Effective Coordination Number Research Articles

Smooth Dispersion Is Physically Appropriate: Assessing and Amending the D4 Dispersion Model.

The addition of dispersion corrections to density functionals is essential for accurate energy and geometry predictions. Among them, the D4 scheme is popular due to its low computational cost and high accuracy. However, due to its design, the D4 correction can occasionally lead to anomalies, such as unphysical curvature and bumps in the potential energy surface. We find these anomalies are common in the D4 model, although observable consequences are rarer than in the D3 model for reasons we explain. Nevertheless, we uncover instances of unphysical local minima and stationary points with the D4 scheme and propose two solutions that yield smoother dispersion energy as a function of nuclear position. One is trivial to implement, based on a smoother reparametrization of Gaussian weighting (D4S) to find the effective coordination number. The other replaces Gaussian weighting with soft linear interpolation (D4SL). These new approaches usually remove artificial extremum points, while maintaining accuracy.

Theoretical model for elastic modulus prediction on basis of atomic structure information: Beginning from amorphous carbon to covalent bonding materials

The mechanical property is one of the most important properties of a material and is determined by the atomic-scale structure. It is thus crucial to establish the theoretical model for the prediction of materials' mechanical properties, benefiting the material design. In our previous work, we proposed an atomic-scale structure-based model for predicting the Young's modulus of hydrogenated amorphous carbon; however, the application range of this model is too narrow to be used practically for the materials development. Therefore, in this work, an extended prediction model of Young's modulus of materials with wide applicability is successfully constructed, based on the two important inherent properties of materials: one is effective coordination number (CNeff) evaluating how densely atoms are structured and the another one is effective bond stiffness (Keff) that is first proposed here and indicates how bonding types contribute the elastic properties. Through the high-throughput molecular dynamics simulations, the predictive model of Young's modulus (E) is determined as E = 3.37Keff (CNeff - 2.0)1.5. Then we demonstrate that this model is valid for a large variety of materials including both amorphous and crystalline structures, and the accuracy is proven by comparing with other work. Overall, this fundamental work may benefit the preparation, development, and utilization of new materials.

Interlocking Evaluation of Mesoscopic Skeleton with the Compaction Degree of Hot-Mix Asphalt.

Asphalt mixtures are multi-phase composites composed of aggregates, bitumen, mineral powders, and voids, and various structures are intertwined during the compaction process. Most of the traditional research focuses on the macro-scale domain, and it is difficult to obtain the internal structure of asphalt mixture in different compaction processes. With the continuous development of digital image technology, the influence of the meso-structure of the asphalt mixture on the compaction quality of the asphalt mixture has become a new means to evaluate the performance of the asphalt mixture. In this paper, different numbers of compactions are selected to represent different stages in the compaction process, the digital images of specimens in different compaction stages are obtained by industrial CT scanning technology. Then, the images are processed and reconstructed in three dimensions using improved image segmentation methods, and the position characteristics and geometric information of coarse aggregate are obtained by combining the Oriented Bounding Box (OBB). The meso-response characteristics of the skeleton structure of the asphalt mixture during compaction were studied. The influence of the internal structure of the mixture on the compaction quality of the mixture was obtained, which is of great significance for the study of improving the durability of the pavement. The results show that the "effective coordination number" (the number of aggregate particles that can transmit force in the skeleton structure) is greatly related to the aggregate size. With the compaction process, the centroid of coarse aggregate in the upper layer of the specimen reflects the overall downward movement trend. The inclination angle of the aggregate spindle tends to be in the range of 80°~100°; the anisotropic amplitude of the xy plane increases, and the direction of the aggregate spindle becomes more and more consistent. With the increase in the number of rotational compactions, these four parameters showed obvious rules, indicating that this meso-characteristic index could well characterize the compaction quality of the asphalt mixture in the compaction process.

Addressing the Conflict between Mobility and Stability in Oxide Thin‐film Transistors

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm2 V-1 s-1 ) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (-1.64/0.76V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

Ca‐vacancy effect on the stability of substitutional divalent cations in calcium‐deficient hydroxyapatite

AbstractFirst‐principles calculations were performed to reveal an effect of Ca vacancies on the stability of substitutional divalent cations M2+ (M = Mg, Zn, Sr) in Ca‐deficient hydroxyapatite (dHAp). M2+ concentrations up to 20 mol% in dHAp were considered, and the most stable substitutional sites and their lowest energy configurations in the dHAp lattice were examined with the aid of a generic algorithm method. It was found that defect formation energies of substitutional M2+ are lower in dHAp than in stoichiometric HAp (sHAp) at all M2+ concentrations. This indicates that these M2+ ions are more favorably involved in dHAp than in sHAp, which is in reasonable agreement with experiment. Detailed analyses on atomic structures in dHAp show that the presence of a Ca vacancy varies its surrounding Ca–O bond lengths over a wide area so that Ca–O polyhedrons with various sizes are produced. As a result, M2+ ions can predominantly occupy Ca sites at which M2+ fits better, depending on the ionic radii of M2+. For Zn2+ substitution in dHAp, its defect formation energy decreases more with the increasing concentrations and has the minimum value at 15 mol%. Such a trend can be understood from changes in effective coordination numbers of Zn in dHAp.

Theoretical investigation of the stability of A55-nBn nanoalloys (A, B = Al, Cu, Zn, Ag)

Nanoalloys have been investigated for a wide range of applications, however, our atomistic understanding of the physical-chemistry properties is still far from complete as quantum-size effects play an important role for particles with about 1nm diameter. In this work, we employed density functional theory calculations to investigate the structural, energetic and electronic properties of 55-atom A55-nBn nanoalloys, where A, B= Al, Cu, Zn, and Ag. For structure generation, we combined clustering algorithm techniques with design principles, which yields a wide range of conformations. From the analyses of the excess energy results, we found that the CuAl, CuAg, CuZn, and AlAg systems are the most energetically favorable, in particular, the Al42Cu13 and Al42Ag13 compositions (onion-like and core–shell structures, respectively). Through Spearman’s correlation analysis, we found that the structural properties (number of under coordinated atoms, effective coordination number, average bond lengths, and chemical order parameter) are the most important descriptors correlated with the energy stability of the nanoalloys (excess energy). Several properties such as the particle volume, binding energy, and average bond length show a linear dependence as a function of composition.

Ab initio screening of two-dimensional CuQ x and AgQ x chalcogenides

Two-dimensional (2D) chalcogenides have attracted great interest from the scientific community due to their intrinsic physical–chemical properties, which are suitable for several technological applications. However, most of the reported studies focused on particular compounds and composition, e.g., MoS2, MoSe2, WS2, and WSe2. Thus, there is an increased interest to extend our knowledge on 2D chalcogenides. Here, we report a density functional theory (DFT) screening of 2D coinage-metal chalcogenides (MQ x ), where M = Cu, Ag, Q = S, Se, Te, x = 0.5, 1.0, 1.5, 2.0, with the aim to improve our atomistic understanding of the physical–chemical properties as a function of cation (M), anion (Q), and composition (x). Based on 258 DFT calculations, we selected a set of 22 stable MQ x monolayers based on phonons analyses, where we identified 9 semiconductors (7 AgQ x and 2 CuQ x ), with band gaps from 0.07 eV up to 1.67 eV, while the remaining systems have a metallic character. Using all 258 systems, we found a logarithmic correlation between the average weighted bond lengths and effective coordination number of cations and anions. As expected, the monolayer cohesive energies increase with the radius of the Q species (i.e., from S to Te). Furthermore, an increase in the anion size diminishes the work function for nearly all MQ x monolayers, which can be explained by the nature of the electronic states at the valence band maximum.

Tunability of the bandgap of SnS by variation of the cell volume by alloying with A.E. elements.

We clarified that the bandgap of inorganic materials is strongly correlated with their effective coordination number (ECoN) via first-principles calculations and experimental confirmations. Tin mono-sulphide (Pnma) and germanium mono-sulphide (Pnma) were selected as model cases since these materials successively alter the ECoN as the cell volume changes and show an uncommon relationship between cell volume and bandgap. Contrary to the common semiconductors, the bandgaps of SnS (Pnma) and GeS (Pnma) have a positive relationship with respect to cell volume. This unique phenomenon was explained by incorporating the concept of ECoN into the theoretical studies. The theory proposed in this study is widely applicable to semiconductors with low-symmetry structures. Further, we experimentally demonstrated that the bandgap of SnS (Pnma) can be broadly tuned by changing the unit cell volume via alloying with alkali-earth (A.E.) metals, which could allow SnS to be applied to Si-based tandem photovoltaics. Alloying with A.E. elements also stabilised Cl as an n-type donor, which enabled n-type conduction in the bandgap-widened SnS film in the SnS-based semiconductors.

Correlation between d-orbital bandwidth and local coordination environment in RE2SiO5 compounds with implications in minimizing the coefficient of thermal expansion anisotropy (RE = Sc, Y, La)

Y2SiO5 is one of the promising environmental barrier coating (EBC) materials that protect the gas turbine engine components from unfavorable reactions at higher temperatures. The Y2SiO5 compound forms in the monoclinic crystal structure (C2/c space group), and one of the drawbacks is its appreciable coefficient of thermal expansion (CTE) anisotropy, which adversely affects its lifetime as the EBC material. The objective of this work is to uncover previously unknown correlation between the electronic structure and crystal structure of RE2SiO5 compounds in the equilibrium and hypothetical C2/c structures (where RE = Sc, Y, or La). Our density functional theory calculations reveal a trend in the RE-cation d-orbital bandwidth as a function of the RE electronic configuration, local RE–O coordination environment, and unit cell volume. We predict that the Y-4d orbital bandwidth can become narrower when Y2SiO5 forms in an open structure with a reduced Y–O effective coordination number. We conjecture that a narrow Y-4d orbital bandwidth may give rise to smaller CTE anisotropy compared to Y2SiO5 in its equilibrium structure. The outcome of this work has potential implications in the rational design of Y2SiO5-based EBCs for use under extreme temperature environments.

Highly active Fe36Co44 bimetallic nanoclusters catalysts for hydrolysis of ammonia borane: The first-principles study

In this paper, Fe36Co44 nanocluster structure is used to catalyze the hydrolysis reaction of ammonia borane to produce H2. Firstly, we complete the construction of Fe36Co44 cluster structure and calculate the electronic properties of the cluster. By comparing the adsorption process of Ammonia Borane (AB) in active sites of the cluster, which have different Effective Coordination Number (ECN), the qualitative relationship between ECN and the catalytic activation of AB is clarified, and the optimal catalytic active site is obtained. Then, from the perspective of different reaction paths, we study the hydrolysis reaction of AB in multiple paths, and obtain 5 different reaction paths and energy profiles. The calculation results show that in the case of NH bond priority break (path 5), the reaction has the minimum rate-determining step (RDS) barrier (about 1.02 eV) and the entire reaction is exothermic (about 0.40 eV). So, path 5 is an optimal catalytic reaction path. This study will have an important guiding significance for the study of the AB hydrolysis reaction mechanism.

Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au-Te and Te-Te bonds in Au1-xAgxTe2 group minerals.

The structure refinement and XANES study of two gold-silver-tellurides [Au1+xAgxTe2, krennerite (x = 0.11-0.13) and sylvanite (x = 0.29-0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08-0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te-Te bonds (bond distances < 2.9 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non-bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te-Te bond in the [Te3]2- anion is not entirely homopolar. Both tellurides can therefore be described as Madelung-type compounds, despite the presence of Te-Te in both structures.

Effective coordination numbers from EXAFS: general approaches for lanthanide and actinide dioxides.

Extended X-ray absorption fine structure (EXAFS) is a comprehensive and usable method for characterizing the structures of various materials, including radioactive and nuclear materials. Unceasing discussions about the interpretation of EXAFS results for actinide nanoparticles (NPs) or colloids were still present during the last decade. In this study, new experimental data for PuO2 and CeO2 NPs with different average sizes were compared with published data on AnO2 NPs that highlight the best fit and interpretation of the structural data. In terms of the structure, PuO2, CeO2, ThO2, and UO2 NPs exhibit similar behaviors. Only ThO2 NPs have a more disordered and even partly amorphous structure, which results in EXAFS characteristics. The proposed new core-shell model for NPs with calculated effective coordination number perfectly fits the results of the variations in a metal-metal shell with a decrease in NP size.

Investigation of the Solid-Solution Limit, Crystal Structure, and Thermal Quenching Mitigation of Sr-Substituted Rb2CaP2O7:Eu2+ Phosphors for White LED Applications.

Rb2CaP2O7:Eu2+ is a bright reddish-orange-emitting phosphor, but its luminescence thermal stability is poor. In this study, we investigated the solid-solution limit and thermal quenching mitigation of Rb2CaP2O7:Eu2+ phosphors by cation substitution with Sr2+ and revisited their crystal structure. First, we carefully investigated the solid solution limit of Sr in the structure of Rb2CaP2O7. The results show that up to 80% of Ca can be substituted by Sr, whereas Ca hardly resides in the structure of Rb2SrP2O7. Consequently, the photoluminescence was fine-tuned from reddish-orange (612 nm) to yellow (580 nm) light emission by increasing the Sr2+ concentration in the solid-solution phosphors Rb2Sr1-xCaxP2O7:Eu2+ under excitation at 342 nm. The mechanism for the blue shift of the emission spectrum was discussed. With the associated modification of the local environment of the activator (as reflected by the changes in the effective coordination number, average bond length, distortion index, and quadratic elongation), the luminescence thermal quenching issue of Rb2CaP2O7:Eu2+ was mitigated by substituting 20% Sr into the Ca site (Rb2Ca0.8Sr0.2P2O7:Eu2+). The integrated intensity of bright orange-emitting Rb2Ca0.8Sr0.2P2O7:Eu2+ (603 nm) at 150 °C retained 53% of its initial value, 1.64 times that of Rb2CaP2O7:Eu2+ (32.3%). Such an enhancement could be attributed to the improved rigidity of the crystal structure due to the local structure modification as evidenced by Rietveld refinement. The cation substitution is an effective approach for mitigating the thermal quenching issue of phosphors.

Adsorption of La on kaolinite (0 0 1) surface in aqueous system: A combined simulation with an experimental verification

A systematic first-principles model study of La(III) adsorption on kaolinite (001) surface was studied using periodic DFT calculations. The effective coordination number, coordination geometry, preferred adsorption position and adsorption type were examined. Based on the binding energy, the form of [La(OH)(H2O)8]2+ would be more stable than [La(H2O)10]3+ in a high pH solution system. Outer-layer and inner-layer adsorption models were applied to account for the different bonding between La and the kaolinite surface. The inner-layer adsorption model was dominated by coordination bonds between La and surface oxygen, while the outer-layer adsorption model was related to hydrogen bonds. The inner-layer adsorption at the Ou site was the optimal adsorption site. The PDOS projections and Mulliken bond populations suggest that the bonding orbital combination of La 5d and Ou 2p was the dominant orbital contribution of La-Ou. Contrary to the stability of La ions in aqueous systems, [La(H2O)10]3+ would be more stable than [La(OH)(H2O)8]2+ after adsorption on the kaolinite (001) surface, which was verified by microcalorimetry experiments.

Li3La[PS4]2: The First Lithium Lanthanum Ortho‐Thiophosphate

AbstractThe first lithium‐containing lanthanum ortho‐thiophosphate(V) with the formula Li3La[PS4]2 crystallizes orthorhombically in the space group Pbca with a=1053.51(6) pm, b=1920.62(11) pm, c=1165.83(7) pm and Z=8. Colorless single crystals were obtained from the reaction of lanthanum trichloride, dilithium sulfide and diphosphorus pentasulfide at 750 °C with lithium chloride as by‐product. The new structure of Li3La[PS4]2 features characteristic [PS4]3− anions and a crystallographically unique La3+‐cation position, surrounded by eight sulfur atoms from four [PS4]3− tetrahedra constructing a trigonal dodecahedron. These [LaS8]13− polyhedra are connected via [PS4]3− tetrahedra to form strongly corrugated layers ( ) spreading out parallel to the ac‐plane with Li+ cations in between residing in four‐, five‐ and sixfold sulfur coordination. Raman‐spectroscopic measurements were performed to visualize further vibrational modes, whereas diffuse reflectance spectroscopy verified the absorption of UV light, owing to the colorless appearance of the crystals. The optical band gap was found to be at 3.25 eV. Energy‐dispersive X‐ray spectroscopy (EDXS) measurements assured the absence of chlorine in the investigated single crystals. Calculations of Effective Coordination Numbers (ECoN) for each crystallographic position confirm the refined coordination environments for all atoms. Further investigations of the Madelung Parts of the Lattice Energies (MAPLE) point out the strong relationship of Li3La[PS4]2 with the ternary compounds of Li3[PS4] and La[PS4].

Progressive deformation and pore network attributes of sandstone at in-situ stress states using computed tomography

Understanding the mechanism of microscopic failure of sandstone is exceptionally crucial for various engineering projects. The current study investigates the real-time high-resolution micro-computed tomography (m-CT) based microscopic failure attributes of sandstone under unconfined axial compression. Specimens of sandstone were analysed under varying loading conditions (zero load, 25%, 50%, 75% of the peak loads, and at peak failure loads) for analysing their effects on various petrophysical properties such as total pores, connected pores, non-connected pores, development of microfractures, and different pore network attributes such as pore radius, pore-volume, pore coordination number (CN), throat radius and throat channel length. Such an investigation enables us to explore the evolution of failure within sandstone under in-situ compression. The results showed the appearance of macrofractures and an increasing pattern of various pore-network attributes in the sandstone after 50% of the peak load. However, the non-connected pores gradually decreased with the rising loading stages. Finally, a negative correlation was obtained between tortuosity of sandsone with effective porosity and coordination number.

Available Active Sites on ε‐Fe3N Nanoparticles Synthesized by a Facile Route for Hydrogen Evolution Reaction

AbstractExploring efficient noble‐metal‐free water‐splitting electrocatalysts from earth‐abundant elements is of great importance to realize wide applications in the generation of hydrogen fuel for clean energy. Here, a facile route is reported to synthesize ε‐Fe3N single‐phase nanoparticles by thermal ammonolysis of Fe precursors. The roles of nitrogen atoms in tailoring the hydrogen evolution reaction (HER) activities of ε‐Fe3N have been systematically investigated. HER activity is enhanced by reducing the effective coordination number of nitrogen atoms from 2.61 to 1.67, where the standard coordination number in ε‐Fe3N is 2. Density functional theory calculations reveal that the reduction of nitrogen content lowers the energy of Tafel process on the (100)‐FeN‐exposed and (110) N‐exposed surfaces. Both surfaces are thermodynamically favored for the HER. Furthermore, the active sites of Tafel process change from the kinetically less favored hollow sites of Fe atoms to the kinetically more favored top site of N atoms and the bridge site of Fe atoms on both (100)‐FeN and (110) N‐exposed surfaces. The findings propose a novel strategy to enhance HER activity by using nitrogen deficiency, which is of great importance for the development of highly active transition metal based electrocatalysts.

Hbox {Ag}_{m} hbox {Rh}_n clusters with m+nle 55

Using a combination of genetic algorithms for the unbiased structure optimization and a Gupta many-body potential for the calculation of the energetic properties of a given structure, we determine the putative total-energy minima for all hbox {Ag}_{m} hbox {Rh}_n clusters with a total number of atoms m+n up to 55. Subsequently, we use various descriptors to analyze the obtained structural and energetic properties. With the help of a similarity function, we show that the pure Ag and Rh clusters are structurally similar for sizes up to around 20 atoms. The same approach gives that the mixed clusters tend to possess a larger structural similarity with the pure Rh clusters than with the pure Ag clusters. However, for clusters with msimeq nge 25, other structures dominate. The effective coordination numbers for the Ag and Rh atoms as well as the radial distributions of those atoms indicate that there is a tendency towards segregation with Rh atoms forming an inner part and the Ag atoms forming a shell. Only few clusters, all with a fairly large total number of atoms, are found to be particularly stable.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance.

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe)0.9 (AgBiTe2 )0.1 , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe)1-x (AgBiTe2 )x . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)1-x (AgBiTe2 )x . (GeSe)0.9 (AgBiTe2 )0.1 has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 μW cm-1 K-2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm-1 K-1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)0.9 (AgBiTe2 )0.1 . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance

AbstractOrthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p‐type polycrystalline rhombohedral (GeSe)0.9(AgBiTe2)0.1 , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8–0.29 in (GeSe)1−x(AgBiTe2)x . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe)1−x(AgBiTe2)x . (GeSe)0.9(AgBiTe2)0.1 has closely lying primary and secondary valence bands (within 0.25–0.30 eV), which results in high power factor 12.8 μW cm−1 K−2 at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm−1 K−1 at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe)0.9(AgBiTe2)0.1. We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3–30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.