Local Structural Properties Research Articles

Internal Electric Fields and Structural Instabilities in Insulating Transition Metal Compounds: Influence on Optical Properties

AbstractThis work reviews new ideas developed in the last two decades which play a key role for understanding the optical properties of insulating materials containing transition metal (TM) cations. Initially, this review deals with compounds involving d4 and d9 ions where the local structure of the involved MX6 complexes (M=dn cation, X=ligand) is never cubic but distorted, a fact widely ascribed to the Jahn‐Teller (JT) effect. Nevertheless, that assumption is often wrong as the JT coupling requires an orbitally degenerate ground state in the initial geometry a condition not fulfilled even if the lattice is tetragonal. For this reason, the equilibrium geometry of d4 and d9 complexes in low symmetry lattices, is influenced by two factors: (i) The effects, usually ignored, of the internal electric field, ER, due to the rest of lattice ions on the active electrons localized in the MX6 unit. (ii) The existence of structural instabilities driven by vibronic interactions that lead to negative force constants. As first examples of these ideas, we show that the equilibrium structure, electronic ground state of KZnF3:Cu2+, K2ZnF4:Cu2+ and K2CuF4 obey to different causes and only in KZnF3:Cu2+ the JT effect takes place. These ideas also explain the local structure and optical properties of CuF2, CrF2 or KAlCuF6 compounds where the JT effect is symmetry forbidden and those of layered copper chloroperovskites where the orthorhombic instability explains the red shift of one d−d transition under pressure. In a second step, this review explores stable systems involving d3, d5 or d9 cations, where the internal electric field, ER, is responsible for some puzzling phenomena. This is the case of ruby and emerald that surprisingly exhibit a different color despite the Cr3+‐O2− distance is the same. A similar situation holds when comparing the normal (KMgF3) and the inverted (LiBaF3) perovskites doped with Mn2+ having the same Mn2+‐F distance but clearly different optical spectra. The role of ER is particularly remarkable looking for the origin of the color in the historical Egyptian Blue pigment based on CaCuSi4O10.

Structural evolution of liquid silicates under conditions in Super-Earth interiors

Molten silicates at depth are crucial for planetary evolution, yet their local structure and physical properties under extreme conditions remain elusive due to experimental challenges. In this study, we utilize in situ X-ray diffraction (XRD) at the Matter in Extreme Conditions (MEC) end-station of the Linear Coherent Linac Source (LCLS) at SLAC National Accelerator Laboratory to investigate liquid silicates. Using an ultrabright X-ray source and a high-power optical laser, we probed the local atomic arrangement of shock-compressed liquid (Mg,Fe)SiO3 with varying Fe content, at pressures from 81(9) to 385(40) GPa. We compared these findings to ab initio molecular dynamics simulations under similar conditions. Results indicate continuous densification of the O-O and Mg-Si networks beyond Earth’s interior pressure range, potentially altering melt properties at extreme conditions. This could have significant implications for early planetary evolution, leading to notable differences in differentiation processes between smaller rocky planets, such as Earth and Venus, and super-Earths, which are exoplanets with masses nearly three times that of Earth.

X-ray diffraction and quasielastic neutron scattering studies of the structure and dynamic properties of water confined in ordered microporous carbon pores

The structure and dynamic properties of capillary-condensed water confined in ordered microporous carbon (OMC) of pores diameter 18.7 Å are investigated over a temperature range of 200–300 K by adsorption/desorption isotherms, X-ray diffraction (XRD), and quasielastic neutron scattering (QENS). The nitrogen adsorption/desorption isotherm of OMC pores at 77 K exhibits an I-type pattern. The water adsorption/desorption isotherm of OMC pores at 298 K is of type V. The XRD data on water confined in OMC reveal that the tetrahedral network structure of water is perturbed from the bulk water structure, but not to such an extent as found for water confined in MCM-41 and Ph-PMO previously reported. With decreasing the temperature, the ∼2.8 Å peak shifts to shorter distances, while the second-neighbor H2O– H2O interaction distances gradually increase from 3.95 Å and 4.49 Å to 4.43 Å and 4.84 Å, indicating a tendency to form a more tetrahedral-like hydrogen-bonded water structure at subzero temperatures. Below 230 K, the hexagonal ice Ih was partially formed in OMC pores. The QENS spectra were analyzed using a jump-diffusion model to translate water molecules. The translational diffusion of water molecules in OMC pores is comparable to that of bulk water at 300 K and higher than that in MCM-41, and the mobility slows as the temperature decreases. The elastic incoherent structure factor (EISF) analysis found evidence for immobile and mobile fractions of confined water. The mobile fraction exhibited jump diffusion, with a jump length consistent with a sphere of confinement radius of 4–8 Å. The results obtained in the present work are compared with those reported for MCM-41 with a hydrophilic interface and Ph-PMO with an amphiphilic interface, and the influence of interface effect on local structure and dynamic properties of confined water is discussed.

Deep Learning Potential Assisted Prediction of Local Structure and Thermophysical Properties of the SrCl2-KCl-MgCl2 Melt.

The local structure and thermophysical properties of SrCl2-KCl-MgCl2 melt were revealed by deep potential molecular dynamicsdriven by machine learning to facilitate the development of molten salt electrolytic Mg-Sr alloys. The short- and intermediate-range order of the SrCl2-KCl-MgCl2 melts was explored through radial distribution functions and structure factors, respectively, and their component and temperature dependence were discussed comprehensively. In the MgCl2-rich system, the intermediate-range order is more pronounced, and its evolution with temperature exhibits a non-Debye-Waller behavior. Mg-Cl is dominated by 4,5 coordination and Sr-Cl by 6,7 coordination, and their coordination geometries exhibit distorted octahedra and distorted pentagonal bipyramids, respectively. A database of thermophysical properties of SrCl2-KCl-MgCl2 melts, including density, self-diffusion coefficient, viscosity, and ionic conductivity, was thus developed, covering the temperature range from 873 to 1173 K.

Local structure and thermophysical property prediction for CaCl2-KCl molten salt with machine learning potentials

CaCl2-KCl molten salt, due to its excellent physical and chemical properties, is considered to have great application prospects in the fields of concentrated solar power (CSP) systems and preparation of metallic calcium. Structure information and thermophysical property have been simulated at various temperatures to explore the effect of the temperature on CaCl2-KCl molten salt. The reliability of machine learning potential molecular dynamics (MLPMD) simulation results is studied by comparison with the first-principles molecular dynamics (FPMD) simulations and experiments. Moreover, the MLPMD simulations lower the barrier to understanding and designing molten salts due to their high efficiency and accuracy.

Intermitted 3D Diffraction Tomography Combined with In situ Laue Diffraction to Characterize Dislocation Structures and Stress Fields in Microbending Cantilevers

The mechanisms of plastic deformation are investigated using different characterization tools as scanning electron microscopy (SEM), transmission electron microscopy, or synchrotron‐based X‐ray techniques like Laue microdiffraction (μLaue). However, structural information can be limited to the specimen surface (SEM), to extremely thin samples (TEM), or depth averaging (μLaue). Until today, a nondestructive in situ investigation of a dislocation population, and de facto, the determination of the local stress tensor in bulk samples, remain challenging. To decompose the depth‐integrated μLaue signals, the so‐called “differential aperture X‐ray microscopy” (DAXM), allowing the 3D determination of the local structural crystal properties, is used. Using this approach, the local crystallographic phase, orientation, and the elastic strain tensor are obtained with 1 μm3 voxel size. In order to accomplish the experiment, a protocol and a new combined in situ mechanical testing rig with a DAXM microscope is created. The experiment is conducted on a severely bent focused ion beam copper single‐crystal microcantilever (10 × 10 × 25 μm3). The local deviatoric strain tensor and the local lattice curvature in the deformed sample are analyzed in 3D. The advantages and resolution limits of the technique are discussed in detail.

Metabolic Network Analysis Reveals Human Impact on Urban Nitrogen Cycles

Human interactions have led to the emergence of a higher complexity of urban metabolic networks; hence, traditional natural- or agriculture-oriented biogeochemical models might not be transferred well to urban environments. Increasingly serious environmental problems require the development of new concepts and models. Here, we propose a basic paradigm for urban–rural complex nitrogen (N) metabolic network reconstruction (NMNR) by introducing new concepts and methodologies from systems biology at the molecular scale, analyzing both local and global structural properties and exploring optimization and regulation methods. Using the Great Hangzhou Areas System (GHA) as a case study, we revealed that pathway fluxes follow a power law distribution, which indicates that human-dominated pathways constitute the principal part of the functions of the whole network. However, only 1.16% of the effective cycling pathways and an average hamming distance of only 5.23 between the main pathways indicate that the network lacks diverse pathways and feedback loops, which could lead to low robustness. Furthermore, more than half of the N fluxes did not pass through core metabolism, causing waste and pollution. We also provided strategies to design network structures and regulate system function: improving robustness and reducing pollution by referring to the characteristics of biochemical metabolic networks (e.g., the bow-tie structure). This method can be used to replace the trial-and-error method in system regulation and design. By decomposing the GHA N metabolic network into 4398 metabolic pathways and the corresponding fluxes with a power law distribution, NMNR helps us quantify the vulnerability in the current urban nitrogen cycle. The basic ideas and methodology in NMNR can be applied to coupled human and natural systems to advance global sustainable development studies, and they can also extend systems biology from the molecule to complex ecosystems and lead to the development of multi-scale unified theory in systems biology.

Correction to: Effect of Crystallization on Local Structure and Optical Properties of Glass with 2.5Li2O–50GeO2–37.5P2O5 Composition

Correction to: Effect of Crystallization on Local Structure and Optical Properties of Glass with 2.5Li2O–50GeO2–37.5P2O5 Composition

Design of Alkali-Activated Materials and Geopolymer for Deep Soilmixing: Interactions with Model Soils.

This study focuses on the use of alkali-activated materials and geopolymer grouts in deep soilmixing. Three types of grouts, incorporating metakaolin and/or slag and activated with sodium silicate solution, were characterized at different scales to understand the development of their local structure and macroscopic properties. The performance of the soilmix was assessed by using combinations of the grouts and model soils with different clay contents. Feret's approach was used to understand the development of compressive strength at different water-to-solid ratios ranging from 0.65 to 1. The results suggested that incorporating calcium reduced the water sensitivity of the materials, which is crucial in soilmixing. Adding soils to grouts resulted in improved mechanical properties, due to the influence of the granular skeleton. Based on strength results, binary soilmix mixtures containing 75% of metakaolin and 25% of slag, with H2O/Na2O ratios ranging from 28 to 42 demonstrated potential use for soilmixing due to the synergistic reactivity of metakaolin and slag. The optimization of compositions is necessary for achieving the desired properties of soil mixtures with higher H2O/Na2O ratios.

Mechanical and structural properties of monatomic zirconium metallic glass under pressure variations and annealing processes: A molecular dynamics study

The aim of this study is to investigate how different pressure levels and annealing times affect the local atomic structure and mechanical properties of pure zirconium metallic glasses. Using molecular dynamics simulations with the embedded atom method, we explored how these changes influence the material's inherent characteristics. Analysis of the radial distribution function, coordination number, and Voronoi tessellation revealed a spectrum of structural arrangements in metallic glass formed across a pressure range of 0–70 GPa. Additionally, the glass transition temperature increased with increasing pressure, accompanied by reduced free volume. The annealing process, ranging from 0 to 5 ns on metallic glass synthesized under 0 GPa pressure, showed the coordination number's significance in achieving a glassy state. Regarding mechanical behavior, both elastic constants and moduli showed a progressive increase with rising pressure, while Young's modulus and hardness displayed enhanced values with longer annealing times.

Effect of Crystallization on Local Structure and Optical Properties of Glass with 12.5Li2O–50GeO2–37.5P2O5 Composition

Effect of Crystallization on Local Structure and Optical Properties of Glass with 12.5Li2O–50GeO2–37.5P2O5 Composition

Elucidating the local structure and electronic properties of a highly active overall alkaline water splitting NixCo1-xO/hollow carbon sphere catalyst

A NixCo1-xO/HCS catalyst with superior water-splitting is presented. High water-splitting reaction kinetics and enhanced durability were observed. The structure-function relationship was investigated with XPS, which demonstrated the presence of dominant Ni2+ and Co2+ species and a functionalized HCS support where nucleation of small metal oxide nanoparticles occurred. The PDF showed broadened Nickel/Cobalt-oxide bonds and expansion of the metal-metal pair distances. The alteration of the Metal-Oxide and Metal-Metal-Oxide bonds favored better HER and OER electrocatalysis, also as supported by DFT. EXAFS showed the existence of the bimetallic oxide catalyst and the stretching of the Ni–O bonds due to the coordination of the Ni–O with the Co. The composite exhibited higher activity and enhanced electrocatalytic mechanism towards water splitting through higher exchange current densities (0.615 and 0.886 mA cm−2) and low charge transfer resistance (14 and 10 Ω) respectively. Chronoamperometry proved the catalyst's stability during prolonged electrolysis.

Insight into the TiO2 on transport behavior and heat transfer of the CaO–SiO2–FeO–MgO system at different basicities: Molecular dynamics simulation and thermodynamics

In order to promote the resource utilization of Ti-containing converter slag and reduce the loss of metallic Ti, it is necessary to clarify the relationship between the oxygen network structure and thermophysical properties of the slag. The present work uses classical molecular dynamics (CMD) simulation to study the effects of basicity (1.0–3.0) and TiO2 content (0%–10 %) on the local structural properties, viscosity, and thermal conductivity of the CaO–SiO2–FeO–MgO–TiO2 (CSFMT) system at 1873 K. The M − O (M = Ca, Si, Fe, Mg, and Ti) bond length calculated based on CMD is consistent with the experimental results, and the [SiO4] tetrahedron and [TiO6] octahedron are the basic structural units of the CSFMT system. The increase in basicity promotes the break of bridging oxygen (BO) Si–O–Si and the formation of non-bridging oxygen Si–O-M. The increase in TiO2 content promotes the formation of Ti–O–Si in the system. The basicity and TiO2 content affect the thermophysical properties of the CSFMT system by promoting and inhibiting the depolymerization of the oxygen network structure, respectively. The diffusion abilities of different atoms in the CSFMT system are in descending order of Fe > Mg > Ca > O > Si > Ti. The viscosity of the CSFMT system decreases with increasing basicity and increases with increasing TiO2 content. Increasing the proportion of BOs can weaken the anharmonicity in the polyhedral structural units and reduce phonon scattering. With increasing basicity and TiO2 content, the effective phonon mean free path decreases and increases, respectively, resulting in a decrease and an increase in the thermal conductivity of the CSFMT system. The changes in the microstructure properties of the CSFMT system can be reasonably explained by the changes in its thermodynamic properties such as enthalpy. It is worth noting that the basicity has a greater effect on the viscosity, while the TiO2 content has a more significant impact on the thermal conductivity of the CSFMT system.

The functional and structural alterations in brain regions related to the fear network model in panic disorder: A resting-state fMRI and T1-weighted imaging study

Abnormal functional connectivity (FC) within the fear network model (FNM) has been identified in panic disorder (PD) patients, but the specific local structural and functional properties, as well as effective connectivity (EC), remain poorly understood in PD. The purpose of this study was to investigate the structural and functional patterns of the FNM in PD. Magnetic resonance imaging data were collected from 33 PD patients and 35 healthy controls (HCs). Gray matter volume (GMV), degree centrality (DC), regional homogeneity (ReHo), and amplitude of low-frequency fluctuation (ALFF) were used to identify the structural and functional characteristics of brain regions within the FNM in PD. Subsequently, FC and EC of abnormal regions, based on local structural and functional features, and their correlation with clinical features were further examined. PD patients exhibited preserved GMV, ReHo, and ALFF in the brain regions of the FNM compared with HCs. However, increased DC in the bilateral amygdala was observed in PD patients. The amygdala and its subnuclei exhibited altered EC with rolandic operculum, insula, medial superior frontal gyrus, supramarginal gyrus, opercular part of inferior frontal gyrus, and superior temporal gyrus. Additionally, Hamilton Anxiety Scale score was positively correlated with EC from left lateral nuclei (dorsal portion) of amygdala to right rolandic operculum and left superior temporal gyrus. Our findings revealed a reorganized functional network in PD involving brain regions regulating exteroceptive-interoceptive signals, mood, and somatic symptoms. These results enhance our understanding of the neurobiological underpinnings of PD, suggesting potential biomarkers for diagnosis and targets for therapeutic intervention.

Accurate detection of subsurface microcavity by bimodal atomic force microscopy

Subsurface detection capability of bimodal atomic force microscopy (AFM) was investigated using the buried microcavity as a reference sample, prepared by partially covering a piece of highly oriented pyrolytic graphite (HOPG) flake with different thickness on a piece of a cleaned CD-R disk substrate. This capability can be manifested as the image contrast between the locations with and without the buried microcavities. The theoretical and experimental results demonstrated that the image contrast is significantly affected by the critical parameters, including the second eigenmode amplitude and frequency as well as local structural and mechanical properties of the sample itself. Specifically, improper parameter settings generally lead to incorrect identification of the buried microcavity due to the contrast reduction, contrast reversal and even disappearance. For accurate detection, the second eigenmode amplitude should be as small as possible on the premise of satisfying the signal-to-noise ratio and second eigenmode frequency should be close to the resonance frequency of the cantilever. In addition, the detectable depth is closely related to microcavity dimension (thickness and width) of the HOPG flake and local stiffness of the sample. These results would be helpful for further understanding of the detection mechanism of bimodal AFM and facilitating its application in nano-characterization of subsurface structures, such as the micro-/nano- channels to direct the flow of liquids in lab-on-a-chip devices.

Influence of oxygen on electrical conductance and local structural properties of BiFeO3 thin films

The effect of oxygen on the electrical conductance and local structural properties of BiFeO3 (BFO) thin films on SiO2/Si substrates grown by RF magnetron sputtering was investigated. The conductivities of BFO were studied in a planar electrode with blue light irradiation. The BFO films grown with oxygen (BFO-O2) show a large conductivity increase, which is 12.66 times more than the BFO grown without oxygen (BFO), and the conductivity change is entirely caused by the BFO thin films. To explain the mechanism of increased electrical conductance, the local structure at the Fe K-edge was investigated by using time-resolved x-ray absorption spectroscopy (TRXAS). The applied voltage and blue light exposure affected the Fe–O bond, while the valence states of Fe atoms in BFO thin films remained unchanged. When the BFO films were irradiated, the bonding distance of the Fe–O bond was deviated, resulting in an oxygen vacancy. These findings imply that BFO thin films with more oxygen components exhibit higher electrical conductivity when exposed to blue light. The results of this research should pave the way for optoelectronic applications to modulate the electrical conductivity driven by oxygen and blue light.

The role of Cu doped Ba0.5Sr0.5Fe0.95Zr0.05O3-δ: Local structure distortion, conductivity, and magnetization

The composition Ba0.5Sr0.5Fe1-xCuxZr0.05O3-δ (BSFCZ-x, x = 0–0.20) was successfully synthesized by the sol-gel self-combustion method using citric acid as fuel combustion. Structure, local structure, electrical, and magnetic properties are investigated. The X-ray diffraction revealed the oxides exhibit a cubic structure (Pm3 m), and the lattice parameter increases with x. The X-ray absorption data are analyzed to determine the oxidation state of B site metal transition Fe and Cu through edge-energy E0 evaluation. Fe performs the oxidation states Fe3+ and Fe4+, similarly to Cu2+ and Cu3+. Furthermore, the pre-edge was analyzed to determine the spin state configuration of the 3d subshell. The local lattice distortion was evaluated through EXAFS, resulting in a distorted FeO6 octahedron elongated to the z-axis, inducing the Jahn-Teller effect. The nonstoichiometric δ-value (3-δ) measurement leads to oxygen vacancy VO⦁⦁ defect. The electrical conductivity of semiconducting behavior increases significantly after Cu is dissolved in BSFZ up to 60 S/cm at 500 °C, from initially 5 S/cm. Furthermore, the magnetic properties show a slight magnetic saturation Ms for BSFZ related to the low spin state configuration of Fe 3d subshell, then significantly increased on BSFCZ samples due to Cu provoking spin state dragged partly to high spin state configuration.

Local Structure, Structural, Vibrational, and Optical Properties of Natural Zircon

Structural properties of Indian metamict single zircon crystals are studied by comparing their properties with those of synthetic zircon synthesized via solid‐state reaction method, which serves as the standard. X‐Ray diffraction (XRD) data reveal changes in lattice parameters and the presence of strain, metamictization, etc. It is found that the degree of crystallinity is ≈63.8% indicating the simultaneous presence of crystalline and disordered regions. These changes are likely due to the presence of several radioactive/nonradioactive elements. The presence of such elements is observed and the sample composition is analyzed through total reflection X‐Ray fluorescence. XRD crystallographic data are further utilized as input for analyzing X‐Ray absorption spectroscopy data recorded around Zr central absorbing atom. X‐Ray absorption near‐edge structure shows slight modification in the white line feature and some disorder compared to the synthetic one. Reduced coordination number observed from extended X‐Ray absorption fine structure suggests the presence of disorder in local structure. Micro‐Raman spectroscopy indicates that crystalline behavior varies across different locations, suggesting that the sample has undergone varying degrees of metamictization, confirming a nonuniform distribution of elements. The work documents and analyses structural evolution within isolated silica tetrahedron network in association of Zr4+ cation as a function of self‐irradiation and recovery over millions of years and correlating the same with optical properties.

Thermal response of encapsulated nanoscale phase change materials with crosslink: A molecular dynamics study

Crosslink plays a key role in improving the thermal and mechanical stability of encapsulated phase change materials (EPCM). In this paper, molecular models of EPCM with different crosslinked density (non-crosslinked, 20 %, 40 %, 60 %, and 80 %) were constructed and then the molecular dynamics method was employed to investigate the thermal response characteristics. Specifically, the local structural characteristics, overall morphology, kinetic properties, and thermo-mechanical properties were analyzed. The results indicate that establishing crosslinked structures effectively enhances the structural steadiness of the EPCM under thermal and mechanical effects, and restricts the radial thermal diffusion of the core substance. More precisely, the presence of crosslinked structure (80 % crosslinked density) led to a 26.61 % improvement in tensile strength, a 5.59 % increase in compression resistance, and a 14.4 % improvement in shear strength than non-crosslinked structure. It is worth mentioning that, the results indicate a remarkable turning point in the occupancy volume change and mechanical properties of the system around the crosslinked density of 40 %.

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials