Heterogeneous slow dynamics and the interaction potential of glass-forming liquids

The role of the intermolecular interaction potential on the dynamic and thermodynamic properties of model glass-forming mixtures is investigated through molecular dynamics simulations. Variations of the repulsive exponent m in the well-studied Lennard-Jones Kob-Andersen mixture are shown to have a negligible effect on the fragility and dynamic correlation volumes when quenches are performed at constant pressure. The number of dynamically correlated particles, estimated from the temperature derivative of a two-point dynamic correlation function, is approximately invariant to m at any fixed relaxation time. Further, the density scaling property of a model tetrahedral network glass-former, based on inverse power law and Lennard-Jones potentials, is investigated. The optimal scaling exponent γ is close to zero and does not superpose the data well. The breakdown of density scaling is consistent with the absence of correlation between fluctuations of the virial and the potential energy. These results emphasize the crucial role of structural many-body correlations in glass-forming systems and show the need of investigations of more complex and realistic model liquids.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

The local atomic environment of oxygen in silicate glasses from molecular dynamics

Recent molecular dynamics (MD) results for (Na₂O)_x(SiO₂)_1−x and (CaO)_x(SiO₂)_1−x glasses show that co-ordination of bridging oxygen (O_b) by modifiers (M) is a normal structural feature. This can be explained as a consequence of the limitation on oxygen co-ordination in network oxides, a common rule of thumb being that total co-ordination of oxygen by (Si + M) is ⩽4. This gives an upper limit on co-ordination of non-bridging oxygen (O_nb) by modifiers of N_OnbM ⩽ m with m = 3, corresponding to N_MOnb ⩽ mv, where v is modifier valence. If modifier co-ordination exceeds this limit, i.e. N_MO > mv, then there is bonding of O_b to modifiers, i.e. N_ObM > 0. This is the case in alkali and alkaline earth silicate crystals (apart from Be and Mg), and is predicted to be a feature of glasses in these systems. An illustration of the influence of oxygen co-ordination is given from MD models of (CaO)_0.33 (SiO₂)_0.67 glass at pressures of 5 and 10 GPa. The main effect of densification is to increase the co-ordination of Ca by O_b. This can be understood because at 0 GPa the co-ordination of O_nb by Ca is already high, with N_OnbCa ∼ 2.7, but the co-ordination of O_b by Ca is less high, with N_ObCa ∼ 1, and so can more easily increase.     

Generation and intermolecular trapping of a cage-annulated cycloalkylidenecarbene

Base-promoted reaction of 11-methylenepentacyclo[5.4.0.0^2,6.0^3,10.0^{5, 9}]undecan-8-one (5) with diethyl diazomethylphosphonate when performed in the presence of excess cyclohexene, resulted in the formation of the corresponding cycloalkylidenecarbene, 6, which subsequently was trapped in situ to afford 8-methylene-11-(7-bicyclo[4.1.0]heptylidene)pentacyclo-[5.4.0.0^{2, 6}.0^{3, 10}.0^{5, 9}]undecane (7, obtained in 44% yield as a mixture of exo, endo isomers). Subsequent reaction of 7 with dichlorocarbene (generated under phase transfer catalytic conditions) produced the corresponding mono- and di-:CCl₂ adducts [i.e., 8 (64% yield) and 9 (5% yield), respectively]. The structure of 9 was established unequivocally via application of single crystal X-ray analysis: Triclinic, P1¯, a = 6.276(2), b = 8.700(2), c = 18.550(3) Å, = 76.52(3), = 87.59(3), = 70.88(4)° Z = 2; D _calc 1.486 g cm^–3.     

Photo-initiation of intermolecular bonding and oxide deposition in Ti-based alkoxide solutions

Photoexposure of a water-stable, Ti-alkoxide, (OC₆H₆N)₂Ti(OC₆H₂(CH₂N(CH₃)₂)₃-2,4,6)₂, dissolved in a water/pyridine solution yielded an insoluble, solid phase, present in the form of suspended particles and a deposited material on the vertical side-wall of the solution container. Raman analysis of these photoproducts confirmed the UV-initiation of hydrolysis and condensation reactions in the alkoxide reagent. The excitation wavelength dependence of the photostructural effects observed provides insight into the mechanisms contributing to these photoinduced phenomena. The results are extended to enable the photopatterned deposition of physical relief structures directly from solution in this system.     

Program for analysis of intermolecular contacts in organic crystals

The ICA (Intermolecular Contacts Analysis) computer program was developed for statistical analysis of intermolecular atom-atom contacts in organic crystals. At present, the program allows one to examine homomolecular crystals belonging to monosystem structural classes. The analysis involves the following steps: selection of supporting contacts (i.e., contacts sufficient for the formation of a three-dimensional framework of intermolecular “touches”), construction of their distributions over atom-atom distances, approximation of the distribution curves by Gaussians, and calculations of the average (most probable) length of the contacts of a particular type and the variance of the distribution. The van der Waals radii of the most important organogenic elements were refined on the basis of the data on approximately 7800 crystal structures retrieved from the Cambridge Structural Database.     

Scaling in the Optical Characteristics of Aperiodic Structures with Self-Similarity Symmetry

The properties of diffraction gratings and multilayered systems constructed using 1D models of quasicrystals are considered based on numerical simulation. It is shown that there is a direct relationship between the self-similarity symmetry of quasicrystals and scaling in the characteristics of the above-mentioned optical devices. The degree of structural correspondence between the graphical representations of the geometric properties of crystals, light diffraction patterns of gratings, and the transmission spectra of multilayered systems is estimated. It is shown that certain types of self-similarity symmetry make the characteristics of aperiodic diffraction gratings highly stable to a change in the size ratio of forming elements.     

Scaling properties of growing surfaces of microcrystalline silicon

Growth-induced roughening of microcrystalline Si (μc-Si) surfaces has been studied from the viewpoint of self-similar and fractal structures in conjunction with crystallographic preferential orientations of μc-Si films. Typically, μc-Si films are prepared by plasma enhanced chemical vapor deposition (PECVD) with various preparation conditions including excitation frequency. Irrespective of preparation conditions, self-similarity of the μc-Si surface roughness derived by an atomic force microscope is well characterized in terms of its scaling exponents. Furthermore, the scaling exponents revealed that growth-induced roughness shows different behaviors in accordance with the crystallographic preferential orientations of the μc-Si films. Experimental results of scaling exponents are discussed regarding the origins of surface roughening in comparison to analytical results and numerical simulation results.     

Secondary dynamics in glass formers: Relation with the structural dynamics and the glass transition

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari–Goldstein process, originating by a local motion of the whole molecule.     

The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study

A series of 27 silicate glasses of various compositions containing 0.2-2 at.% iron were synthesized at various oxygen fugacity values. The glasses were examined using X-ray absorption fine structure (XANES) spectroscopy at the Fe K-edge in order to determine iron oxidation state and first-neighbor coordination number. Spectral information extracted from the pre-edge region and principal component analysis (PCA) of the XANES region, together with a spectral inversion, were used to derive the end-member spectral components for Fe(II) and Fe(III). Linear trends in the pre-edge features were observed for most compositional series of the glasses examined as a function of Fe(II)/Fe(III) content. These linear trends are believed to be due to the similarity of average coordination numbers for both Fe(II) and Fe(III) end-members in each series. This result is consistent with model simulations of the XANES region and molecular dynamics (MD) simulations for the two end-member compositions which also show that Fe(II) and Fe(III) have similar average coordination numbers. These simulations also suggest the presence of five-coordinated Fe(III) in the melt phase. Based on a bond valence analysis of these MD simulations, a simple model is proposed to help predict the speciation of iron in oxide and silicate glasses and melts.     

Nickel complexes of 1,10-phenanthroline and 2,2′-bipyridyl with strong intermolecular interactions

Nickel complexes of 1,10-phenanthroline and 2,2′-bipyridyl which can have strong intermolecular interactions have been prepared in the present work. The complexes with intermolecular interactions appear pink, red or red-violet. Two of the complexes prepared Ni(Bip)₂ · Ni(CN)₄ · x H₂O and Ni(Phen)₂ · Ni(CN)₄ · y H₂O indeed show strong absorption bands at 470 nm and 500 nm, respectively.     

Macrosteps dynamics and the growth of crystals and epitaxial layers

Step pattern stability of the vicinal surfaces during growth was analyzed using various surface kinetics models. It was shown that standard analysis of the vicinal surfaces provides no indication on the possible step coalescence and therefore could not be used to elucidate macrostep creation during growth. A scenario of the instability, leading go macrostep creation, was based on the dynamics of the step train, i.e. the step structure consisting of the high (train) and low (inter-train) density of the steps. The critical is step motion at the rear of the train which potentially leads to the step coalescence i.e. creation of the double and multiple step. The result of the analysis shows that the decisive factor for the step coalescence is the step density ratio in and out of the train. The ratio lower than 2 prevents double step formation irrespective of the kinetics. For higher ratio the coalesce depends on step kinetics: fast incorporation from lower terrace stabilizes the single steps, fast incorporation from upper leads to step coalescence. The double step is slower than the single steps, so the single steps behind catch up creating multistep and finally macrostep structure. The final surface structure consists of the macrosteps and superterraces, i.e. relatively flat vicinal segments. The macrostep alimentation from lower superterrace leads to emission of the single steps which move forward. Thus the single step motion is dominant crystal growth mode in the presence of the macrosteps. These steps finally are absorbed by the next macrostep. The absorption and emission of single steps sustain the macrostep existence, i.e. the macrostep fate is determined the single step dynamics. The condition for single step emission was derived. In addition, the macrosteps are prone to creation of the overhangs which results from surface dynamics coupling to impingement from the mother phase. The angular preferential access of the bulk material to the macrostep edge, leads to the overhang instability and creation of inclusions and dislocations.     

Correlation of mesogenic properties with intermolecular interaction energy for homologous series of HnCBP

Intermolecular interaction energy between a pair of molecules of homologous series 4, 4′-disubstituted biphenyl of the general formula HO?(CH₂)_n?O?C₆H₄?C₆H₄?CN(n = 3 ? 11) (H_nCBP) has been evaluated under various interacting conditions viz. stacking, in-plane and terminal interaction. Molecular geometry of the studied molecules was fully optimized without any constraint and checked for imaginary frequencies using hybrid density functional B3LYP combined with 6–31 g^** basis set. Electronic structure of the molecules obtained through these calculations has been utilized to calculate electrostatic and polarization energies under Rayleigh-Schrodinger perturbation theory modified with multi-centered multi-pole expansion method. Dispersion and repulsion energies have been evaluated using Kitaigorodskii formula. The identified minimum energy complexes have been further utilized to evaluate interaction energy under super molecular approach by employing M06 and DFT-D methods. A comparative analysis of the results has been reported with a view to examine suitability of different methods to study molecular aggregations in moderately large organic systems.     

Liquid crystalline G phase of self assembled donor-acceptor molecules by intermolecular hydrogen bonding

The interaction with donor p-n-alkylbenzoic acid and acceptor nonyl-p-hydroxy benzoate molecules was studied. The likely association of these molecules to complexes is studied by intermolecular hydrogen bonding. The textures are observed by polarizing optical microscope with corresponding transitions confirmed by differential scanning calorimetry. The molecular complexes exhibited enantiotropic crystal G phases in both heating and cooling cycles. The relevant functional groups C═O, C-O and OH in formation of molecular complexes are attributed with spectral shifts in infrared spectra and further with absorption studies. The proton NMR studies convinced the structural aspects of hydrogen bonded structure. Crystal parameters were studied with powdered X-ray diffraction. The results show that constituent molecules self organize through intermolecular hydrogen bonding in the formation of crystal G phase.     

Molecular dynamics study of the melting and crystallization of nanoparticles

Crystallography Reports - The size effects during the melting and crystallization of Lennard-Jones nanoparticles and metal clusters are investigated using two independently developed computer...     

Crystal habits of cubic insulin from porcine pancreas and evaluation of intermolecular interactions by macrobond and EET analyses

Experimental and theoretical investigations of the crystal growth of cubic insulin from porcine pancreas were carried out. Three different crystal habits, cube, cube and dodecahedron, and rhombic dodecahedron forms, were obtained. Judging from the results of macroseeding, the most stable of these forms would be the rhombic dodecahedron surrounded by {1 1 0} faces. Molecular interactions in the crystal were evaluated by the macrobond analysis and electrostatic energy of transfer (EET) analysis. The calculated surface energy of {1 1 0} was lower than {1 0 0} by around 40%, which was consistent with the experimental results. Atomic EET supported the macrobond analysis, where the number of hydrogen bonds was primarily evaluated. At the same time, it was suggested that the parameters of the interaction energies of electrically neutral pairs would be smaller than those of charged pairs. The contribution from the bound water molecules to the crystal stabilization was discussed.     

Experimental investigation of the atomic dynamics of amorphous and icosahedral alloys

Most recent results from the experimental investigation of the single particle dynamics and the collective atomic motions in amorphous metals and in icosahedral alloys using neutron inelastic scattering techniques are briefly presented. The extension of the dispersion curve of short wavelength collective motions into the first pseudo Brillouin zone by neutron Brillouin scattering and the investigation of crystal field excitations in an amorphous metal and its dependence on structural relaxation are discussed. Comparative studies of the atomic dynamics of polydomain icosahedral alloys, approximant polycrystals, and polycrystals with similar chemical composition but different structure are discussed. It is shown that the generalized vibrational density of states and the total dynamic structure factor of the icosahedral alloys investigated so far are rather different from any crystalline sample except for the case of higher order approximants with very large units cells.     

Structure and dynamics of non-crystalline materials at the Institut Laue Langevin

The determination of the structural and dynamical properties of non-crystalline materials is usually a difficult task because any scientific insight must be achieved without the mathematical amenities useful in the crystalline state. Special experimental techniques need to be employed and among them, neutron scattering is one of the most powerful. The Institut Laue Langevin (ILL) operates a nuclear reactor, which is the finest steady-state source of neutrons in the world. In this work we introduce a large suite of instruments available at the ILL for structural and dynamical studies on non-crystalline solids or, more generally, amorphous systems, including liquids.