On the mechanisms of ionic conductivity in BaLiF3: A molecular dynamics study

The mechanisms of ionic conductivity in BaLiF3 are investigated using molecular simulations. Direct molecular dynamics simulations of (quasi) single crystalline super cell models hint at the preferred mobility mechanism which is based on fluoride interstitial (and to a smaller extent F- vacancy) migration. Analogous to previous modeling studies, the energy related to Frenkel defect formation in the ideal BaLiF3 crystal was found as 4-5 eV which is in serious controversy to the experimentally observed activation barrier to ionic conductivity of only 1 eV. However, this controversy could be resolved by incorporating Ba2+ ↔ Li+ exchange defects into the elsewise single crystalline model systems. Indeed, in the neighborhood of such cation exchange defects the F- Frenkel defect formation energy was identified to reduce to 1.3 eV whilst the cation exchange defect itself is related to a formation energy of 1.0 eV. Thus, our simulations hint at the importance of multiple defect scenarios for the ionic conductivity in BaLiF3. © 2011 the Owner Societies

Fullerene van der waals Oligomers as electron traps

Density functional theory calculations indicate that van der Waals fullerene dimers and larger oligomers can form interstitial electron traps in which the electrons are even more strongly bound than in isolated fullerene radical anions. The fullerenes behave like super atoms , and the interstitial electron traps represent one-electron intermolecular σ-bonds. Spectroelectrochemical measurements on a bis-fullerene-substituted peptide provide experimental support. The proposed deep electron traps are relevant for all organic electronics applications in which non-covalently linked fullerenes in van der Waals contact with one another serve as n-type semiconductors

Polymorphic Phase Transitions:Macroscopic Theory and Molecular Simulation

Transformations in the solid state are of considerable interest, both for fundamental reasons and because they underpin important technological applications. The interest spans a wide spectrum of disciplines and application domains. For pharmaceuticals, a common issue is unexpected polymorphic transformation of the drug or excipient during processing or on storage, which can result in product failure. A more ambitious goal is that of exploiting the advantages of metastable polymorphs (e.g. higher solubility and dissolution rate) while ensuring their stability with respect to solid state transformation. To address these issues and to advance technology, there is an urgent need for significant insights that can only come from a detailed molecular level understanding of the involved processes. Whilst experimental approaches at best yield time- and space-averaged structural information, molecular simulation offers unprecedented, time-resolved molecular-level resolution of the processes taking place. This review aims to provide a comprehensive and critical account of state-of-the-art methods for modelling polymorph stability and transitions between solid phases. This is flanked by revisiting the associated macroscopic theoretical framework for phase transitions, including their classification, proposed molecular mechanisms, and kinetics. The simulation methods are presented in tutorial form, focusing on their application to phase transition phenomena. We describe molecular simulation studies for crystal structure prediction and polymorph screening, phase coexistence and phase diagrams, simulations of crystal-crystal transitions of various types (displacive/martensitic, reconstructive and diffusive), effects of defects, and phase stability and transitions at the nanoscale. Our selection of literature is intended to illustrate significant insights, concepts and understanding, as well as the current scope of using molecular simulations for understanding polymorphic transitions in an accessible way, rather than claiming completeness. With exciting prospects in both simulation methods development and enhancements in computer hardware, we are on the verge of accessing an unprecedented capability for designing and developing dosage forms and drug delivery systems in silico, including tackling challenges in polymorph control on a rational basis

Indentation and self-healing mechanisms of a self-assembled monolayer:a combined experimental and modeling study

A combination of in situ vibrational sum-frequency generation (SFG) spectroscopy and molecular-dynamics (MD) simulations has allowed us to study the effects of indentation of self-assembled octadecylphosphonic acid (ODPA) monolayers on α-Al2O3(0001). Stress-induced changes in the vibrational signatures of C–H stretching vibrations in SFG spectra and the results of MD simulations provide clear evidence for an increase in gauche-defect density in the monolayer as a response to indentation. A stress-dependent analysis indicates that the defect density reaches saturation at approximately 155 MPa. After stress is released, the MD simulations show an almost instantaneous healing of pressure-induced defects in good agreement with experimental results. The lateral extent of the contact areas was studied with colocalized SFG spectroscopy and compared to theoretical predictions for pressure gradients from Hertzian contact theory. SFG experiments reveal a gradual increase in gauche-defect density with pressure before saturation close to the contact center. Furthermore, our MD simulations show a spatial anisotropy of pressure-induced effects within ODPA domains: molecules tilted in the direction of the pressure gradient increase in tilt angle while those on the opposite side form gauche-defects

Deciphering the molecular mechanism of water boiling at heterogeneous interfaces

Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites’ location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms

Aus der Geburtsstube von Nanokristallen: Computersimulationen der Aggregation von Ionen und der Entstehung geordneter Strukturen

The study of crystal nucleation represents a considerable challenge to both experiment and theory. Crystallisation from solutions is initiated by the association of only a few ions. The resulting aggregates are the embryonic precursors to crystals and exhibit diameters of less than a nanometre. While experimental studies offer a wide variety of insights at the macroscopic scale, the atomistic level of detail often remains elusive. On the other hand, computer simulation approaches may easily achieve microscopic resolution and hence appear particularly suited for analysis of the mechanisms of ion aggregation. On the basis of atomistic models, new insights are obtained into the early steps of ion association and the self-organisation of disordered aggregates into crystalline structures.Das Studium der Nukleation von Kristallen stellt eine immense Herausforderung sowohl an die Experimentatoren als auch an die Theoretiker dar. Die Bildung eines Kristalls aus einer Lösung beginnt mit dem Zusammenschluss einzelner Ionen zu kleinen Aggregaten. Diese embryonalen Vorstufen von Kristallen umfassen nur einige Teilchen und weisen Durchmesser von weniger als einem Nanometer auf. Experimentelle Untersuchungen sind oftmals auf die makro- und mesoskopische Größenskala beschränkt und können vergleichsweise wenige Informationen über die atomaren Aggregationsprozesse liefern. Molekulare Simulationen verlaufen im Gegensatz dazu unmittelbar auf der atomaren Detailstufe und stellen so eine hervorragende Ergänzung zum Experiment dar. Im Computer werden dabei Modellszenarien entwickelt, die Aufschlüsse über die elementaren Schritte der Aggregation von Ionen geben können und aufzeigen, wie sich zunächst ungeordnete Agglomerate allmählich zu periodisch geordneten Strukturen organisieren

Directed Dehydration as Synthetic Tool for Generation of a New Na4 SnS4 Polymorph: Crystal Structure, Na+ Conductivity, and Influence of Sb-Substitution

We present the convenient synthesis and characterization of the new ternary thiostannate Na4 SnS4 (space group I41/acd ) by directed removal of crystal water molecules from Na4 SnS4 ⋅14 H2 O. The compound represents a new kinetically stable polymorph of Na4 SnS4 , which is transformed into the known, thermodynamically stable form (space group P4‾21c ) at elevated temperatures. Thermal co-decomposition of mixtures with Na3 SbS4 ⋅9 H2 O generates solid solution products Na4-x Sn1-x Sbx S4 (x=0.01, 0.10) isostructural to the new polymorph (x=0). Incorporation of Sb5+ affects the bonding and local structural situation noticeably evidenced by X-ray diffraction, 119 Sn and 23 Na NMR, and 119 Sn Mössbauer spectroscopy. Electrochemical impedance spectroscopy demonstrates an enormous improvement of the ionic conductivity with increasing Sb content for the solid solution (σ25°C =2×10-3 , 2×10-2 , and 0.1 mS cm-1 for x=0, 0.01, and 0.10), being several orders of magnitude higher than for the known Na4 SnS4 polymorph

Directed Dehydration as Synthetic Tool for Generation of a New Na4_{4}SnS4_{4} Polymorph: Crystal Structure, Na+^{+} Conductivity, and Influence of Sb‐Substitution

We present the convenient synthesis and characterization of the new ternary thiostannate Na$_{4}$SnS$_{4}$4 (space group I4$_{1}$/acd ) by directed removal of crystal water molecules from Na$_{4}$SnS$_{4}$⋅14 H$_{2}$O. The compound represents a new kinetically stable polymorph of Na$_{4}$SnS$_{4}$, which is transformed into the known, thermodynamically stable form (space group P$\overline{4}$2$_{1}$c) at elevated temperatures. Thermal co-decomposition of mixtures with Na$_{3}$SbS$_{4}$⋅9 H$_{2}$O generates solid solution products Na$_{4}-x$Sn$_{1-x}$Sb$_{x}$S$_{4}$ (x=0.01, 0.10) isostructural to the new polymorph (x=0). Incorporation of Sb$^{5+}$ affects the bonding and local structural situation noticeably evidenced by X-ray diffraction, $_{119}$Sn and $_{23}$Na NMR, and $_{119}$Sn Mössbauer spectroscopy. Electrochemical impedance spectroscopy demonstrates an enormous improvement of the ionic conductivity with increasing Sb content for the solid solution (σ$_{25°}$C=2×10$_{-3}$, 2×10$_{-2}$, and 0.1 mS cm$_{-1}$ for x=0, 0.01, and 0.10), being several orders of magnitude higher than for the known Na$_{4}$SnS$_{4}$ polymorph