jump back and forth between the solid and liquid states with an average period that is longer than required for thermal equilibration. The jump frequency must be between 10(4) and 10(7) s(-1) for the bimodal distribution to be observable in our experiments. The (NaCl)nNa+ clusters can dissociate by an unusual thermally activated process where melting and freezing raise the internal energy to generate hot solid clusters that can sublime before they cool to the ambient temperature.     

Ab initio molecular dynamical investigation of the finite temperature behavior of the tetrahedral Au19 and Au20 clusters

Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au19 and Au20 clusters. Au20 has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap, and an atomic packing similar to that of the bulk gold (Li, J.; et al. Science 2003, 299, 864). Our results show that the geometry of Au19 is exactly identical with that of Au20 with one missing corner atom (called a vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold clusters exhibit dramatic differences. Au20 undergoes a clear and distinct solid-like to liquid-like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au19 has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving a series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au19 between 650 to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au20 is destroyed around 800 K, and the cluster is clearly in a liquid-like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature that has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.     

Imaginary time Gaussian dynamics of the Ar3 cluster

Semiclassical Gaussian approximations to the Boltzmann operator have become an important tool for the investigation of thermodynamic properties of clusters of atoms at low temperatures. Usually, numerically expensive thawed Gaussian variants are applied. In this article, we introduce a numerically much cheaper frozen Gaussian approximation to the imaginary time propagator with a width matrix especially suited for the dynamics of clusters. The quality of the results is comparable to that of thawed Gaussian methods based on the single-particle ansatz. We apply the method to the argon trimer and investigate the dissociation process of the cluster. The results clearly show a classical-like transition from a bounded moiety to three free particles at a temperature T ≈ 20 K, whereas previous studies of the system were not able to resolve this transition. Quantum effects, i.e., differences with the purely classical case manifest themselves in the low-temperature behavior of the mean energy and specific heat as well as in a slight shift of the transition temperature. We also discuss the influence of an artificial confinement of the atoms usually introduced to converge numerical computations. The results show that restrictive confinements often implemented in studies of clusters can influence the thermodynamic properties drastically. This finding may have implications on other studies of atomic clusters.     

Raman study of phase transitions in (n-C4H9NH3)2BiCl5

The Raman spectra of (n-C₄H₉NH₃)₂BiCl₅ were recorded and analysed from 4 K to 390 K. The phase transition (α → β) at 370 K to the metastable form is manifested by changes in the low-frequency Raman spectra, indicating the changes in the anionic structure of the crystal. The phase transition at 215 K is clearly manifested by the temperature evolution of the internal modes of the butylammonium cation. The phase transition is likely to be due to a reorientational motion of the cation.     

Temperature and Composition Dependent Structural Evolution: Thermodynamics of CunAg135−n (n = 0–135) Nanoalloys during Cooling

Molecular dynamics simulations are performed to investigate the changes of packing structures, and thermodynamic quantities including internal energy, entropy, and free energy are used to determine temperature regime and transition time of atomic packing structures. The simulation results show different packing structures as the component composition changes, and there are different packing patterns during cooling. For these Cu-Ag alloy clusters containing only a small number of atoms of Cu, they present FCC packing structures in different parts at high temperatures, and then there are transformations to icosahedral structures. With the increase in content of Cu atoms, there is a transition mechanism from molten state to icosahedron. When the content of Cu atoms is appropriate, core-shell structures can be formed at room temperature.     

Gas-phase dissociation pathways of multiply charged peptide clusters

Numerous studies of cluster formation and dissociation have been conducted to determine properties of matter in the transition from the condensed phase to the gas phase using materials as diverse as atomic nuclei, noble gasses, metal clusters, and amino acids. Here, electrospray ionization is used to extend the study of cluster dissociation to peptides including leucine enkephalin with 7–19 monomer units and 2–5 protons, and somatostatin with 5 monomer units and 4 protons under conditions where its intramolecular disulfide bond is either oxidized or reduced. Evaporation of neutral monomers and charge separation by cluster fission are the competing dissociation pathways of both peptides. The dominant fission product for all leucine enkephalin clusters studied is a proton-bound dimer, presumably due to the high gas-phase stability of this species. The branching ratio of the fission and evaporation processes for leucine enkephalin clusters appears to be determined by the value of z²/n for the cluster where z is the charge and n the number of monomer units in the cluster. Clusters with low and high values of z²/n dissociate primarily by evaporation and cluster fission respectively, with a sharp transition between dissociation primarily by evaporation and primarily by fission measured at a z²/n value of 0.5. The dependence of the dissociation pathway of a cluster on z²/n is similar to the dissociation of atomic nuclei and multiply charged metal clusters indicating that leucine enkephalin peptide clusters exist in a state that is more disordered, and possibly fluid, rather than highly structured in the dissociative transition state. The branching ratio, but not the dissociation pathway of [somatostatin₅ + 4H]⁴⁺ is altered by the reduction of its internal disulfide bond indicating that monomer conformational flexibility plays a role in peptide cluster dissociation.     

The abnormal changes of electrical resistivity in liquid Pb–In alloys

Electrical resistivity of liquid lead and indium (Pb–In) alloys with different compositions has been measured using the four-probe method in a large temperature range. Marked turning points on each resistivity–temperature (ρ–T) curve of the liquid Pb–In alloys can be observed far above the liquidus. The unusual variation of the resistivity of Pb–In melts suggests a structural transition of these melts, for resistivity is a sensitive parameter to the structure. Moreover, the DSC experiment of Pb–In melts supports the existence of a liquid–liquid (L–L) structure transition in Pb–In melts. Such a L–L structural transition can be described in terms of the gradual disappearance of atomic bonds corresponding to the crystal structure and/or to a reduction of the size of pre-formed atomic clusters. This implies an increase of disorder in the high temperature melts. The transition temperatures depend on the composition of Pb–In melts and the onset transition temperatures of the intermediate phase (α) Pb–63%In and Pb–70.6%In melts are higher than that of other compositions.     

A new type of antiferrodistortive structure in a double perovskite with Pb

We have found a new structural transition in Pb(2)MnReO(6) at 410 K. Above this temperature, Pb(2)MnReO(6) is cubic with disordered and dynamic atomic displacements manifested in the large thermal parameters of Pb and O atoms. Below 410 K, the antiferrodistortive shift of 2/3 of Pb(2+) cations away from the high-symmetry cubic site produces a new type of monoclinic cell. The unit cell expands at the transition and the heat capacity shows a peak with thermal hysteresis. These features agree with a first order transition. The entropy content of the transition is quite low indicating that the structural disorder has not been completely removed in the low temperature phase. The monoclinic phase of Pb(2)MnReO(6) shows thermally activated conductivity which does not vary when an external magnetic field is applied. A change in the slope of the resistivity curve, observed at the structural phase transition temperature, is related to a slight difference in the activation energy between both phases. It suggests that the condensation of the distortions likely affects the conduction mechanism. The isothermal magnetization measurements reveal the presence of ferromagnetic contributions below 85 K. The ac magnetic susceptibility shows a dynamic peak at 50 K and, in addition, zero-field-cooled and field-cooled magnetization curves diverge strongly below 80 K. These features might be signature of magnetic inhomogeneity. Magnetic loops, obtained at 5 K, do not show saturation in fields up to 9 T. Furthermore, the measured coercivity increases sharply at low temperature indicating an abrupt change in the magnetic anisotropy. We show that all these magnetic properties point out to a ferrimagnetic ordering of Mn and Re atoms in an intermediate valence state.     

Cryogenic CO2 formation on oxidized gold clusters synthesized via reactive layer assisted deposition

Gas-phase Au atoms deposited onto a multilayer film of molecular oxygen produce atomic oxygen bound to gold clusters. After removal of molecular O2, temperature programmed desorption and molecular beam techniques show that the atomic oxygen readily reacts with CO to produce CO2. At present, the structure and size distribution of these clusters are unknown. Nevertheless, CO2 forms on these clusters upon exposure to CO at temperatures as low as 35 K. Furthermore, above 120 K, the reaction goes to completion with initial reaction yields as high as 50%.     

The properties of ion-water clusters. II. Solvation structures of Na+, Cl-, and H+ clusters as a function of temperature

Ion-water-cluster properties are investigated both through the multistate empirical valence bond potential and a polarizable model. Equilibrium properties of the ion-water clusters H+(H2O)100, Na+(H2O)100, Na+(H2O)20, and Cl-(H2O)17 in the temperature region 100-450 K are explored using a hybrid parallel basin-hopping and tempering algorithm. The effect of the solid-liquid phase transition in both caloric curves and structural distribution functions is investigated. It is found that sodium and chloride ions largely reside on the surface of water clusters below the cluster melting temperature but are solvated into the interior of the cluster above the melting temperature, while the solvated proton was found to have significant propensity to reside on or near the surface in both the liquid- and solid-state clusters.     

Atomic‐Level Mechanisms of Nucleation of Pure Liquid Metals during Rapid Cooling

To obtain a material with the desired performance, the atomic‐level mechanisms of nucleation from the liquid to solid phase must be understood. Although this transition has been investigated experimentally and theoretically, its atomic‐level mechanisms remain debatable. In this work, the nucleation mechanisms of pure Fe under rapid cooling conditions are investigated. The local atomic packing stability and liquid‐to‐solid transition‐energy pathways of Fe are studied using molecular dynamics simulations and first‐principle calculations. The results are expressed as functions of cluster size in units of amorphous clusters (ACs) and body‐centered cubic crystalline clusters (BCC‐CCs). We found the prototypes of ACs in supercooled liquids and successfully divided these ACs to three categories according to their transition‐energy pathways. The information obtained in this study could contribute to our current understanding of the crystallization of metallic melts during rapid cooling.     

Friction and adhesion between C60 single crystal surfaces and AFM tips: effects of the orientational phase transition

We have investigated the nanotribological properties of C60 single crystal (111) and (100) surfaces around its orientational order-disorder phase transition temperature, approximately 260 K, by atomic force microscopy and frictional force microscopy (AFM/FFM) in high vacuum. Results show that for both surfaces across the phase transition temperature, the friction force and the adhesive force between a C60 coated AFM tip and the C60 crystal surfaces exhibit discontinuous behavior. The friction force within the applied external load range in the low temperature phase is significantly larger than that in the high temperature phase, with no obvious change in the slope of the friction force curves (the friction coefficient) in the low and high temperature phases. The abrupt change in friction was found to be caused mainly by the abrupt change in adhesion, which, in turn, can be qualitatively understood through changes in the van der Waals interaction and the short-range Coulomb interaction associated with the structural changes across the phase transition. Compared to most other degrees of freedom, the rotation of C60 molecules was found to have little effect on friction and is an ineffective energy dissipation channel.     

A new Monte Carlo method for getting the density of states of atomic cluster systems

A novel Monte Carlo flat histogram algorithm is proposed to get the classical density of states in terms of the potential energy, g(E(p)), for systems with continuous variables such as atomic clusters. It aims at avoiding the long iterative process of the Wang-Landau method and controlling carefully the convergence, but keeping the ability to overcome energy barriers. Our algorithm is based on a preliminary mapping in a series of points (called a σ-mapping), obtained by a two-parameter local probing of g(E(p)), and it converges in only two subsequent reweighting iterations on large intervals. The method is illustrated on the model system of a 432 atom cluster bound by a Rydberg type potential. Convergence properties are first examined in detail, particularly in the phase transition zone. We get g(E(p)) varying by a factor 10(3700) over the energy range [0.01 < E(p) < 6000 eV], covered by only eight overlapping intervals. Canonical quantities are derived, such as the internal energy U(T) and the heat capacity C(V)(T). This reveals the solid to liquid phase transition, lying in our conditions at the triple point. This phase transition is further studied by computing a Lindemann-Berry index, the atomic cluster density n(r), and the pressure, demonstrating the progressive surface melting at this triple point. Some limited results are also given for 1224 and 4044 atom clusters.     

Dielectric characterization of the B7 and a BX phase

The first clear experimental evidence of a dielectric relaxation in the B₇ phase is given. The associated molecular process is probably the reorientation of the bent-shaped molecules about the long molecular axis. It is also shown that the dielectric constant strongly increases at the phase transition I-B₇ pointing to a co-operative motion of ferroelectric clusters. Furthermore, a phase transition B₇-B_X is observed. The values of about 10 for the high frequency limit of the dielectric constants of the B₇ and B_X phases and the static dielectric constants of the isotropic phase are in agreement. Thus, in all the phases the main dipoles can reorient faster than 10^-8 s, i.e. the experimental limit. Only in the supercooled B_X phase were the dynamics of the transversal dipoles measured. Using atomic force microscopy, focal-conic domains modulated by parallel lines have been observed at room temperature.     

Noncontact atomic force microscopy studies of ultrathin films of amorphous solid water deposited on Au(111)

Noncontact atomic force microscopy was used to study the morphological changes of an ultrathin amorphous solid water (ASW) film as a function of deposition temperature, annealing temperature, and annealing time. ASW deposited at 80 or 108 K on Au(111) formed truncated hemispherical clusters of increasing size during annealing at 134 K; these clusters were inferred to be crystalline. The number of nuclei present at the outer surface of the film after deposition was greater for higher deposition temperature. For lower cluster densities, depletion of the ASW film around the clusters was observed when the clusters became larger and dendritic growth was observed when the apparent cluster footprint radius exceeded 100 nm.