Local Structural Properties Research Articles

Material Development and Physical Properties of BiS2-Based Layered Compounds

In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development (probably all the compounds) and physical properties of BiS2-related materials developed in the last six years. The key parameters for the emergence of bulk superconductivity in the BiS2-based compounds are carrier doping to the parent phase (band insulator), pressure-induced structural transition, in-plane chemical pressure, and the suppression of in-plane disorder. We systematically review those parameters and the method of designing new superconductors with BiS2 layers. In addition, studies of BiS2-based compounds as thermoelectric materials are briefly reviewed.

Comparison of local crystal structure and magnetic properties of cation substituted LiNiO2 compositions

We investigate the change in local crystal structure and its impact on the magnetic ground states of LiNi0.5X0.5O2 (X = Mn, Co) and LiNi0.75Y0.25O2 (Y = Fe, Al) and contrast it against LiNiO2. The solid-state synthesis route adopted here introduces some anti-site disorder in the final compositions due to the volatile nature of Li ions. Powder X-ray diffraction (XRD) profiles were hence recorded using a rotating anode X-ray source to determine the percentage of anti-site disorder, and phase purity of all the compositions. Rietveld refinement of the XRD profiles confirms the symmetry and provides the estimation of unit cell parameters. Near-edge X-ray absorption spectra verified the oxidation state of various cations present in the compositions. Analysis of the extended X-ray absorption fine structure spectra identify the signatures of Jahn–Teller distortion in Ni3+ rich compositions. Changes in NiO6 octahedral environment has a much deeper impact on the magnetism of these compositions.

Study of the local structure and electrical properties of gallium substituted LLZO electrolyte materials

The solid-state lithium batteries are more stable than the batteries with liquid electrolyte, however their performance are also worse especially due to the low ionic conduction of their electrolyte. Garnet-type Li7La3Zr2O12 is a promising solid-state electrolyte candidate for lithium batteries. In this work the influence of gallium substitution on the electrical, crystal and electronic structure properties in the Li7La3Zr2O12 material were studied. Li7-3xGaxLa3Zr2O12 solid electrolytes were synthesized by solid state reaction method and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impedance spectroscopy and X-ray absorption spectroscopy (XAS) techniques. XRD and XAS analyses showed that the x = 0.05 sample is formed in the tetragonal phase with a space group of I41/acd:2 and the rest are formed in the cubic phase with a space group of I-43d due to strong coupling between outer shell electrons of the oxygen (O) and gallium (Ga) atoms. Electrochemical impedance spectroscopy (EIS) studies indicated that the tetragonal phase has the highest ionic conductivity, 3.04 × 10−6 S cm−1, among all other cubic phases.

Local structure and vibrational properties of molten lead halides PbX2 (X = Cl, Br, I) from ab initio molecular dynamics

Ab initio molecular dynamics (AIMD) was applied to calculate the local structure and dynamics parameters of molten lead halides PbX2 (X = Cl, Br, I). The thoroughness of our approach is substantiated by the variation of both cell size and temperature. The total duration of AIMD runs amounted to 600 ps. It was found that molten lead chlorides exhibit weak binding of the local structure. The latter was analyzed by means of radial distribution functions for each pair of salt constituents. The radial distribution functions exhibit pronounced liquid-like damped oscillations and weakly depend on the cell size. In terms of vibrational properties, we highlight the frequencies of 125 cm−1 and 80 cm−1 which relate to the Pb-Br and Pb-I vibrations respectively, both of a short lifetime. These results are additionally supported by the calculated self-diffusion coefficients which concur closely with experimental values.

The effect of polymorphism on the structural, dynamic and dielectric properties of plastic crystal water: A molecular dynamics simulation perspective.

We have employed molecular dynamics simulations based on the TIP4P/2005 water model to investigate the local structural, dynamical, and dielectric properties of the two recently reported body-centered-cubic and face-centered-cubic plastic crystal phases of water. Our results reveal significant differences in the local orientational structure and rotational dynamics of water molecules for the two polymorphs. The probability distributions of trigonal and tetrahedral order parameters exhibit a multi-modal structure, implying the existence of significant local orientational heterogeneities, particularly in the face-centered-cubic phase. The calculated hydrogen bond statistics and dynamics provide further indications of the existence of a strongly heterogeneous and rapidly interconverting local orientational structural network in both polymorphs. We have observed a hindered molecular rotation, much more pronounced in the body-centered-cubic phase, which is reflected by the decay of the fourth-order Legendre reorientational correlation functions and angular Van Hove functions. Molecular rotation, however, is additionally hindered in the high-pressure liquid compared to the plastic crystal phase. The results obtained also reveal significant differences in the dielectric properties of the polymorphs due to the different dipolar orientational correlation characterizing each phase.

Tuning infrared plasmon resonances in doped metal-oxide nanocrystals through cation-exchange reactions

Metal-oxide nanocrystals doped with aliovalent atoms can exhibit tunable infrared localized surface plasmon resonances (LSPRs). Yet, the range of dopant types and concentrations remains limited for many metal-oxide hosts, largely because of the difficulty in establishing reaction kinetics that favors dopant incorporation by using the co-thermolysis method. Here we develop cation-exchange reactions to introduce p-type dopants (Cu+, Ag+, etc.) into n-type metal-oxide nanocrystals, producing programmable LSPR redshifts due to dopant compensation. We further demonstrate that enhanced n-type doping can be realized via sequential cation-exchange reactions mediated by the Cu+ ions. Cation-exchange transformations add a new dimension to the design of plasmonic nanocrystals, allowing preformed nanocrystals to be used as templates to create compositionally diverse nanocrystals with well-defined LSPR characteristics. The ability to tailor the doping profile postsynthetically opens the door to a multitude of opportunities to deepen our understanding of the relationship between local structure and LSPR properties.

Cu-Exchanged Ferrierite Zeolite for the Direct CH4 to CH3OH Conversion: Insights on Cu Speciation from X-Ray Absorption Spectroscopy

The direct stepwise transformation of CH4 to CH3OH over Cu-exchanged zeolites has been an intensively researched reaction as it can provide a solution for the utilization of this abundant feedstock. Up to date a commercial process is far from realization, which is why an understanding of the Cu speciation in zeolites as a function of reaction conditions as well as the development of a mechanistic view of the reaction are necessary to further advance the field. Herein we study Cu-exchanged ferrierite zeolite for the direct CH4 to CH3OH conversion by utilizing X-ray absorption spectroscopy (XAS), in order to assess the local structure and electronic properties of Cu through the reaction. A Cu-FER sample with a Cu/Al = 0.20 and Si/Al = 11 was subjected to three reaction cycles yielding ultimately 96 µmol $$_{{{\text{C}}{{\text{H}}_3}{\text{OH}}}}/{{\text{g}}_{{\text{zeolite}}}}$$ . Normalized to the Cu loading, this accounts for 0.33 mol $$_{{{\text{C}}{{\text{H}}_3}{\text{OH}}}}$$ /molCu, making the sample comparable to very active Cu-MOR materials reported in the literature. During O2 activation, a transient self-reduction regime of CuII to CuI ions was identified; eventually leading to mostly framework interacting CuII species. CH4 loading leads to a reduction of these CuII containing species; which are finally partially reoxidized during H2O-assisted CH3OH extraction. The speciation after CH4 activation as well as H2O-assisted CH3OH extraction was assessed via linear combination fitting analysis of the XAS data.

Heterogeneity of water structure and dynamics at the protein-water interface.

In this molecular dynamics simulation study, we analyze the local structural and dynamic properties of water hydrating the protein ubiquitin on a spatial grid with 1 Å resolution. This allows for insights into the spatial distribution of water number densities, molecular orientations, translations, and rotations as a function of distance from the protein surface. Water molecule orientations follow a heterogeneous distribution with preferred local orientations of water dipoles and O-H bond vectors up to 10-15 Å distances from the protein, while local variations of the water number density converge to homogeneous bulk-like values within less than 8 Å. Interestingly, we find that the long-ranged orientational structure of water does not impact either the translational or rotational dynamics of water. Instead, heterogeneous distributions of local dynamical parameters and averaged dynamical retardation factors are only found close to the protein surface and follow a distance dependence comparable to heterogeneities in the local water number density. This study shows that the formation of nanodomains of preferred water orientations far from the protein does not significantly impact dynamical processes probed as a non-local average in most experiments.

Structure and transport properties of zirconia crystals co-doped by scandia, ceria and yttria

This work is a study of the effect of co-doping (ZrO2)0.9(Sc2O3)0.1 solid solution with yttria and/or ceria on the phase composition, local structure and transport properties of the crystals. The solid solution crystals were grown using directional melt crystallization in cold crucible. We show that ceria co-doping of the crystals does not stabilize the high-temperature cubic phase in the entire crystal bulk, unlike yttria co-doping. Ceria co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals increases their conductivity, whereas the addition of 1 mol.% yttria tangibly reduces the conductivity. Equimolar co-doping of the (ZrO2)0.9(Sc2O3)0.1 crystals with ceria and yttria changes the conductivity but slightly. Optical spectroscopy of the local structure of the crystals identified different types of optical centers. We found that the fraction of the trivalent cations having a vacancy in the first coordination sphere in the ceria co-doped crystals is smaller compared with that in the yttria co-doped crystals.

Thermal and structural instability of sodium-iron carbonophosphate ball milled with carbon

Pristine Na3FePO4CO3 (NFPC) with the monoclinic structure and the P21/m space group was prepared by hydrothermal synthesis at 120 °C. To increase the conductivity of NFPC, it was ball milled with carbon using a SPEX 8000 mill. Crystal and local structure, morphology, thermal stability, conductivity and electrochemical properties of NFPC and NFPC/C composites were studied by XRD, DSC/TG, FTIR, Mӧssbauer spectroscopy, 23NMR spectroscopy, magnetic measurements, SEM, EIS and galvanostatic cycling. It has been shown that the as-prepared NFPC is stable below 500 °C and then decomposes to Fe3O4 and Na3PO4. Ball milling of NFPC with and without carbon leads to its partial decomposition with the formation of nanosized superparamagnetic Fe3O4 particles and a significant structural disordering, though the crystal symmetry maintains unchanged. Due to high sensitivity of NFPC to air, pristine sample contains some portion of the Fe3+ ions; it increases after ball milling. As a result, all samples are able to cycle starting both with charge and discharge. NFPC shows high stability upon cycling with the specific discharge capacity close to the theoretical one (96 mA·h·g−1 for one-electron reaction). Though the capacity of the NFPC/C composites is slightly lower at low cycling rate than that of pristine NFPC, they show better high-rate performance due to improved conductivity via the formation of the highly conductive carbon matrix. As-established low lattice volume variation upon (de)intercalation of the sodium ions along with realization of a single-phase mechanism is responsible for a long cycle life of the NFPC/C cathode material.

Simulating the NaK Eutectic Alloy with Monte Carlo and Machine Learning

Combining atomistic simulations and machine learning techniques can expedite significantly the materials discovery process. We present an application of such methodological combination for the prediction of the melting transition and amorphous-solid behavior of the NaK alloy at the eutectic concentration. We show that efficient prediction of these properties is possible via machine learning methods trained on the topological local structural properties. The configurations resulting from Monte Carlo annealing of the NaK eutectic alloy are analyzed with topological attributes based on the Voronoi tessellation and using expectation-maximization clustering and Random Forest classification. We show that the Voronoi topological fingerprints make an accurate and fast prediction of the alloy thermal behavior by cataloguing the atomic configurations into three distinct phases: liquid, amorphous solid, and crystalline solid. Melting is found at 230 K by the sharp split of configurations classified as crystalline solid and as liquid. With the proposed metrics, an arrest-motion temperature is identified at 130–140 K through a top down clustering of the atomic configurations catalogued as amorphous solid. This statistical learning paradigm is not restricted to eutectic alloys or thermodynamics, extends the utility of topological attributes in a significant way, and harnesses the discovery of new material properties.

Electronic and Magnetic Properties of Lanthanum and Strontium Doped Bismuth Ferrite: A First-Principles Study

While bismuth ferrite BiFeO3 (BFO) is a well studied multiferroic material, its electronic and magnetic properties in the presence of A-site dopants have not been explored widely. Here we report the results of a systematic study of the local electronic structure, spontaneous polarization, and magnetic properties of lanthanum (La) and strontium (Sr) doped rhombohedral bismuth ferrite within density functional theory. An enhanced ferroelectric polarization of 122.43 μC/cm2 is predicted in the uniformly doped BiLaFe2O6. We find that substitution of Sr in the A-site drives the system into a metallic state. The nature of magnetism arises mainly from the B-site Fe exhibiting a G-type antiferromagnetic ordering. Our study finds that upon dopant substitution, the local magnetic moment is decreased and its magnitude is dependent on the distance between the Fe and the dopant atom. The correlation between the local moment and the distance between the Fe and the dopant atom is discussed.

Molecular simulation of the structure and physical properties of alkali nitrate salts for thermal energy storage

Molecular dynamics simulation method with an improved flexible nitrate model was used in this paper to study the structure and properties of molten LiNO3, NaNO3, and KNO3. Both local structure and physical properties including densities, thermal conductivities, viscosities and heat capacities of the salts were simulated comprehensively. Different simulation methods were used and the results were compared with experimental data. The relationship between the structure and properties of the salts were discussed. Besides, a binary mixture of NaNO3 and KNO3 was also simulated to verify the applicability of the force field and simulation method for salt mixtures. The results indicate that the force field and simulation methods adopted in this paper work well in predicting the properties of both single nitrate salts and their mixtures. This paper did a basis job for further investigation into the simulation of nitrate salt mixtures used in thermal energy storage.

Local structural changes due to the electric field‐induced migration of oxygen vacancies at Fe‐doped SrTiO3 interfaces

AbstractWe report on our study of dc voltage‐induced structural changes at reduced and oxidized Fe‐doped SrTiO3 (Fe:STO) electrode interfaces using second harmonic generation (SHG) together with photoluminescence (PL) method. We show that oxygen vacancy defects play a critical role in determining the local electrical and structural properties of interfacial depletion regions at Schottky junctions. The SHG results show that the dc electric field causes oxygen ions and vacancies to displace toward the anode and cathode in the low field regime, respectively. This process forms electrostrictive distortions within local interfacial depletion regions which are described by Fe:Ti‐O bond stretching and bending. Differences in the EFISHG responses from the oxidized and reduced crystal interfaces are explained according to local oxygen vacancy concentrations and dynamics and their effects on the Schottky barrier heights and depletion region widths at each interface. These results are further supported by our PL measurements. Oxygen ion migration toward the Fe:STO surface leads to enhanced fluorescence intensities from in‐gap acceptor states. We demonstrate that SHG and PL measurements are well‐suited for understanding and resolving the underlying causes of dielectric breakdown processes and device failure brought on by dc electric field and ionic defect migrations in perovskite‐type electroceramics.

Investigation of local geometrical structure, electronic state and magnetic properties of PLD grown Ni doped SnO2 thin films

We have investigated the ferromagnetic behavior, electronic states and local geometrical structure of Ni (2 and 10 at %) doped SnO2 thin films. The films were successfully fabricated with the help of pulsed laser deposition (PLD) technique on Si (100) substrate under ultrahigh vacuum (UHV) condition. X-ray diffraction (XRD) results revealed the single phase character of SnO2 rutile lattice structure with P42/mnm space group. The inclusion of Ni ions into SnO2 matrix induced oxygen vacancy (Vo), enhanced the distortion in octahedral local symmetry and reduced the oxidation state of the host Sn4+ (SnO2) to Sn3+ (Sn2O3), these details have been estimated by Raman scattering, Near edge X-ray absorption fine structure (NEXAFS) spectra at Ni L3,2 and O K edges. Further quantitative details on the local geometrical structure around Ni ions were obtained via fitting the experimental Fourier transforms EXAFS spectra |X(R)| with FEFF6 code. The magnetization measurements performed at room temperature (RT) infers that Ni doped SnO2 films displayed ferromagnetic (FM) signal, and there was no significant difference in the saturation moments even with increase in the Ni content. Hence, the similarity in the observed magnetic behavior of the films seems relevant to the same crystal growth condition (UHV) and might not be limited directly to the Ni dopant concentrations. The FM signal and the role of surface defects have been discussed in the light of spin-split impurity band percolation mechanism.

Molecular dynamics simulation on the effect of water uptake on hydrogen bond network for OH− conduction in imidazolium-g-PPO membrane

OH− conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake. To investigate the effect of water uptake on the hydrogen bond network for OH− conduction, a series of molecular dynamics simulations based on all-atom force field were performed on the hydrated imidazolium-g-PPO membranes with different water uptakes. The systems were well verified by comparing the membrane density and OH− conductivity with previous experiments. By means of local structural properties and pair-potential energy, reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane. Increasing water uptake enhances the hydration structures of water and OH−, and facilitates the reorganization of the hydrogen bond network. Water and OH− are nearly saturated with water when the water uptake reaches λ = 10, where well-connected hydrogen bond network is produced. Further increasing water uptake has much less contribution to improving the hydrogen bond network, but inevitably swells the membrane channel. This work provides a molecular-level insight into the effect of water uptake on the hydrogen bonding structures and dynamics of OH− and water confined in the imidazolium-g-PPO membrane.

Photoluminescence of graphene quantum dots doped with different elements

Graphene quantum dots (GQDs), as a new graphene material, possess advantageous chemical-physical properties. Owing to their distinctive photoluminescence (PL) property and low biological toxicity, GQDs have been shown to be good candidates for applications in bioimaging and medical analysis. However, the prepared GQDs have their own slew of problems, including low luminescent efficiency, uncertain luminescent mechanism and lack of effective methods to tuning the luminescence property. Heteroatom doping is a good strategy, which could partially address these above problems. Because the doping atoms will be located in the internal structure of carbon nanomaterials thus change their local electronic configuration, polarizability, defect degree and band structure, etc., the physical and chemical properties of GQDs could be well tuned. At present, GQDs doped with Cl, F, N, S, B, P, etc. have been prepared. These obtained GQDs with heteroatom doping not only exhibited tunable optical properties, but also showed enormous potential applications in the field of photocatalysis. Herein, in order to further improve the PL performance of GQDs and extend their application, we prepared pure GQDs and single elemental-doped GQDs with Cl, N, P and S by using the electrochemical method and hydrothermal method. Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy, (XPS) and Raman measurements were used to characterize their elemental composition, surface element state and structural defect. Based on the experimental results, the position and bonding state of heteroatoms in doped GQDs were analyzed. The doping amounts in these doped GQDs are different, i.e., 1.35% of Cl-GQDs, 7.95% of N-GQDs, 10.08% of P-GQDs and 3.25% of S-GQDs, respectively. The degree of defect state is decreased in the order as follows: P-GQDs>S-GQDs>GQDs>Cl-GQDs>N-GQDs. Meanwhile, the PL performance was tested, and the fluorescent quantum efficiencies were calculated to be 8.2% for Cl-GQDs, 5.3% for N-GQDs, 4.0% for GQDs, 2.8% for S-GQDs, and 0.037% for P-GQDs, respectively. It can be concluded that the diverse doping atoms play different roles for the improvement of PL performance. The doping of Cl and N can form the luminescent center, which improves the fluorescent intensity of GQDs. Especially for the Cl doping, the fluorescent intensity of Cl-GQDs is increased twice compared to the pure GQDs, due to larger atomic radius and more outer shell electron of Cl than that of the others doped elementals. In the S-GQDs, the doped S could become small quenching centers, which decreased the fluorescent intensity slightly compared to that of pure GQDs. However, different from the above doped GQDs, P-GQDs showed the negligible fluorescence. Because the P atoms mainly are present in the surface functional groups of P-GQDs, these P-atoms induced defects may become the large fluorescent quenching center and greatly decrease their PL intensity. Besides the measurement and analysis of luminescence intensity, the emission peak positions and corresponding excitation wavelengths of these doped GQDs were also well studied. The excitation wavelength of pure GQDs is located at 340 nm, while the doped GQDs are all located at 360 nm. The strongest emission peaks of Cl-GQDs and N-GQDs are around 450 nm, while the strongest emission peaks of S-GQDs and pure GQDs are around 430 nm. These results indicate that the doping of heteroatoms could change the band gap of GQDs to some extent.

A graph-based approach for the structural analysis of road and building layouts

ABSTRACTA better understanding of the relationship between the structure and functions of urban and suburban spaces is one of the avenues of research still open for geographical information science. The research presented in this paper develops several graph-based metrics whose objective is to characterize some local and global structural properties that reflect the way the overall building layout can be cross-related to the one of the road layout. Such structural properties are modeled as an aggregation of parcels, buildings, and road networks. We introduce several computational measures (Ratio Minimum Distance, Minimum Ratio Minimum Distance, and Metric Compactness) that respectively evaluate the capability for a given road to be connected with the whole road network. These measures reveal emerging sub-network structures and point out differences between less-connective and more-connective parts of the network. Based on these local and global properties derived from the topological and graph-based representation, and on building density metrics, this paper proposes an analysis of road and building layouts at different levels of granularity. The metrics developed are applied to a case study in which the derived properties reveal coherent as well as incoherent neighborhoods that illustrate the potential of the approach and the way buildings and roads can be relatively connected in a given urban environment. Overall, and by integrating the parcels and buildings layouts, this approach complements other previous and related works that mainly retain the configurational structure of the urban network as well as morphological studies whose focus is generally limited to the analysis of the building layout.

Экстремальная и топологическая нелинейная оптика открытых систем

AbstractA review of results of an investigation of the theory of optical wave packets with extreme properties with respect to a controllable pulse shape or to the complexity of the internal structure of radiation beam pulses is presented. Special attention is paid to the manifestations of dissipative effects of the electromagnetic energy inflow and outflow. Precisely these factors lead to peculiar rules of conservation of purely electromagnetic quantities in dissipative media, in which the electromagnetic energy irreversibly decreases in the case of absorption and increases with gain. These rules impose certain restrictions on the possibility of transformation of the shape of pulses and allow one to qualitative describe their evolution. A higher efficiency of the action of unipolar or quasi-unipolar radiation pulses on classical and quantum microobjects is shown and possible ways of formation of these pulses are discussed. For bulk laser media with saturable absorption, the topological properties of localized radiation structures and their transformations with a smooth change in the parameters of the medium (pump level) are described. The preservation of the topological structure upon changes in parameters within the stability range yields the possibility of their distinguishing when coding information by topological solitons of the considered type.

Controlling the structure and magnetic properties of cluster-assembled metallic glasses

The local structure and magnetic properties of amorphous materials are controlled by the impact energy of amorphous clusters.