Changes In Local Order Research Articles

Surface properties of clinker phases and clay minerals characterized by inverse gas chromatography (IGC) and their link to reactivity

This paper presents a preliminary study of the characterization of the surface energy properties of clinker phases (C3S and C3A), kaolin and metakaolin by Inverse Gas Chromatography (IGC). For this, a reliable measurement methodology was developed. By looking at changes in the whole series of results (dispersive surface energy, specific polar interaction parameter, acid and base constants, morphology index, nanoroughness and adsorption energy distribution function), it is possible to discern changes between the same powders with different surface treatments. A promising correlation between surface properties and the reactivity of studied materials have been found. However, based on the IGC characterization, the increased reactivity of metakaolin compared to the raw kaolinite seems to be strongly linked to the change in local order rather than significant changes in the surface energetics, although a change in the acid/base nature of the surface has been observed.

Amphoteric behavior of component and microstructure feature on CaO-Al2O3-TiO2 ternary melt by molecular dynamics simulation

Amphoteric oxides, Al2O3 and TiO2, are essential compositions in metallurgical melts and have an important role in regulating slag performance. In this study, the structural characteristics of the CaO-Al2O3-TiO2 ternary melt, such as local structural order and particle coordination changes, are investigated by molecular dynamics simulation. The effects of Al2O3 and TiO2 on the microstructure of various melts and their role differences are analyzed. The results show that the amphoteric behavior of TiO2 dominate with low titanium concentration ranging from 3 mol% to 18 mol%. It promotes the change of Al2O3 from acidic to alkaline. Meanwhile, the value of (NBO/T) increases and the melt’s viscosity decreases. In addition, the amphoteric behavior of TiO2 dominate in the high titanium system, ranging from 52 mol% to 66 mol% of TiO2 concentration. It promotes the conversion of TiO2 to alkaline. Meanwhile, the melt polymerization and viscosity are enhanced. As for the ‘aluminum avoidance principle’, it only applies to low-titanium system and invalid in high-titanium system.

On the devitrification of Cu–Zr–Al alloys: Solving the apparent contradiction between polymorphic liquid-liquid transition and phase separation

In this work, the crystallization pathways and kinetics of the Cu47.5Zr45.1Al7.4 bulk metallic glass (BMG) are studied systematically using a broad collection of laboratory and synchrotron-based techniques, such as differential scanning calorimetry, transmission electron microscopy, in situ high-energy synchrotron X-ray diffraction (HEXRD), simultaneous small- and wide-angle X-ray scattering, and atom probe tomography. The crystallization sequence during heating at conventional slow rates and isothermal annealing near the glass transition temperature was determined to consist in preferential formation of (Cu, Al)10Zr7 followed by the formation of CuZr2 and AlCu2Zr induced by long-range atomic diffusion. In situ HEXRD shows that the formation of the latter two phases is accompanied by the partial dissolution of the primary Cu-rich phase. Microstructural characterizations of annealed samples suggest that a new liquid phase with enhanced local order and obscure compositional changes has formed from the homogeneous supercooled liquid before crystallization. A concept analogous to the disorder-to-order transition in crystalline materials is applied here to describe the observed transition in liquid, resolving the longstanding apparent contradiction between observing a nanoscale phase separation versus a liquid–liquid phase transition during devitrification. Furthermore, we suggest that the formation of an intermediate liquid phase prior to crystallization is the structural origin of the rapid devitrification kinetics during the primary crystallization. On the basis of our combined experimental results, a scheme is proposed to generalize the complex multistep devitrification mechanisms of the Cu47.5Zr45.1Al7.4 BMG during isothermal annealing or slow heating. This work may provide new insights into the crystallization behavior and thermal stability of Cu–Zr–Al BMGs in terms of thermodynamics and kinetics and might lead to deeper understanding of the underlying mechanisms of the often observed rapid crystallization processes of deeply undercooled metallic liquids.

Membrane hydrophobicity determines the activation free energy of passive lipid transport

The collective behavior of lipids with diverse chemical and physical features determines a membrane’s thermodynamic properties. Yet, the influence of lipid physicochemical properties on lipid dynamics, in particular interbilayer transport, remains underexplored. Here, we systematically investigate how the activation free energy of passive lipid transport depends on lipid chemistry and membrane phase. Through all-atom molecular dynamics simulations of 11 chemically distinct glycerophospholipids, we determine how lipid acyl chain length, unsaturation, and headgroup influence the free energy barriers for two elementary steps of lipid transport: lipid desorption, which is rate limiting, and lipid insertion into a membrane. Consistent with previous experimental measurements, we find that lipids with longer, saturated acyl chains have increased activation free energies compared to lipids with shorter, unsaturated chains. Lipids with different headgroups exhibit a range of activation free energies; however, no clear trend based solely on chemical structure can be identified, mirroring difficulties in the interpretation of previous experimental results. Compared to liquid-crystalline phase membranes, gel phase membranes exhibit substantially increased free energy barriers. Overall, we find that the activation free energy depends on a lipid’s local hydrophobic environment in a membrane and that the free energy barrier for lipid insertion depends on a membrane’s interfacial hydrophobicity. Both of these properties can be altered through changes in lipid acyl chain length, lipid headgroup, and membrane phase. Thus, the rate of lipid transport can be tuned through subtle changes in local membrane composition and order, suggesting an unappreciated role for nanoscale membrane domains in regulating cellular lipid dynamics.

Melittin Induces Local Order Changes in Artificial and Biological Membranes as Revealed by Spectral Analysis of Laurdan Fluorescence.

Antimicrobial peptides (AMPs) are a class of molecules widely used in applications on eukaryotic and prokaryotic cells. Independent of the peptide target, all of them need to first pass or interact with the plasma membrane of the cells. In order to have a better image of the peptide action mechanism with respect to the particular features of the membrane it is necessary to better understand the changes induced by AMPs in the membranes. Laurdan, a lipid membrane probe sensitive to polarity changes in the environment, is used in this study for assessing changes induced by melittin, a well-known peptide, both in model and natural lipid membranes. More importantly, we showed that generalized polarization (GP) values are not always efficient or sufficient to properly characterize the changes in the membrane. We proved that a better method to investigate these changes is to use the previously described log-normal deconvolution allowing us to infer other parameters: the difference between the relative areas of elementary peak (ΔSr), and the ratio of elementary peaks areas (Rs). Melittin induced a slight decrease in local membrane fluidity in homogeneous lipid membranes. The addition of cholesterol stabilizes the membrane more in the presence of melittin. An opposite response was observed in the case of heterogeneous lipid membranes in cells, the local order of lipids being diminished. RS proved to be the most sensitive parameter characterizing the local membrane order, allowing us to distinguish among the responses to melittin of both classes of membrane we investigated (liposomes and cellular membranes). Molecular simulation of the melittin pore in homogeneous lipid bilayer suggests that lipids are more closely packed in the proximity of the melittin pore (a smaller area per lipid), supporting the experimental observation.

Single-pulse (100 ps) extended x-ray absorption fine structure capability at the Dynamic Compression Sector.

Determining real-time changes in the local atomistic order is important for a mechanistic understanding of shock wave induced structural and chemical changes. However, the single event and short duration (nanosecond times) nature of shock experiments pose challenges in obtaining Extended X-ray Absorption Fine Structure (EXAFS) measurements-typically used for monitoring local order changes. Here, we report on a new single pulse (∼100 ps duration) transmission geometry EXAFS capability for use in laser shock-compression experiments at the Dynamic Compression Sector (DCS), Advanced Photon Source. We used a flat plate of highly oriented pyrolytic graphite (HOPG) as the spectrometer element to energy disperse x rays transmitted through the sample. It provided high efficiency with ∼15% of the x rays incident on the HOPG reaching an x-ray area detector with high quantum efficiency. This combination resulted in a good signal-to-noise ratio (∼103), an energy resolution of ∼10 eV at 10 keV, EXAFS spectra covering 100 s of eV, and a good pulse to pulse reproducibility of our single pulse measurements. Ambient EXAFS spectra for Cu and Au are compared to the reference spectra, validating our measurement system. Comparison of single pulse EXAFS results for ambient and laser shocked Ge(100) shows large changes in the local structure of the short lived state of shocked Ge. The current DCS EXAFS capability can be used to perform single pulse measurements in laser shocked materials from ∼9 keV to 13 keV. These EXAFS developments will be available to all users of the DCS.

Managing Local Order in Conjugated Polymer Blends via Polarity Contrast

The optoelectronic landscape of conjugated polymers is intimately related to their molecular arrangement and packing, with minute changes in local order, such as chain conformation and torsional backbone order/disorder, frequently having a substantial effect on macroscopic properties. While many of these local features can be manipulated via chemical design, the synthesis of a series of compounds is often required to elucidate correlations between chemical structure and macromolecular ordering. Here, we show that blending semiconducting polymers with insulating commodity plastics enables controlled manipulation of the semiconductor backbone planarity. The key is to create a polarity difference between the semiconductor backbone and its side chains, while matching the polarity of the side chains and the additive. We demonstrate the applicability of this approach through judicious comparison of regioregular poly(3-hexylthiophene) (P3HT) with two of its more polar derivatives, namely the diblock copolymer poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) and the graft polymer poly[3-but(ethylene oxide)thiophene] (P3BEOT), as well as their blends with poly(ethylene oxide) (PEO). Proximity between polar side chains and a similarly polar additive reduces steric hindrance between individual chain segments by essentially ‘expelling’ the side chains away from the semiconducting backbones. This process, which has been shown to be facilitated via exposure to polar environments such as humid air/water vapor, facilitates backbone realignment to-wards specific chain arrangements and, in particular, planar backbone configurations.

Relaxation of dynamically disordered tetragonal platelets in the relaxor ferroelectric 0.964Na1/2Bi1/2TiO3−0.036BaTiO3

The local dynamics of the lead-free relaxor $0.964\mathrm{Na}_{1/2}\mathrm{Bi}_{1/2}\mathrm{TiO}_3-0.036\mathrm{BaTiO}_3$ (NBT-3.6BT) have been investigated by a combination of quasielastic neutron scattering (QENS) and ab initio molecular dynamics simulations. In a previous paper, we were able to show that the tetragonal platelets in the microstructure are crucial for understanding the dielectric properties of NBT-3.6BT [F. Pforr et al., Phys. Rev. B 94, 014105 (2016)]. To investigate their dynamics, ab initio molecular dynamics simulations were carried out using $\mathrm{Na}_{1/2}\mathrm{Bi}_{1/2}\mathrm{TiO}_3$ with 001 cation order as a simple model system for the tetragonal platelets in NBT-3.6BT. Similarly, 111-ordered $\mathrm{Na}_{1/2}\mathrm{Bi}_{1/2}\mathrm{TiO}_3$ was used as a model for the rhombohedral matrix. The measured single crystal QENS spectra could be reproduced by a linear combination of calculated spectra. We find that the relaxational dynamics of NBT-3.6BT are concentrated in the tetragonal platelets. Chaotic stages, during which the local tilt order changes incessantly on the timescale of several picoseconds, cause the most significant contribution to the quasielastic intensity. They can be regarded as an excited state of tetragonal platelets, whose relaxation back into a quasistable state might explain the frequency dependence of the dielectric properties of NBT-3.6BT in the 100 GHz to THz range. This substantiates the assumption that the relaxor properties of NBT-3.6BT originate from the tetragonal platelets.

From two- to three-dimensional alumina: Interface templated films and formation of γ−Al2O3 (111) nuclei

Oxide thin films are of fundamental importance due to their applications in materials science, optics, corrosion protection, catalysis, and microelectronics. A multistep oxidation procedure is employed to precisely tune the alumina (${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$) thickness on a NiAl(110) alloy from two atomic bilayers to $1.5\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. Structural changes are analyzed with x-ray photoelectron diffraction and low-energy electron diffraction. The long-range order does not relate to any bulk structure and is imposed by the crystallized interface. The large unit cell formed at the interface persists in thicker films. In contrast, the local order changes at a thickness above $0.5\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ from the complex structure of this prelayer under the formation of subnanometer-sized $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(111)-type nuclei. The band structure is monitored with angle-resolved photoelectron spectroscopy. Increasing film thickness results in a slight decrease of the work function, but does not lead to significant changes of the electronic band structure. The presented multistep procedure opens a route for the synthesis of thin film structures in general and in particular provides fundamental insight in the surface structure of spinel-based transition ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$.

SrTi1−yFeyO3 samples obtained by hydrothermal method: The effect of the amount of Fe on structural and photocatalytic properties

This manuscript describes the synthesis and the photocatalytic efficiency of SrTiO3 (STO) and SrTi1−yFeyO3 (STFO, with 2.5, 5, 10, 25 and 40mol% of Fe) materials evaluated by monitoring the photodegradation of methylene blue (MB) dye under visible and UVC irradiation. Samples synthesized by conventional hydrothermal method were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption isotherms, UV–Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS) and Mössbauer spectroscopy. The results show that a controlled introduction of Fe into Ti sites caused changes in morphology, local order, band gap, textural properties and photoactivity of the STO compound. By increasing the Fe content, the surface roughness was also increased when compared with the STO sample. Furthermore, the highest amount of Fe caused significant changes in the STO lattice, especially in relation to the local order around Ti atoms. The Mössbauer analysis confirmed that Ti4+ atoms were replaced by Fe3+. The sample with the lower amount of Fe (2.5mol%) presented photoactivity similar to that of pure STO. The samples containing intermediate amounts of Fe (5 and 10mol%) presented good performances on the methylene blue (MB) dye photodegradation, while others with higher amounts of Fe (25 and 40mol%) led to a large distortion of the crystalline structure, which disfavored the photocatalytic performance. The enhanced photoactivity of the some STFO samples was related to three combined effects: an improvement of the textural property; a decrease in the rate of electron-hole recombination; and a broadening of the photocatalyst optical absorption band, which is important for the photodegradation under visible irradiation. These positive characteristics were predominant for both 5mol% and 10mol% STFO samples. The one with 5mol% has presented the best performance for MB photodegradation, which was mainly attributed to its best textural properties.

Tiny changes in local order identify the cluster formation threshold in model fluids with competing interactions.

We use Monte Carlo simulations to carry out a thorough analysis of structural correlations arising in a relatively dense fluid of rigid spherical particles with prototype competing interactions (short-range attractive and long-range repulsive two-Yukawa model). As the attraction strength increases, we show that the local density of the fluid displays a tiny reversal of trend within specific ranges of interparticle distances, whereupon it decreases first and increases afterwards, passing through a local minimum. Particles involved in this trend display, accordingly, distinct behaviours: for a sufficiently weak attraction, they seem to contribute to the long-wave oscillations typically heralding the formation of patterns in such fluids; for a stronger attraction, after the reversal of the local density has occurred, they form an outer shell of neighbours stabilizing the existing aggregation seeds. Following the increment of attraction, precisely in correspondence of the local density reversal, the local peak developed in the structure factor at small wavevectors markedly rises, signalling-in agreement with recent structural criteria-the onset of a clustered state. A detailed cluster analysis of microscopic configurations fully validates this picture.

An Interesting Class of Porous Polymer--Revisiting the Structure of Mesoporous α-D-Polysaccharide Gels.

The processes involved in the transformation of non-porous, native polysaccharides to their highly porous equivalents introduce significant molecular complexity and are not yet fully understood. In this paper, we propose that distinct changes in polysaccharide local short-range ordering promotes and directs the formation of meso- and micro-pores, which are investigated here using N2 sorption, FTIR, and solid-state (13)C NMR. It is found that an increase in the overall double helical amylose content, and their local association structures, are responsible for formation of the porous polysaccharide gel phase. An exciting consequence of this local ordering change is elegantly revealed using a (19)F NMR experiment, which identifies the stereochemistry-dependent diffusion of a fluorinated chiral probe molecule (1-phenyl-2,2,2-trifluoroethanol) from the meso- to the micro-pore region. This finding opens opportunities in the area of polysaccharide-based chiral stationary phases and asymmetric catalyst preparation.

Microwave-assisted hydrothermal synthesis of Ag2(W1 -xMox)O4 heterostructures: Nucleation of Ag, morphology, and photoluminescence properties

Ag2W1-xMoxO4 (x=0.0 and 0.50) powders were synthesized by the co-precipitation (drop-by-drop) method and processed using a microwave-assisted hydrothermal method. We report the real-time in situ formation and growth of Ag filaments on the Ag2W1-xMoxO4 crystals using an accelerated electron beam under high vacuum. Various techniques were used to evaluate the influence of the network-former substitution on the structural and optical properties, including photoluminescence (PL) emission, of these materials. X-ray diffraction results confirmed the phases obtained by the synthesis methods. Raman spectroscopy revealed significant changes in local order–disorder as a function of the network-former substitution. Field-emission scanning electron microscopy was used to determine the shape as well as dimensions of the Ag2W1-xMoxO4 heterostructures. The PL spectra showed that the PL-emission intensities of Ag2W1-xMoxO4 were greater than those of pure Ag2WO4, probably because of the increase of intermediary energy levels within the band gap of the Ag2W1-xMoxO4 heterostructures, as evidenced by the decrease in the band-gap values measured by ultraviolet–visible spectroscopy.

Dependence of local order on cooling rate investigated in a colloidal glass

The cooling rate with which liquid is cooled tremendously affects the properties of amorphous solids, but little information on underlying mechanism for this dependence is available, mainly due to the lack of clear characterization on microstructural variation. We built a colloidal glass to directly investigate the effect of cooling rate on local order. By quantifying effective cooling rate and structural properties, we show how local order changes with cooling rate. Furthermore we demonstrate that free-volume distribution results in local-order evolution.

Nature of electrical resistivity and structural stability in N-doped GeTe models for reliable phase-change materials

We study the effects of nitrogen (N)-doping on the electrical memory reliability of GeSbTe phase-change materials based on GeTe prototype models. We find that the loss of secondary bonding (e.g., resonant interlayer bonding) determines the feasibility of various types of N-doping that can be easily adopted by the GeSbTe system. We give a more generalized explanation beyond compliance with the formation of local Ge3N4 motifs. The nitrogen-induced change in local order produces crystalline GeTe with shallow states near the valence band edge. These states are localized on the nearest-neighbor Ge sites, thus reducing the conductivity in the crystalline phase. This trend carries over to c-GeTe with Ge vacancy. The N-doped amorphous GeTe models exhibit enhanced degrees of band tail overlap, which pins the Fermi energy in the mid-gap and thus gives rise to even higher resistivities in the amorphous phases.

Calorimetric and magnetic study for Ni50Mn36In14 and relative cooling power in paramagnetic inverse magnetocaloric systems

The non-stoichiometric Heusler alloy Ni50Mn36In14 undergoes a martensitic phase transformation in the vicinity of 345 K, with the high temperature austenite phase exhibiting paramagnetic rather than ferromagnetic behavior, as shown in similar alloys with lower-temperature transformations. Suitably prepared samples are shown to exhibit a sharp transformation, a relatively small thermal hysteresis, and a large field-induced entropy change. We analyzed the magnetocaloric behavior both through magnetization and direct field-dependent calorimetry measurements. For measurements passing through the first-order transformation, an improved method for heat-pulse relaxation calorimetry was designed. The results provide a firm basis for the analytic evaluation of field-induced entropy changes in related materials. An analysis of the relative cooling power (RCP), based on the integrated field-induced entropy change and magnetizing behavior of the Mn spin system with ferromagnetic correlations, shows that a significant RCP may be obtained in these materials by tuning the magnetic and structural transformation temperatures through minor compositional changes or local order changes.

Temperature effects on atomic pair distribution functions of melts

Using molecular dynamics simulations, we investigate the temperature-dependent evolution of the first peak position/shape in pair distribution functions of liquids. For metallic liquids, the peak skews towards the left (shorter distance side) with increasing temperature, similar to the previously reported anomalous peak shift. Making use of constant-volume simulations in the absence of thermal expansion and change in inherent structure, we demonstrate that the apparent shift of the peak maximum can be a result of the asymmetric shape of the peak, as the asymmetry increases with temperature-induced spreading of neighboring atoms to shorter and longer distances due to the anharmonic nature of the interatomic interaction potential. These findings shed light on the first-shell expansion/contraction paradox for metallic liquids, aside from possible changes in local topological or chemical short-range ordering. The melts of covalent materials are found to exhibit an opposite trend of peak shift, which is attributed to an effect of the directionality of the interatomic bonds.

Structural change of carbon supported Pt nanocatalyst subjected to a step-like potential cycling in PEM FC

In this paper we present detailed X-ray absorption fine structure (XAFS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations of the changes in the local geometric and electronic structure of Pt nanoparticles used as a cathode catalyst in proton exchange membrane fuel cell (PEMFC), working under controlled potential cycling conditions. The body of the results obtained suggests that in the first stage of PEMFC operation, small particle dissolution was a dominant process. Subsequent 100h of work led to the progressive agglomeration of nanoparticles followed by a pronounced growth of the mean nanoparticle size. At the same time, high-quality XAFS spectra analysis demonstrated that negligible changes in structural local ordering and a slight increase in Pt 5d-electron density occurred during the whole FC operation period under consideration.

Alpha Decays Impact on Nuclear Glass Structure

This paper reviews the main results on the long-term behavior of SON68 nuclear glass towards alpha decay accumulation. The effects of the radiation damage induced by alpha decay were investigated by doping a glass with a short-lived actinide 244Cm, by using ionic irradiations and by molecular dynamic simulations.The analysis of the behavior of the glass structure subjected to ballistic effects with various spectroscopic studies has identified some local order changes around born and silicon atoms. Moreover a modification of the medium-range order has also been demonstrated through changes in the bond angles between network formers and broadening of the ring size distributions, indicating increasing disorder of the glass structure. This structural evolution induced by alpha decays would be driven by the reconstruction of the glass disorganized by displacement cascades of the recoil nuclei, freezing a glass structure with a higher fictive temperature. This “ballistic disordering (BD) fast quenching” event induce a new glassy state characterized by a higher enthalpy state.

The determinants of hydrophobic mismatch response for transmembrane helices

Hydrophobic mismatch arises from a difference in the hydrophobic thickness of a lipid membrane and a transmembrane protein segment, and is thought to play an important role in the folding, stability and function of membrane proteins. We have investigated the possible adaptations that lipid bilayers and transmembrane α-helices undergo in response to mismatch, using fully-atomistic molecular dynamics simulations totaling 1.4μs. We have created 25 different tryptophan-alanine-leucine transmembrane α-helical peptide systems, each composed of a hydrophobic alanine–leucine stretch, flanked by 1–4 tryptophan side chains, as well as the β-helical peptide dimer, gramicidin A. Membrane responses to mismatch include changes in local bilayer thickness and lipid order, varying systematically with peptide length. Adding more flanking tryptophan side chains led to an increase in bilayer thinning for negatively mismatched peptides, though it was also associated with a spreading of the bilayer interface. Peptide tilting, bending and stretching were systematic, with tilting dominating the responses, with values of up to ~45° for the most positively mismatched peptides. Peptide responses were modulated by the number of tryptophan side chains due to their anchoring roles and distributions around the helices. Potential of mean force calculations for local membrane thickness changes, helix tilting, bending and stretching revealed that membrane deformation is the least energetically costly of all mismatch responses, except for positively mismatched peptides where helix tilting also contributes substantially. This comparison of energetic driving forces of mismatch responses allows for deeper insight into protein stability and conformational changes in lipid membranes.