Partial Coordination Numbers Research Articles

High temperature breakdown of the Stokes-Einstein relation in a computer simulated Cu-Zr melt.

Transport properties and the Stokes-Einstein (SE) relation in liquid Cu8Zr3 are studied by molecular dynamics simulation with a modified embedded atom potential. The critical temperature Tc of mode coupling theory (MCT) is derived as 930 K from the self-diffusion coefficient D and viscosity η. The SE relation breaks down around TSE = 1900 K, which is far above Tc. At temperatures below TSE, the product of D and η fluctuates around a constant value, similar to the prediction of MCT near Tc. The influence of the microscopic atomic motion on macroscopic properties is investigated by analyzing the time dependent liquid structure and the self-hole filling process. The self-holes for the two components are preferentially filled by atoms of the same component. The self-hole filling dynamics explains the different breakdown behaviors of the SE relation in Zr-rich liquid CuZr2 compared to Cu-rich Cu8Zr3. At TSE, a kink is found in the temperature dependence of both partial and total coordination numbers for the three atomic pair combinations and of the typical time of self-hole filling. This indicates a strong correlation between liquid structure, atomic dynamics, and the breakdown of SE relation. The previously suggested usefulness of the parameter d(D1/D2)/dT to predict TSE is confirmed. Additionally we propose a viscosity criterion to predict TSE in the absence of diffusion data.

Influence of Nickel on the Structure of Al0.878Si0.122 Liquid Eutectic

We investigate the atomic structure of aluminum-based alloys both by the X-ray diffraction method and by the reverse Monte-Carlo technique. We compute the total and partial structural factors, the pair correlation functions, and partial coordination numbers. It is shown that Al0.88Si0.12 eutectic melt consists of microregions based on aluminum and an Al–Si solution. Adding nickel to the eutectic leads to the formation of Al–Ni chemically ordered structural regions where nickel is mainly surrounded by aluminum atoms.

Electron energy loss spectroscopy equation for spectra with overlapping oscillations and its solution by a regularization method

The integral equation of EELFS spectra with the overlapping oscillations for two-component systems is proposed. To solve this equation Tikhonov’s regularization method with the iterative procedure is applied. The atomic pair correlation functions of the investigated objects are obtained from the experimental EELFS spectra of Cu50Ni50 and Cu50Mn50 alloys by using Tikhonov’s regularization method. The experimental results agree well with the known crystallographic data (partial interatomic distances, coordination numbers and Debye–Waller factors).

Short-range order of germanium selenide glass

Chalcogenide Ge20Se80 glass was prepared using the melt-quench technique. The radial distribution function is obtained from X-ray diffraction data in the scattering vector interval 0.28 ≤ K ≤ 6.87 A−1. Reverse Monte Carlo (RMC) simulations are useful to compute the partial pair distribution functions, partial structure factors, S ij(K), and total structure factor. Values of r 1/r 2 ratio and bond angle (Θ) indicate that Ge(Se½)4 tetrahedra units connected by chains of the chalcogen atoms are present. The partial structure factors have shown that homopolar Ge—Ge and Se—Se bonds are behind the appearance of the first sharp diffraction peak in the total structure factor. Tetrahedral Ge(Se½)4 structural units connected by Se-Se chains have been confirmed by the simulated values of the partial coordination numbers and the bond angle distributions. Finally, Raman spectra measurements have strongly supported the conclusions obtained either from the calculated Fourier data or from RMC simulations.

Characterization of atomic-level structure in Fe-based amorphous and nanocrystalline alloy by experimental and modeling methods

The atomic structure of Fe70Nb10B20 alloy in “as-cast” state and after annealing was investigated using high-energy X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and high resolution transmission electron microscopy (HRTEM). The HRTEM observations allowed to indicate some medium-range order (MRO) regions about 2nm in size and formation of some kinds of short-range order (SRO) structures represented by atomic clusters with diameter ca. 0.5nm. The Reverse Monte Carlo (RMC) method basing on the results of XRD measurements was used in modeling the atomic structure of Fe-based alloy. The structural model was described by peak values of partial pair correlation functions and coordination numbers determined by Mössbauer spectroscopy investigations. The three-dimensional configuration box of atoms was obtained from the RMC simulation and the representative Fe-centered clusters were taken from the calculated structure. According to the Gonser et al. approach, the measured spectra of alloy studied were decomposed into 5 subspectra representing average Fe–Fe coordination numbers. Basing on the results of disaccommodation of magnetic permeability, which is sensitive to the short order of the random packing of atoms, it was stated that an occurrence of free volume is not detected after nanocrystallization process.

Short-range order of germanium selenide glass

Chalcogenide Ge 20Se80 glass was prepared using the melt-quench technique. The radial distribution function is obtained from X-ray diffraction data in the scattering vector interval 0⋅28 ≤ K ≤ 6⋅87 Å −1. Reverse Monte Carlo (RMC) simulations are useful to compute the partial pair distribution functions, g ij (r), partial structure factors, S ij (K), and total structure factor. Values of r 1/r 2 ratio and bond angle (Θ) indicate that Ge (Se1/2)4 tetrahedra units connected by chains of the chalcogen atoms are present. The partial structure factors have shown that homopolar Ge–Ge and Se–Se bonds are behind the appearance of the first sharp diffraction peak (FSDP) in the total structure factor. Tetrahedral Ge (Se1/2)4 structural units connected by Se–Se chains have been confirmed by the simulated values of the partial coordination numbers and bond angle distributions. Finally, Raman spectra measurements have strongly supported the conclusions obtained either from the calculated Fourier data or from RMC simulations.

Pressure-induced structural transition in amorphous GeO2: a molecular dynamics simulation

We studied the structural and dynamical properties of amorphous germanium dioxide (GeO2) from low to high pressure by means of the classical molecular dynamics technique. The simulations were done in the micro-canonical ensemble, with systems at densities ranged from 3.16 to 6.79 g/cm3, using a pairwise potential. The network topology of the systems is analyzed at atomic level through partial pair correlations, coordination number and angular distributions. The dynamic properties were characterized by means of the vibrational density of states. According the density increases, a structural transformation from a short-range order, defined by a building block composed by a basic (GeO4) tetrahedron, to a basic (GeO6) octahedron is observed. The vibrational density of states also presents important changes when the density increases, with a low frequency band lessened, and a high density band wider and flatter.

Atomic bond proportions and relations to physical properties in metallic glasses

Identifying the underlying mechanisms of the special physical properties of metallic glasses (MGs) is of fundamental interest. The complexity and uncertainty in the structure of MGs have been obstacles to develop an efficient structure–property relationship. In this letter, we present a simplified structural parameter, atomic bond proportion, instead of exploring the complicated atomic structure to predict the properties of MGs. Based on statistical calculations and partial coordination number methods, the proportion of different types of atomic bonds for a series of MGs (e.g., Ca60Mg15Zn25, Zr67Ni33, and Al90Y10) is successfully given, and the results from both methods significantly agree with each other. The weighted averages of the physical properties corresponding to the constituent atomic bonds were used to analyze the relationship between the composition and physical properties. The predicted atomic bond proportion may better elucidate the structure and properties of these materials.

Atomic structure of Pd81Si19 glassy alloy under high pressure

In situ high-pressure X-ray diffraction experiments and reverse Monte Carlo simulations on Pd81Si19 glassy alloy were performed up to ∼35GPa. High-quality structure factor data with a q range up to 13.5Å−1 were obtained. The pressure dependence of the relative volume of Pd81Si19 glassy alloy can be well described by a third-order Birch–Murnaghan equation of state with bulk modulus of B0=229GPa and a pressure derivative of B0′=2.7. The continuous and monotonic change of relative volume as a function of pressure does not indicate any first-order phase transitions in Pd81Si19 glassy alloy in the studied pressure range. The pressure dependence of the atomic structures was systematically investigated by means of bond angle distribution, the Honeycutt and Andersen index, Voronoi tessellation, total and partial coordination numbers, and local chemical ordering analyses. It is found that the topological and chemical atomic structures of the Pd81Si19 glassy alloy do not change much with pressure, indicating the stability of this material under pressure.

Structural and dynamical heterogeneity of undercooled Fe75Cu25 melts with miscibility gap

Molecular dynamics simulation (MD) based upon the developed embedded atom method (EAM) has been performed to explore the structural and dynamical heterogeneity of Fe75Cu25 melts. The results show that the melts separate into Cu-rich droplets surround by the Fe-rich matrix controlled by nucleation and growth mechanism. The larger undercoolings suggest the higher nucleation rate and growth rate of droplets. The growth of droplet is achieved by the aggregation and coagulation of neighbor droplet with the characteristics of collective movement for homogeneous atoms. A sharp increase of SCC (q=0) is found at all simulated temperature, which means concentration fluctuation on large length scales are much pronounced. Both partial pair correlation functions (PPCFs) and coordination number (CN) confirm that liquid–liquid (L–L) phase separation is a successive process with a stronger interaction of homogeneous pairs than that of heterogeneous pairs in Fe75Cu25 melts. The studies above characterize the phase separation of metal melts on the atomic scale.

Amorphous LiLaTiO3 As Solid Electrolyte for Lithium Ion Batteries

Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4.5×10-6, 6.9×10-6, 1.3×10-5, and 3.8×10-5 S/cm at 30°C, 50°C, 70°C, and 90°C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30-90°C. The amorphous structure of LLTO was predicted by ab-initio molecular dynamics (AIMD) silumations and analyzed by partial pair distribution functions and coordination numbers. It is observed that the local atomic environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacements obtained from the AIMD simulation results, and the ionic conductivities were evaluated through the Einstein’s relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volume.

Amorphous LiLaTiO3 as Solid Electrolyte Material

Amorphous lithium lanthanum titanate (LLTO) thin films were successfully prepared by a sol-gel method with an all-alkoxide based route. The thin film resistance was determined from the complex spectra by fitting experimental data to the equivalent circuit. The ionic conductivities of amorphous LLTO thin films were 4.5 × 10−6, 6.9 × 10−6, 1.3 × 10−5, and 3.8 × 10−5 S/cm at 30°C, 50°C, 70°C, and 90°C, respectively. The activation energy of lithium ion conduction in the thin film was evaluated to be 0.36 eV in the temperature range of 30–90°C. The structure of amorphous LLTO was predicted by the ab-initio molecular dynamics (AIMD) simulations and analyzed by partial pair distribution functions, coordination numbers and Voronoi tessellation. It is observed that the local environment of Ti in amorphous LLTO is quite similar to that in the crystal state but the atomic packing is much less dense. The ionic diffusivities were derived from the mean square displacement curves and the conductivities were evaluated from the Nernst-Einstein's relation, showing good agreement with experimental data. The good ionic conductivity of amorphous LLTO is attributed to its open and disordered structure with large excessive volumes.

Acidic properties of aqueous phosphoric acid solutions: a microscopic view

We report on new neutron and x-ray diffraction data on D2O:D3PO4 solutions at two concentrations, 1:1 and 3:1. The experimental datasets were modelled simultaneously by the reverse Monte Carlo (RMC) method. From the resulting models, partial radial distribution functions (prdf) and coordination numbers were obtained. The acidity was found to decrease with increasing D3PO4 concentration. The ratio of dissociated acidic protons was estimated by dedicated simulation runs using average coordination number constraints. It was found that in the saturated solution the ratio of dissociated protons cannot exceed 20%.

Structural property comparison of Ca–Mg–Zn glasses to a colloidal proxy system

We compare various structural properties of the Ca–Mg–Zn ternary metallic glass system at compositions Ca60MgXZn40−X (where X=25, 20, 15, 10) to a colloidal proxy system containing “Red”, “Green” and “Blue” particles at the corresponding compositions R60GXB40−X. The methacrylate-based polymer colloid particles have the same relative radius ratio as their atomic counterparts and surface electrostatic charges are employed to mimic atomic interactions. The structures of these colloidal suspensions are investigated through laser scanning confocal microscopy, with each particle species labeled with red, green and blue fluorescent dyes. We find qualitative agreement with number densities and many characteristics of the partial radial distribution functions and partial coordination numbers between the atomic and colloid systems. In general, coordination numbers agree within experimental error. Total coordination numbers for Red-centered clusters are found to be slightly higher than the corresponding Ca-centered clusters, while Green- and Blue-centered clusters have slightly fewer neighbors than Mg- and Zn-centered clusters. These and other differences are noted and possible explanations of these are offered, as well as ways that the proxy system might be improved to more accurately reproduce the structure of the metallic system.

Ab initio molecular dynamics simulation of the liquid and amorphous structure of Mg65Cu25Gd10 alloy

The liquid and amorphous structures of Mg65Cu25Gd10 alloy were studied by using molecular dynamics methods within the frame of density functional theory. The generalized and partial pair correlation functions, structure factors, coordination numbers and bond pairs for this alloy were analyzed. It is shown that this alloy exhibit typical characterization of liquid structure at the temperature higher than 750K, and of amorphous structure with shoulders on the second diffuse peaks of the pair correlation functions curves at room temperature. The local short and medium range ordering tends to be increased with the decrease of temperature. Both the liquid and the amorphous structures are mainly composed of icosahedral type of bond pairs. Perfect and distorted icosahedra can be differentiated from the atomic configuration of the amorphous alloy.

Structure and dynamics of undercooled FeNi

Molecular dynamics simulation has been performed to explore the structural and dynamical properties of undercooled FeNi melt based on the embedded atom method (EAM), namely due to G. Bonny. The simulated partial pair correlation function (PPCF) and coordination number (CN) of liquid FeNi indicate that no stronger interaction of heterogenic atom pairs than those of the same atom pairs happen in melts. FeNi melt is close to an ideal mixing system without chemical short-range order (CSRO). The undercooled FeNi exhibits icosahedral short-range order (ISRO) that is reflected directly as a large number of 1551, 1541 bonded pairs as well as snapshot of icosahedron clusters. The mean squared displacement (MSD) and self diffusion coefficient of Fe and Ni is quite similar, which is consistent with the strong glass formation liquid of Ni36Zr64. Our work gives the detailed structure and dynamics information of undercooled FeNi melt.

Chemical order in liquid As2Se3 at high temperatures obtained by X-ray scattering and reverse Monte Carlo modeling

X-ray scattering experiments for liquid As2Se3 were carried out at high temperatures up to 1673K at 6MPa, using synchrotron radiation at SPring-8 in Japan, and the structure factor with reasonably nice statistics is obtained. The first peak position in the pair distribution function slightly shifts to smaller distance and the first minimum at 2.7Å develops, as the metallic properties are enhanced with increasing temperatures. A first sharp diffraction peak at 1.2Å−1 in the structure factor becomes broad with an increasing temperature. The partial structure factors in liquid As2Se3 were deduced by Reverse Monte Carlo modeling constrained by partial coordination number fractions reported by the first principles molecular dynamics simulations. Chemical order correlated with the medium range order expected by the first sharp diffraction peak is discussed on the basis of results from the Reverse Monte Carlo modeling.

Short range order in bimetallic nanoalloys: An extended X-ray absorption fine structure study

Partial coordination numbers measured by extended X-ray absorption fine structure (EXAFS) spectroscopy have been used for decades to resolve between different compositional motifs in bulk and nanoscale bimetallic alloys. Due to the ensemble-averaging nature of EXAFS, the values of the coordination numbers in nanoparticles cannot be simply interpreted in terms of the degree of alloying or segregation if the compositional distribution is broad. We demonstrate that a Cowley short range order parameter is an objective measure of either the segregation tendency (e.g., a core-shell type) or the degree of randomness (in homogeneous nanoalloys). This criterion can be used even in the case when the clusters are random but have broad compositional distributions. All cases are illustrated using the analyses of EXAFS data obtained in three different nanoscale bimetallic systems: Pt(core)-Pd(shell), Pd(core)-Pt(shell), and Pt-Pd random alloy.

The structure of Cu–Zr glasses using a colloidal proxy system

We present a novel experimental technique for studying the structure of metallic glasses (specifically Cu–Zr) through a proxy system of charged colloidal particles. A dense suspension of two types of colloidal particles that match the relative size ratio and approximate the attractive interaction of Cu and Zr atoms is created. Confocal microscopy images are analyzed to find the location of tens of thousands of particles at each composition studied. The particle locations are used to determine the packing fraction, partial radial distribution functions and partial coordination numbers of each component as well as the shape of clusters (central atom and first shell neighbors) that form. These results are compared to experimental and simulation literature on Cu–Zr systems, with good agreement found among all structural properties.

Local order and dynamic properties of liquid AuxSi1−x alloys by molecular dynamics simulations

Molecular dynamics simulations are performed to examine structural and dynamic properties of liquid Au-Si alloys around the eutectic composition, with interactions described via a modified embedded-atom model suitable for the liquid properties. The local structure as defined by the partial pair-correlation functions, coordination numbers, and partial structure factors is found to display a strong evolution with composition. In addition, a structural study using a three-dimensional pair-analysis technique evidences a strong evolution of the icosahedral short-range order over the range of concentrations, 0 < x(Si) < 0.5. In examining the dynamic properties of these alloys, we show a strong interplay between the structural changes and the evolution of the self-diffusion coefficients as a function of composition.