Structural Crossover Research Articles

A dispersion-corrected DFT insight into the structural, electronic and CH4 adsorption properties of small tin-oxide clusters

The main goal of this paper is to investigate the structural and electronic properties of small tin-oxide clusters, SnxOy (x = 2–6), before and after methane (CH4) physisorption. To this end, we employ the dispersion-corrected density functional theory (DFT-D2). At first, we demonstrate the relative binding stability of SnxOy clusters with a fixed number of tin atoms via analysis of their binding energy per atom and second-order energy differences. Our results show that the highest relative stability occurs at y = 2, y = 3, y = 4, y = 6 and y = 8 for Sn2Oy, Sn3Oy, Sn4Oy, Sn5Oy and Sn6Oy clusters, respectively. Furthermore, with the cluster size increasing, the ring-shape to cube-like structural crossover is occurred. The higher average coordination and total number of bonds in cube-like clusters make their interatomic bonds weaker and longer. On the other hand, the cube-like clusters, in comparison with ring-shape ones, possess higher chemical potential and lower chemical hardness that lead to their more chemical reactivity. We then proceed to study the CH4 adsorption properties of the resulted most stable clusters. The lowest-energy structures of CH4-adsorbed clusters have been found by comparing the adsorption energy of different configurations. It is found that Van der Waals interactions lead to exothermic physisorption of CH4 on each tin-oxide cluster with the adsorption energy ranging from −101 to −191 meV. In addition, upon CH4 adsorption on SnxOy clusters, the energy gap is decreased which shows an enhancement in the conductivity of the clusters. As a result, the CH4 physisorption ability of these small studied tin-oxide clusters provides their application feasibility as the low-temperature CH4 gas sensors.

Dynamics, thermodynamics and structure of liquids and supercritical fluids: crossover at the Frenkel line

We review recent work aimed at understanding dynamical and thermodynamic properties of liquids and supercritical fluids. The focus of our discussion is on solid-like transverse collective modes, whose evolution in the supercritical fluids enables one to discuss the main properties of the Frenkel line separating rigid liquid-like and non-rigid gas-like supercritical states. We subsequently present recent experimental evidence of the Frenkel line showing that structural and dynamical crossovers are seen at a pressure and temperature corresponding to the line as predicted by theory and modelling. Finally, we link dynamical and thermodynamic properties of liquids and supercritical fluids by the new calculation of liquid energy governed by the evolution of solid-like transverse modes. The disappearance of those modes at high temperature results in the observed decrease of heat capacity.

Interplay between structural and magnetic-electronic responses of FeAl2O4 to a megabar: Site inversion and spin crossover

X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe M\"ossbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic spectral component in the M\"ossbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the post-spinel structure. There is also a much steeper volume change dV/V ~ 10% in the range 45-50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ~106 GPa, the M\"ossbauer data evidences a diamagnetic Fe low-spin abundance of ~50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ~33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly magnetism survives in such a spin-diluted post-spinel lattice at high densities. The R(300 K) data decreases by only two orders of magnitude from ambient pressure to the vicinity of ~100 GPa. Despite a ~26% unit-cell volume densification from the lattice compressibility, structural transitions and spin crossover.

Temperature-induced structural evolution in liquid Sn85Zn15 eutectic alloy

Temperature-dependent atomistic structural evolution in liquid Sn85Zn15 eutectic alloy has been investigated by using in situ high energy X-ray diffraction and ab initio molecular dynamics simulations. We reveal a structural crossover at around 900 K in liquid Sn85Zn15 eutectic alloy, below and above which the liquids exhibit different changes in first peak position shifts on S(q) and G(r) curves, the nearest coordination number and activation energies for self-diffusion. This structural crossover is likely ascribed to the spatial connectivity variation of local clusters. Our finding will trigger more studies on the liquid-to-liquid structural transition in metallic liquids.

Interfacial structural crossover and hydration thermodynamics of charged C60 in water.

Classical molecular dynamics simulations of the hydration thermodynamics, structure, and dynamics of water in hydration shells of charged buckminsterfullerenes are presented in this study. Charging of fullerenes leads to a structural transition in the hydration shell, accompanied by creation of a significant population of dangling O-H bonds pointing toward the solute. In contrast to the well accepted structure-function paradigm, this interfacial structural transition causes nearly no effect on either the dynamics of hydration water or on the solvation thermodynamics. Linear response to the solute charge is maintained despite significant structural changes in the hydration shell, and solvation thermodynamic potentials are nearly insensitive to the altering structure. Only solvation heat capacities, which are higher thermodynamic derivatives of the solvation free energy, indicate some sensitivity to the local hydration structure. We have separated the solvation thermodynamic potentials into direct solute-solvent interactions and restructuring of the hydration shell and analyzed the relative contributions of electrostatic and nonpolar interactions to the solvation thermodynamics.

Dynamical crossover of water confined within the amphiphilic nanocores of aggregated amyloid β peptides.

It is believed that the self-assembly of amyloid beta (Aβ) peptides in the brain is the cause of Alzheimer's disease. Atomistic molecular dynamics simulations of aqueous solutions of Aβ protofilaments of different sizes at room temperature have been carried out to explore the dynamic properties of water confined within the core and at the exterior surface of the protofilaments. Attempts have been made to understand how the non-uniform distortion of the protofilaments associated with their structural crossover influences the diffusivity and the hydrogen bonding environment of the confined water molecules. In contrast to the homogeneous solvent dynamical environment at the exterior surface, the calculations revealed heterogeneously restricted motions of water confined within the distorted cores of the protofilaments. Importantly, it is demonstrated that the structural crossover of the aggregates observed for the decamer is associated with a dynamical transition of water confined within its core. A direct one-to-one correlation between the heterogeneously restricted core water motions and the kinetics of the breaking and formation of hydrogen bonds quantitatively demonstrated that a modified hydrogen bond arrangement within the cores of higher order Aβ protofilaments is the origin behind the crossover in core water mobility. A fraction of the water molecules forming short-lived water-water hydrogen bonds within the core of the crossover protofilament decamer are believed to diffuse away easily from the core and thus play a crucial role in further growth of the protofilament by facilitating the binding of new peptide monomers.

Anomalously high heat capacity of core-softened liquids

ABSTRACTHeat capacity is among the most important thermodynamic characteristics of a substance. The behaviour of the heat capacity is well understood in gases and crystals, but not in liquids. A common view on the heat capacity of liquids is that it should be close to the solid-like values close to the melting line and it should approach the gas values in the limit of high temperatures. However, some liquids can show anomalously high magnitudes of isochoric heat capacity. In the present paper, I show that core-softened liquids can demonstrate unusually high magnitude of the heat capacity induced by a structural crossover of the liquid.

Temperature dependent structural evolution in liquid Ag50Ga50 alloy

The temperature dependence of atomic structural evolution in liquid Ag50Ga50 alloy has been studied using an in situ high energy x-ray diffraction (XRD) experiment combined with first-principles molecular dynamics (FPMD) simulations. The experimental data show a reversible structural crossover at the temperature of about 1050 K. Changes in both electrical resistivity and absolute thermoelectric power at about 1100 K strongly support the XRD results. Additionally, FPMD simulations reveal the abnormal temperature dependent behavior of partial coordination number and atomic diffusivity at about 1200 K, elucidating that the partition experimentally observed changes in structure and properties could be linked with the repartition between Ag and Ga atoms in the liquid at around 1050–1200 K. This finding will trigger more studies on the structural evolution of noble-polyvalent metals in particular and metallic liquids in general.

Near-Wall Molecular Ordering of Dilute Ionic Liquids.

The interfacial behavior of ionic liquids promises tunable lubrication as well as playing an integral role in ion diffusion for electron transfer. Diluting the ionic liquids optimizes bulk parameters, such as electric conductivity, and one would expect dilution to disrupt the near-wall molecular ordering. We study this ordering in the ionic liquids [Emim]+[NTf2]−, [Emim]+[DCA]−, and [C4mpyr]+[NTf2]−, diluted in the solvent dimethyl sulfoxide. We found a structural crossover from well-ordered ionic liquids to a well-ordered solvent with increasing dilution, but this occurs nonlinearly, with solvent molecules initially space-filling and solvating and later disrupting the ionic layers. This is of key importance for ionic liquids as optimized tunable nanolubricants.

Structural evolution in liquid calcium under pressure

The atomic structure evolution of liquid Ca at 3500K as a function of pressure up to 100GPa has been studied by ab initio molecular dynamics simulations. The pair correlation function, structure factor and respective peak positions reveal two crossovers at ~10GPa and ~50GPa in liquid Ca while the coordination number shows four pressure regions: about 0.1–5GPa, about 5–20GPa, about 20–50GPa and above ~50GPa and heat capacity shows two different pressure dependences separated at about 30GPa, i.e., below about 30GPa a negative one and a positive value above about 30GPa. It is also observed below 10GPa the first nearest neighbor CaCa distance decreases more sharply as compared to above 10GPa, this behavior is attributed to the gradual occupation of the interstitial sites. Comparisons of the pressure-dependent bond angle distribution and bond orientation order data of liquid Ca with solid crystalline phases imply that pressure-dependent atomic structural evolution in liquid calcium is, to some extent, similar to that of solid crystalline calcium reported by Teweldeberhan and Bonev [Phys. Rev. B 78 140101(R) (2008)]. These sluggish structural crossovers that have been observed in liquid Ca under pressure will trigger more studies in other disordered systems.

Liquid-to-liquid crossover in the GaIn eutectic alloy

Liquid-liquid crossover is promising and closely related to the atomic dynamics during heating and cooling processes. Here we reveal a reversible structural crossover in the liquid $\mathrm{G}{\mathrm{a}}_{85.8}\mathrm{I}{\mathrm{n}}_{14.2}$ eutectic alloys by using in situ synchrotron x-ray diffraction and ab initio molecular dynamics simulation. A kink always appears on the temperature dependent behaviors of density, ratio of the second peak position to the first in the pair correlation function, coordination number, heat capacity, free energy, and atomic diffusivity in the temperature range of about 400--550 K. It is likely ascribed to atomic rearrangements of Ga and In atoms from a relative random packing at high temperatures to a relative nonuniform packing at low temperatures, in which In atoms prefer to have more In neighbors. This observation will promote more understanding of the liquid structure of eutectic alloys.

Experimental evidence of the Frenkel line in supercritical neon

Recent research suggests that the supercritical state consists of liquidlike and gaslike states where particle dynamics and key system properties are qualitatively different. We report experimental evidence of the structural crossover in supercritical neon at pressure and temperature conditions significantly exceeding the critical point values: 250$\,P_c$ and 6.6$\,T_c$. The experimental results show a crossover of the medium-range order structure evidenced by the change of the structure factor with pressure. We also observe the crossover of the short-range order structure indicated by changes in the coordination number. The relative width of the crossover is fairly narrow and is smaller than 10-12 \% in pressure and temperature. By comparing our experimental results with molecular dynamics simulations, we suggest that the observed crossover can be attributed to the Frenkel line and discuss the relationship between the structural crossover and qualitative changes of dynamical and thermodynamic properties of supercritical matter.

Direct links between dynamical, thermodynamic, and structural properties of liquids: Modeling results.

We develop an approach to liquid thermodynamics based on collective modes. We perform extensive molecular-dynamics simulations of noble, molecular, and metallic liquids, and we provide direct evidence that liquid energy and specific heat are well-described by the temperature dependence of the Frenkel (hopping) frequency. The agreement between predicted and calculated thermodynamic properties is seen in the notably wide range of temperature spanning tens of thousands of Kelvin. The range includes both subcritical liquids and supercritical fluids. We discuss the structural crossover and interrelationships between the structure, dynamics, and thermodynamics of liquids and supercritical fluids.

Pinned-to-sliding transition and structural crossovers for helically confined charges.

We explore the nonequilibrium dissipative dynamics of a system of identical charged particles trapped on a closed helix. The particles are subject to an external force accelerating them along the underlying structure. The effective interactions between the charges induce a coupling of the center of mass to the relative motion which in turn gives rise to a pinned-to-sliding transition with increasing magnitude of the external force. In the sliding regime we observe an Ohmic behavior signified by a constant mobility. Within the same regime a structural transition of the helical particle chain takes place with increasing the helix radius leading to a global change of the crystalline arrangement. The resulting crystal is characterized by the existence of multiple defects whose number increases with the helix radius.

Assembly of hard spheres in a cylinder: a computational and experimental study.

Hard spheres are an important benchmark of our understanding of natural and synthetic systems. In this work, colloidal experiments and Monte Carlo simulations examine the equilibrium and out-of-equilibrium assembly of hard spheres of diameter σ within cylinders of diameter σ≤D≤ 2.82σ. Although phase transitions formally do not exist in such systems, marked structural crossovers can nonetheless be observed. Over this range of D, we find in simulations that structural crossovers echo the structural changes in the sequence of densest packings. We also observe that the out-of-equilibrium self-assembly depends on the compression rate. Slow compression approximates equilibrium results, while fast compression can skip intermediate structures. Crossovers for which no continuous line-slip exists are found to be dynamically unfavorable, which is the main source of this difference. Results from colloidal sedimentation experiments at low diffusion rate are found to be consistent with the results of fast compressions, as long as appropriate boundary conditions are used.

Dynamical and orientational structural crossovers in low-temperature glycerol.

Mean-square displacements of hydrogen atoms in glass-forming materials and proteins, as reported by incoherent elastic neutron scattering, show kinks in their temperature dependence. This crossover, known as the dynamical transition, connects two approximately linear regimes. It is often assigned to the dynamical freezing of subsets of molecular modes at the point of equality between their corresponding relaxation times and the instrumental observation window. The origin of the dynamical transition in glass-forming glycerol is studied here by extensive molecular dynamics simulations. We find the dynamical transition to occur for both the center-of-mass translations and the molecular rotations at the same temperature, insensitive to changes of the observation window. Both the translational and rotational dynamics of glycerol show a dynamic crossover from the structural to a secondary relaxation at the temperature of the dynamical transition. A significant and discontinuous increase in the orientational Kirkwood factor and in the dielectric constant is observed in the same range of temperatures. No indication is found of a true thermodynamic transition to an ordered low-temperature phase. We therefore suggest that all observed crossovers are dynamic in character. The increase in the dielectric constant is related to the dynamic freezing of dipolar domains on the time scale of simulations.

Structural crossover in a supercooled metallic liquid and the link to a liquid-to-liquid phase transition

Time-resolved synchrotron measurements were carried out to capture the structure evolution of an electrostatically levitated metallic-glass-forming liquid during free cooling. The experimental data shows a crossover in the liquid structure at ∼1000 K, about 115 K below the melting temperature and 150 K above the crystallization temperature. The structure change is characterized by a dramatic growth in the extended-range order below the crossover temperature. Molecular dynamics simulations have identified that the growth of the extended-range order was due to an increased correlation between solute atoms. These results provide structural evidence for a liquid-to-liquid-phase-transition in the supercooled metallic liquid.

Direct observation in 3d of structural crossover in binary hard sphere mixtures.

For binary fluid mixtures of spherical particles in which the two species are sufficiently different in size, the dominant wavelength of oscillations of the pair correlation functions is predicted to change from roughly the diameter of the large species to that of the small species along a sharp crossover line in the phase diagram [C. Grodon et al., J. Chem. Phys. 121, 7869 (2004)]. Using particle-resolved colloid experiments in 3d we demonstrate that crossover exists and that its location in the phase diagram is in quantitative agreement with the results of both theory and our Monte-Carlo simulations. In contrast with previous work [J. Baumgartl et al., Phys. Rev. Lett. 98, 198303 (2007)], where a correspondence was drawn between crossover and percolation of both species, in our 3d study we find that structural crossover is unrelated to percolation.

AutoT&T v.2: An Efficient and Versatile Tool for Lead Structure Generation and Optimization.

In structure-based drug design, automated de novo design methods are helpful tools for lead discovery as well as lead optimization. In a previous study ( J. Chem. Inf. 2011 , 51 , 1474 - 1491 ) we reported a new de novo design method, namely, Automatic Tailoring and Transplanting (AutoT&T). It overcomes some intrinsic problems in conventional fragment-based buildup methods. In this study, we describe an upgraded version, namely, AutoT&T2. Structural operations conducted by AutoT&T2 have been largely optimized by introducing several new algorithms. As a result, its overall speed in multiround optimization jobs has been improved by a few thousand fold. With this improvement, it is now practical to conduct structural crossover among multiple lead molecules using AutoT&T2. Three different test cases are described in this study that demonstrate the new features and versatile applications of AutoT&T2. The AutoT&T2 software suite is available to the public. Besides, a Web portal for running AutoT&T2 online is provided at http://www.sioc-ccbg.ac.cn/software/att2 for testing.

The Frenkel Line: a direct experimental evidence for the new thermodynamic boundary.

Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to change only in a monotonic way even moving around the critical point, not only along isotherms or isobars. Conversely, we observe structural crossovers for the first time in a deeply supercritical sample through diffraction measurements in a diamond anvil cell and discover a new thermodynamic boundary on the pressure-temperature diagram. We explain the existence of these crossovers in the framework of the phonon theory of liquids using molecular dynamics simulations. The obtained results are of prime importance since they imply a global reconsideration of the mere essence of the supercritical phase. Furthermore, this discovery may pave the way to new unexpected applications and to the exploration of exotic behaviour of confined fluids relevant to geo- and planetary sciences.