Local Structural Order Research Articles

Homeotropically Aligned Monodomain-like Smectic-A Structure in Liquid Crystalline Epoxy Films: Analysis of the Local Ordering Structure by Microbeam Small-Angle X-ray Scattering.

For the development of functional thin films with high thermal conductivity, the local ordering structure of a cured liquid crystalline epoxy resin (LCER) droplet was investigated by using synchrotron radiation microbeam small-angle X-ray scattering. The cured LCER in the vicinity of a substrate with low surface free energy was revealed to form a polydomain smectic-A (SmA) structure in which the normal direction of the layers was random in each domain, although the alignment was planar near the air interface. On the other hand, the cured LCER on a substrate with high surface free energy formed a homeotropically aligned SmA structure in the region within 21 μm from the surface of the substrate. Therefore, a 20 μm thick LCER film was fabricated and found to form a homeotropically aligned monodomain-like SmA structure throughout the whole film with a high thermal conductivity (0.81–5.8 W m–1 K–1). This film with a high thermal conductivity is expected to be applicable for adhesion and precoating materials for electrical and electronic devices.

Effects of the post-annealing reductive-atmosphere-sintered (K0.48Na0.52)NbO3 lead-free piezoceramics

Post-annealing in a weak oxidative atmosphere (a partial pressure of oxygen P(O2) = 1 × 10-6 MPa in high purity N2) was applied to the reductive-atmosphere-sintered (K0.48Na0.52)NbO3 lead-free piezoceramics. Structural characterization indicated that the Nb–O bond length and local structure order degree of the (K0.48Na0.52)NbO3 ceramics increased after post-annealing. The dielectric and impedance spectra analysis revealed that the oxygen vacancies could be compensated after the post-annealing process. The piezoelectric performances of the KNN ceramics post-annealed at 900 °C for 4 h were comparable with that of counterpart sintered in the air. These results indicate that the post-annealing treatment may provide a prospective approach for the fabrication of base metal electrode-cofired KNN multilayer piezoelectric devices.

Matching vacancy formation energy and defect levels with the density of amorphous Ga2O3

In this study, we present six structural models for amorphous Ga2O3 (a-Ga2O3) with different mass densities from ab initio molecular dynamics simulations. The detailed local structural order, non-perfect coordination number, electronic and vibrational properties of a-Ga2O3 as well as the energetics and stability of O vacancy with various charge states have been systemically investigated. As the density increases, the first peak in the radial distribution function is weakened and the range of vibrational frequencies is narrowed, while the average coordination number and band gap gradually increase. Density also plays an important role in oxygen vacancy formation in a-Ga2O3. In the low-density a-Ga2O3, oxygen vacancy is more stable and the average formation energy of oxygen vacancy (VO) with neutral state is 3.32–3.70 eV lower than that in crystalline β-Ga2O3 (4.20 eV). In the high-density a-Ga2O3, VO in the 2 + charge state may form spontaneously in the case of lower Fermi energy and the thermodynamic transition energy for VO was found to shift to higher Fermi energy. Hence, the electron trap levels observed in experiment might be correlated with local structural distortion and non-perfect coordination number. Moreover, highly localized valence tail has been observed in all the six a-Ga2O3 systems. Thus, it would be hard to shift the Fermi level to the valence band, leading to rarely observed p-type conductivity in amorphous Ga2O3. These results not only establish a fundamental picture on the origin of a wide range of defect levels in Ga2O3 thin films deposited under different conditions, but also provide useful evidence to identify them with oxygen vacancy states.

Local structural changes in Ce1-xLnxO2-δ (Ln = La, Gd) solid electrolytes

The local structural changes in La and Gd in doped ceria were investigated using the XAFS technique. In addition, Ce1-xLnxO2-δ (Ln = La, Gd; LDC) (0 ≤ x ≤ 0.5) was synthesized through a coprecipitation method and the local structures were analyzed based on the XAFS spectra for the first and second neighbor shells of the constituent cations. For these two types of materials, the variation in the coordination number of the Ln and Ce cations showed different trends as a function of the dopant concentration, x. For LDC, a local structural ordering was formed at x ≥ 0.3 similar to a pyrochlore-type structure, in which the oxygen vacancies are preferentially located adjacent to La3+. For Ce1-xGdxO2-δ (GDC), no distinct differences in the coordination number of the cations were observed.The combined XAFS and XRD results successfully showed that the weighted average distances of the longer Ln-Ln (Ln = La for LDC and Ln = Gd for GDC) and shorter Ce-Ce at each composition, as obtained through XAFS analysis, fit extremely well with the lattice parameter obtained using XRD. A model of the local structural change was derived to explain the variation in the electrical conductivity during the early stage of Ln doping.

Effect of pre-orientation on formation of microstructure of lamella crystal and the stress response of semicrystalline polymers: Molecular dynamics simulations

Molecular dynamics simulations have been used to study the effect of the pre-orientation on the microstructure of lamella crystal and the stress response of polyvinyl alcohol (PVA) semicrystalline polymer under stretching. For the different pre-oriented systems, nucleation is demonstrated to be a two-step process, however, in a different intermediate order. For the isotropic PVA polymer melt, the segment needs more time to adjust its inter-chain structure, therefore, the nucleation is assisted by local order structures, while the nucleation of the oriented PVA melt is promoted by density fluctuation. The nucleation process is the result of coupling effect of conformational and orientational ordering. The transformation from flexible chains into conformational ordered segments circumvents the entropic penalty under the shear flow, which is the most peculiar and rate-limited step in polymer crystallization. Therefore, the current work suggests that the acceleration of the nucleation rate by shear deformation is mainly attributed to the different kinetic pathway via conformational/orientational ordering-density fluctuation-nucleation. From the different pre-oriented PVA semicrystalline polymers, we know that the higher oriented degree corresponds to a higher number of Tie chains and lower Loop chains, and the higher number of Tie chains corresponds to a stronger stress-strain response. And the detailed molecular structural evolution of semicrystalline polymer under stretching is also given in this work.

Large and Robust Charge-to-Spin Conversion in Sputtered Disordered WTe <sub>x</sub>

Topological insulators have recently shown great promise for ultralow-power spin-orbit torque (SOT) devices thanks to their large charge-to-spin conversion efficiency originating from the spin-momentum-locked surface states. Recently, Weyl semimetals emerge as an alternative SOT source with spin-polarized surface as well as bulk states, robustness against magnetic and structural disorder, and higher electrical conductivity for integration in metallic magnetic tunnel junctions. Here, we report that long-range disordered sputtered WTex thin films exhibit local chemical and structural order as those of Weyl semimetal WTe2 and conduction behavior that is consistent with semi-metallic Weyl fermion. We find large charge-to-spin conversion properties in thermally annealed sputtered WTex films that are comparable with those in crystalline WTe2 flakes. Besides, the strength of unidirectional spin Hall magnetoresistance in annealed WTex/Mo/CoFeB heterostructure is 5 to 20 times larger than typical SOT/ferromagnet bilayers reported at room temperature. We further demonstrate room temperature field-free magnetization switching at a current density of 1 – 2 MA/cm2. These large charge-to-spin conversion properties that are robust in the presence of long-range disorder and thermal annealing pave the way for industrial integration of such topological materials. Our results open a new class of sputtered semimetals for memory and computing based on magnetic tunnel junctions as well as broader planar heterostructures consisting of SOT layer/ferromagnet interfaces.

Local structure order parameters and site fingerprints for quantification of coordination environment and crystal structure similarity.

Structure characterization and classification is frequently based on local environment information of all or selected atomic sites in the crystal structure. Therefore, reliable and robust procedures to find coordinated neighbors and to evaluate the resulting coordination pattern (e.g., tetrahedral, square planar) are critically important for both traditional and machine learning approaches that aim to exploit site or structure information for predicting materials properties. Here, we introduce new local structure order parameters (LoStOPs) that are specifically designed to rapidly detect highly symmetric local coordination environments (e.g., Platonic solids such as a tetrahedron or an octahedron) as well as less symmetric ones (e.g., Johnson solids such as a square pyramid). Furthermore, we introduce a Monte Carlo optimization approach to ensure that the different LoStOPs are comparable with each other. We then apply the new local environment descriptors to define site and structure fingerprints and to measure similarity between 61 known coordination environments and 40 commonly studied crystal structures, respectively. After extensive testing and optimization, we determine the most accurate structure similarity assessment procedure to compute all 2.45 billion structure similarities between each pair of the ≈70 000 materials that are currently present in the Materials Project database.

Short- and Medium-Range Orders in Al <sub>90</sub>Tb <sub>10</sub> Glass and Their Relation to the Structures of Competing Crystalline Phases

Molecular dynamics simulations using an interatomic potential developed by artificial neural network deep machine learning are performed to study the local structural order in Al90Tb10 metallic glass. We show that more than 80% of the Tb-centered clusters in Al90Tb10 glass have short-range order (SRO) with their 17 first coordination shell atoms stacked in a ‘3661’ or ‘15551’ sequence. Medium-range order (MRO) in Bergman-type packing extended out to the second and third coordination shells is also clearly observed. Analysis of the network formed by the ‘3661’ and ‘15551’ clusters show that ~82% of such SRO units share their faces or vertexes, while only ~6% of neighboring SRO pairs are interpenetrating. Such a network topology is consistent with the Bergman-type MRO around the Tb-centers. Moreover, crystal structure searches using genetic algorithm and the neural network interatomic potential reveal several low-energy metastable crystalline structures in the composition range close to Al 90 Tb 10 . Some of these crystalline structures have the ‘3661’ SRO while others have the ‘15551’ SRO. While the crystalline structures with the ‘3661’ SRO also exhibit the MRO very similar to that observed in the glass, the ones with the ‘15551’ SRO have very different atomic packing in the second and third shells around the Tb centers from that of the Bergman-type MRO observed in the glassy phase.

Molecular dynamics investigations on the effect of Na2O on the structure and properties of blast furnace slag under different basicity conditions

The effect of Na2O on the structure and properties of SiO2-Al2O3-CaO-MgO-Na2O slag system was studied by molecular dynamics simulation. The local structure order was studied and the microstructure bond length in the blast furnace slag was summarized. It was found that Na2O has the ability to promote the coordination number of SiO pair and AlO pair. And through the analysis of the structural unit, it was found that Na2O leads to a decrease in the degree of structure complexity of the slag system by promoting the transition of BO to TO, NBO and FO. And Na2O could also promote the conversation of 4-coordinated Al to 5-coordinated Al. Then, after analyzing the transport properties of the system, it is found that the self-diffusion coefficient of each atom has the following relationship: Na > Mg > Ca > Al > O > Si. And the increase of Na2O content leads to an increase in the total self-diffusion coefficient of the slag system, resulting in a decrease in viscosity of the blast furnace slag system. Therefore, to some extent, Na2O can adjust the viscosity of the blast furnace slag.

Luminescence reveals variations in local structural order of calcium carbonate polymorphs formed by different mechanisms

In nature, calcium carbonate (CaCO3) in the form of calcite and aragonite nucleates through different pathways including geogenic and biogenic processes. It may also occur as pyrogenic lime plaster and laboratory-precipitated crystals. All of these formation processes are conducive to different degrees of local structural order in CaCO3 crystals, with the pyrogenic and precipitated forms being the least ordered. These variations affect the manner in which crystals interact with electromagnetic radiation, and thus formation processes may be tracked using methods such as X-ray diffraction and infrared spectroscopy. Here we show that defects in the crystal structure of CaCO3 may be detected by looking at the luminescence of crystals. Using cathodoluminescence by scanning electron microscopy (SEM-CL) and laser-induced fluorescence (LIF), it is possible to discern different polymorphs and their mechanism of formation. We were thus able to determine that pyrogenic calcite and aragonite exhibit blue luminescence due to the incorporation of distortions in the crystal lattice caused by heat and rapid precipitation, in agreement with infrared spectroscopy assessments of local structural order. These results provide the first detailed reference database of SEM-CL and LIF spectra of CaCO3 standards, and find application in the characterization of optical, archaeological and construction materials.

Alignment of worm-like micelles at intermediate and high shear rates

HypothesisRheology combined with Small-Angle Neutron Scattering (Rheo-SANS) can determine the local structural order in Worm-Like Micelle (WLM) solutions when the shear rate increases beyond the ending of the gradient shear banding. There, micelles are supposedly aligned, but viscosity reveals a transition regime as the shear rate increases. ExperimentsThe mixture of 3-[dimethyl(tetradecyl)azaniumyl]propane-1-sulfonate (TDPS), sodium dodecylsulfate (SDS) (R = [SDS]/[TDPS] = 0.55), and a water solution of NaCl (0.2 mol/L), was studied with mechanical rheology and Small-Angle Neutron Scattering (SANS) in the quiescent fluid and under flow. FindingsThe system self-assembles in WLMs and presents gradient shear banding. SANS patterns of the bands formed during the shear banding were obtained in a Couette geometry along the 1–2 plane, as well as the orientation parameter along the gap. At very high shear rates, in the paranematic phase, we found an apparent transition on the flow curves with its corresponding change in the orientation parameter. The origin of this transition is unclear, but we present possible explanations of why we observe it.

The Origin of Optical Contrast in Sb2Te3‐Based Phase‐Change Materials

The optical contrast between the crystalline and amorphous forms of phase‐change materials (PCMs) is so marked as to make it ideal for data storage. Various explanations have been given, including smearing of bands, changes in the local structural order and bonding, and resonant bonding. A consensus has not been reached. Herein, the dielectric function of crystalline and amorphous Sb2Te3, a topological insulator and prototype PCM, is studied using density functional theory and ellipsometry measurements; an expression for interplay between the crystalline electric field and the spin–orbit interaction on the valence band orbitals is derived. It is found that the crystalline electric field and spin–orbit interaction are responsible for this difference and the unusually large refractive index in the crystalline phase. Upon amorphization, a second‐order electronic phase transition is induced by the spin–orbit interaction's effect on the pz orbitals. This mechanism explains how a substantial optical gap can open in systems that have a small electronic band gap.

Local structure and orientational ordering in liquid bromoform

The neutron diffraction data of liquid bromoform (CHBr3) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl3), CHBr3 displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr3 molecules, although to a lesser extent than in chloroform. Consistent with this, and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr3 is smaller than that of CHCl3. The net antiparallel alignment of dipole moments in CHBr3, as suggested by dielectric spectroscopy, must be due to weak but persistent long-range orientation correlations in CHBr3, which counteract the local polar stacking.

The generalized Vogel-Fulcher-Tamman equation for describing the dynamics of relaxor ferroelectrics

Relaxor ferroelectrics (RF) are outstanding materials owing to their extraordinary dielectric, electromechanical, and electro-optical properties. Although their massive applications, they remain to be one of the most puzzling solid-state materials because understanding their structural local order and relaxation dynamics is being a long-term challenge in materials science. The so-called Vogel-Fulcher-Tamman (VFT) relation has been extensively used to parameterize the relaxation dynamics in RF, although no microscopic description has been firmly established for such empirical relation. Here, we show that VFT equation is not always a proper approach for describing the dielectric relaxation in RF. Based on the Adam-Gibbs model and the Grüneisen temperature index, a more general equation to disentangle the relaxation kinetic is proposed. This approach allows to a new formulation for the configurational entropy leading to a local structural heterogeneity related order parameter for RF. A new pathway to disentangle relaxation phenomena in other relaxor ferroics could have opened.

Anion-Dependent Perturbations in Ionic Liquid Structure Probed By High-Energy X-Ray Scattering

Room-temperature ionic liquids (ILs) are molten salts at room temperature, and their wide electrochemical windows and high ionic conductivities make them promising next-generation electrolytes for energy storage devices. The vast potential cation-anion pairings result in tunable structure and organization of the bulk ILs, which ultimately dictates their properties. However, the amorphous structure of bulk ILs remains poorly understood due to the typically large and flexible nature of the molecules and complex interactions between them. We employed high-energy synchrotron X-ray scattering to study the structure of a set of archetypal imidazolium-based ILs. We sought to understand perturbations in the structure across 3 popular anions possessing distinctly different geometric features and dipole moments (i.e., PF6 -, OTf-, TFSI-), while also systematically varying the length of the alkyl chain on the cation. In particular, we collected scattering at both high and low angles to compare local and longer-range structural order, respectively, and we leveraged pair distribution function (PDF) analysis to transform our X-ray data to real-space density fluctuations for intuitive inspection and quantitative comparison with simulations. Both classical and ab-initio molecular dynamics (MD) cross-validated these detailed intra- and intermolecular density distributions rendered by our X-ray experiments. This enabled us to use MD to decouple and characterize cation-anion, cation-cation, and anion-anion interactions as well as H-bonding type interactions for each IL studied. Developing such analytical frameworks for detailed analysis of bulk IL structure as a function of different ions is important toward developing guidelines for engineering cations and anions for targeted applications. Furthermore, understanding the influence of interchanging ions is a prerequisite for applications in which ILs interface with other species, such as other types of ions, nanoparticles, or electrode interfaces.

Revealing key structural features hidden in liquids and glasses

A great success of solid state physics comes from the characterization of crystal structures in the reciprocal (wave vector) space. The power of structural characterization in Fourier space originates from the breakdown of translational and rotational symmetries. However, unlike crystals, liquids and amorphous solids possess continuous translational and rotational symmetries on a macroscopic scale, which makes Fourier space analysis much less effective. Lately, several studies have revealed local breakdown of translational and rotational symmetries even for liquids and glasses. Here, we review several mathematical methods used to characterize local structural features of apparently disordered liquids and glasses in real space. We distinguish two types of local ordering in liquids and glasses: energy-driven and entropy-driven. The former, which is favoured energetically by symmetry-selective directional bonding, is responsible for anomalous behaviours commonly observed in water-type liquids such as water, silicon, germanium and silica. The latter, which is often favoured entropically, shows connections with the heterogeneous, slow dynamics found in hard-sphere-like glass-forming liquids. We also discuss the relationship between such local ordering and crystalline structures and its impact on glass-forming ability. The knowledge of local structures of non-crystalline materials is of fundamental importance in the understanding of their physical properties. This Review describes recent advances in local structural analysis methods, which shed new light on mysterious phenomena in liquids and glasses.

Chemical Design Principles for Cache-Type Sc–Sb–Te Phase-Change Memory Materials

Enhanced crystal nucleation in a Sc–Sb–Te phase-change material has enabled subnanosecond switching in phase-change memory devices, making cache-type nonvolatile memory feasible. However, the microscopic mechanisms remain to be further explored. In this work, we present a systematic ab initio study of the relevant parent compounds, namely, Sc2Te3 and Sb2Te3. Despite similar bond lengths and angles in the amorphous phases of the two compounds, Sc2Te3 displays a much more ordered amorphous network without homopolar bonds. As a result, the local structural order in amorphous Sc2Te3 is dominated by square motifs, remarkably similar to those of the metastable rocksalt-like phase. Chemical bonding analysis indicates more robust Sc–Te bonds compared with Sb–Te bonds in the amorphous phase, as well as a substantial role of electrostatic interactions in Sc2Te3 but not in Sb2Te3. The robustness of Sc–Te bonds explains the enhanced nucleation in Sc–Sb–Te compounds. Finally, we discuss an alloying strategy of Sc2Te3 ...

Anomalous Dynamics of a Nanoconfined Gas in a Soft Metal-Organics Framework.

Dynamics of confined molecules within porous materials is equally important as local structural order, and it is necessary to quantify it and to reveal the microscopic mechanisms ruling it for better control of adsorption applications. In this study, molecular dynamics simulations were carried out to investigate the translational and the rotational dynamics of methanol trapped into the flexible NH2-MIL-53(Al) metal-organics framework (MOF). Indeed, atomistic simulation is nowadays a relevant tool to explore matter at the nanoscale. Very recently it has been shown that the NH2-MIL-53(Al) MOF material was capable to undergo a reversible structural transition (breathing phenomenon) by combining adsorption and thermal stimuli. This flexibility can drastically affect the dynamics of confined molecules and therefore the successful conduct of adsorption applications such as gas storage and separation. Rotational and translational dynamics of confined methanol through nanoporous flexible NH2-MIL-53(Al) MOF were then deeply investigated by exploring a broad range of dynamical properties to extract the molecular mechanisms ruling them. This study allowed us to shed light on the interplay of dynamics of confined fluids and flexibility of porous material and to highlight the physical insights in diffusion mechanisms of confined molecules. Anomalous translational diffusion was evidenced due to a dynamical heterogeneity caused by a combination of a localized dynamics at the subnanometric scale and translational jumps between nanodomains in a zigzag scheme between the hydroxide group of the NH2-MIL-53(Al). Actually, the non-Fickian dynamics of methanol is the result of the specific host-guest interactions and the MOF flexibility involving the pore opening. Eventually, decoupling between both rotational and translational dynamics related to breaking in the Stokes-Einstein relation was highlighted.

Effects of cooling rate on the atomic structure and glass formation process ofCo90Zr10metallic glass investigated by molecular dynamics simulations

In this study, the atomic structure and the glass formation process of Co90 Zr10 metallic glass alloy were studied by molecular dynamic simulation based on the embedded atom method using four different cooling rates. The average atomic volume and the potential energy of the system were observed to be strongly dependent on the cooling rate during rapid solidification, and the glass transition temperature decreased with decreasing cooling rate. The radial distribution functions and the structure factors derived from molecular dynamics simulations at 300 K agreed well with the experimental and other molecular dynamics results. The coordination numbers calculated at 300 K were consistent with the experimental results in the literature. The local structural atomic orders of the system have been characterized by Honeycutt-Andersen indices and the Voronoi tessellation method. We realized that the FCC and icosahedral shortrange order increases with decreasing cooling rate, the icosahedral clusters are predominant in the Co90 Zr10 metallic glass, and the fraction of the icosahedral polyhedra increases with decreasing cooling rate and temperature. We have seen that Co-centered Voronoi polyhedrals such as , , and play a dominant role in the development of the icosahedral order of the Co90 Zr10 alloy.

Dynamic slowing-down and crystal nucleation in a supercooled metallic glass former induced by local icosahedral order

A long-standing question in condensed matter and material science is understanding how local structural order determines the fate of a liquid on cooling, i.e., whether it crystallizes or experiences dynamic slowing down leading to glass formation. Here we address this issue by focusing on a metallic glass former with concurring crystal and amorphous local order based on icosahedral symmetry. Using atomistic simulations, we study its deep supercooled regime in which we clearly identify spontaneous crystal nucleation close to the glass transition. We find that regions of slow dynamics characterized by a high degree of icosahedral orientational order play a central role in the nucleation process. Our study brings a new perspective into the microscopic mechanism of crystallization from the supercooled phase, which could be of significance for the in silico design of materials with selected glass-forming ability.