Concentration-concentration Partial Structure Factor Research Articles

Concentration dependence of the bulk and surface properties of liquid AlZn alloys at 1000 K

The properties of mixing of the segregating liquid AlZn alloys at 1000 K have been investigated using the joint effect of size ratio and enthalpic contribution as well as entropic effect. The theoretical values of the bulk properties i.e. thermodynamic properties such as excess free energy of mixing, free energy of mixing, entropy of mixing, heat of mixing as well as the structural properties like concentration-concentration partial structure factor (or concentration fluctuations in the long wavelength limit) and short-range order parameter have been compared with the corresponding experimental data available in the literature. Again Butler model has been applied to study the surface properties such as the surface concentration, surface tension and surface concentration-concentration partial structure factor (or surface concentration fluctuations) of liquid AlZn alloys at 1000 K. The theoretical data of the surface tension have been compared with the experimental data available in the literature which show a qualitative agreement.

Structure of the Intermediate Phase Glasses GeSe3 and GeSe4: The Deployment of Neutron Diffraction With Isotope Substitution

The method of neutron diffraction with isotope substitution was used to measure the full set of partial pair-correlation functions for each of the network-forming glasses GeSe$_3$ and GeSe$_4$, which lie at the boundaries of the so-called intermediate phase in the Ge-Se system. The results show the formation of chemically ordered networks, where selenium chains are cross-linked by Ge(Se$_4$)$_{1/2}$ tetrahedra, in contrast to glassy GeSe$_2$ where the chemical order is broken. In all of these materials, the Ge-centered structural motifs are distributed unevenly on an intermediate length scale, as indicated by the appearance of a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor. The new experimental work provides benchmark results for guiding in the development of realistic structural models, which can be used to explore the network rigidity and other structure-related properties of the glass. In this context, there are discrepancies between experiment and the predictions of first-principles molecular dynamics simulations that are particularly marked in respect of the Ge-Ge correlation functions, which are sensitive to the connectivity of the Ge-centered structural motifs.

Structure of amorphous GeSe9 by neutron diffraction and first-principles molecular dynamics: Impact of trajectory sampling and size effects

The structure of glassy GeSe9 was investigated by combining neutron diffraction with density-functional-theory-based first-principles molecular dynamics. In the simulations, three different models of N = 260 atoms were prepared by sampling three independent temporal trajectories, and the glass structures were found to be substantially different from those obtained for models in which smaller numbers of atoms or more rapid quench rates were employed. In particular, the overall network structure is based on Sen chains that are cross-linked by Ge(Se4)1/2 tetrahedra, where the latter are predominantly corner as opposed to edge sharing. The occurrence of a substantial proportion of Ge-Se-Se connections does not support a model in which the material is phase separated into Se-rich and GeSe2-rich domains. The appearance of a first-sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor does, however, indicate a non-uniform distribution of the Ge-centered structural motifs on an intermediate length scale.

Investigation of size effects on the structure of liquid GeSe2 calculated via first-principles molecular dynamics

The structural properties of liquid GeSe(2) have been calculated by first-principles molecular dynamics by using a periodic simulation box containing N = 480 atoms. This has allowed a comparison with previous results obtained on a smaller system size (N = 120) [M. Micoulaut, R. Vuilleumier, and C. Massobrio, Phys. Rev. B 79, 214205 (2009)]. In the domain of first-principles molecular dynamics, we obtain an assessment of system size effects of unprecedented quality. Overall, no drastic differences are found between the two sets of results, confirming that N = 120 is a suitable size to achieve a realistic description of this prototypical disordered network. However, for N = 480, short range properties are characterized by an increase of chemical order, the number of Ge tetrahedra coordinated to four Se atoms being larger. At the intermediate range order level, size effect mostly modify the low wavevector region (k ~1 Å(-1)) in the concentration-concentration partial structure factor.

Intermediate range order in vitreous silica from a partial structure factor analysis

By combining the methods of isotopic substitution in neutron diffraction and high-energy x-ray diffraction, we have determined partial structure factors of vitreous ${\text{SiO}}_{2}$. A discussion of the effect of systematic and statistical errors is presented. The experimental results are found to be in good (but not exact) agreement with existing ab initio and classical molecular-dynamics simulations. No first sharp diffraction peak (FSDP) is observed in the concentration-concentration partial structure factor, ruling out the void-cluster-based model as a possible explanation for the origin of intermediate range order. However, the data are consistent with a model in which the intermediate range order arises from the periodicity of boundaries between a succession of small cages in the network, and the second diffraction peak is associated with chemical ordering of ${\text{SiO}}_{4}$ tetrahedra within continuous regions of the network between cages. The cage model is used to explain compositional trends in the FSDP height for neutron and x-ray data on ${\text{BeO-SiO}}_{2}$ glasses.

Structure of Mn40Zn60 liquid alloy

In this work we present the structural study of the Mn40Zn60 liquid alloy, whose two components have a high vapour pressure. The structure has been measured by neutron diffraction. The investigation of the chemical order in this alloy was readily made possible. This is due to the manganese negative scattering length which allows a good contrast. A magnetic correction has been considered since manganese is paramagnetic in the liquid state. An ‘effective’ spin is obtained and its value is discussed. The interpretation of the atomic structure is done in the frame of the Bhatia-Thornton formalism, (SNN (q), SNC (q), SCC (q)) which allows to separate topological, size and chemical effects. It appears clearly that manganese ions and zinc ions have approximately the same radius in the alloy as SNC (q) ≅ 0. The Bhatia-Thornton number-number partial structure factor SNN (q) has been approximated by a linear combination of the experimental structure factors of the two alloy pure components. In the frame of this assumption, the Bhatia-Thornton concentration-concentration partial structure factor SCC (q) is obtained, and shows clearly that this alloy is hetero-coordinated. The hard sphere model cannot explain the structure of this alloy. Its behaviour is compared to other manganese-polyvalent alloys and the general trends are discussed.

Charge fluctuations and concentration fluctuations at intermediate-range distances in the disordered network-forming materialsSiO2,SiSe2, andGeSe2

We calculate the concentration-concentration partial structure factor ${S}_{\mathrm{CC}}(k)$ and the charge-charge structure factor ${S}_{\mathrm{zz}}(k)$ of liquid $\mathrm{Si}{\mathrm{O}}_{2}$, amorphous $\mathrm{Si}{\mathrm{Se}}_{2}$ and liquid $\mathrm{Ge}{\mathrm{Se}}_{2}$ using first-principles molecular dynamics. These systems are characterized by the occurrence of intermediate range order, as evidenced by a first sharp diffraction peak (FSDP) at low $k$ values in the total neutron structure factor. We show that a FSDP in the concentration-concentration partial structure factor ${S}_{\mathrm{CC}}(k)$ is generally associated with a small departure from chemical order. This feature tends to vanish either when sufficiently high levels of structural disorder set in, or, oppositely, when the chemical order is essentially perfect. For none of these networks, a FSDP is observed in the charge-charge structure factor ${S}_{\mathrm{zz}}(k)$, i.e., fluctuations of charge do not occur over intermediate range distances. The constraint of charge neutrality is at the very origin of the appearance of fluctuations of concentration. These are observed when the atoms occur in configurations with different coordinations.

Chemical Order of Manganese-Antimony Liquid Alloys Obtained Experimentally by the Neutron “Null Matrix” Method

The Bhatia-Thornton concentration-concentration partial structure factor S CC ( q ) is a strong indicator of the order in liquid alloys. Manganese is one of the four metals, which has a negative neutronic scattering length, while the antimony one is positive. Thus for a defined composition (Mn 60 Sb 40 ) the linear combination of the two neutronic scattering lengths weighted by the atomic compositions is zero (zero alloy). We present here the results of neutron diffraction on the Mn 60 Sb 40 "null matrix" alloy at 950C, which is proportional to the Bhatia-Thornton S CC ( q ) ("null matrix" method). The total structure factor of Mn 40 Sb 60 at 800C has also been measured. The main peak of the experimental S CC ( q ) is a proof of a strong chemical order in this alloy. This order is confirmed in the real space by the Fourier transform of the structure factor. To interpret our experimental results, one generally uses effective potentials determined with the pseudopotential formalism. But transition metal pseudopotentials are not easy to handle especially in alloys. We used different simple effective potential models: hard spheres with constant diameters, hard spheres with composition dependent diameters, shouldered hard spheres models. We first show that the hard sphere model cannot reproduce the experimental results if we postulate that the hard sphere diameters do not change on alloying. Then we fit the hard sphere diameters on the experimental structure factor of the alloy. This schema can correspond physically to a charge transfer between the two components. We show that it can no more explain the experiment. The addition of attractive and (or) repulsive contributions to the different interatomic potentials give satisfactory results. The best results are obtained with attractive contributions between unlike atoms and repulsive ones between identical atoms. We have also shown that the parameters obtained for Mn 60 Sb 40 give also good results for the structure factor at a second composition: Mn 40 Sb 60 .

An exact quasi-chemical equation for excess heat capacity with W-shaped concentration dependence

A reliable equation for the excess heat capacity C V E at constant volume is derived from Guggenheim's quasi-chemical lattice theory of liquid mixtures for the case of equal-sized molecules, with the assumption that the interchange energy depends on the temperature. Using suitable parameters, a W-shaped concentration dependence for C V E is predicted. The availability of this exact quasi-chemical equation permits the study of the theoretical dependence of C V E on temperature and concentration, a comparison with previous approximate results and better analysis of the molecular meaning of the W-shapes found in the experimental excess heat capacities C P E at constant pressure. Moreover, within this theory, the long-wavelength limit of the Bhatia-Thornton concentration-concentration partial structure factor S cc(0) is also derived and its correlation with C V E or C P E is discussed.

The structure of molten and glassy 2:1 binary systems: an approach using the Bhatia—Thornton formalism

A systematic analysis of those liquid binary 2:1 systems (denoted MX 2 ), for which experimental partial structure factors are available from the isotopic substitution method in neutron diffraction, is made using the Bhatia-Thornton (BT) formalism.Particular attention is paid to the origin of the first sharp diffraction peak (FSDP ), which occurs in the measured diffraction patterns for some of the MX 2 systems, since it appears, from recent studies, that this feature is a signature of directional bonding. It is found that FSDPS can occur in all three BT partial structure factors S xB (k). A FSDP feature in the concentration-concentration partial structure factor S cc (k) is not, however, pronounced except in the case of MgCl 2 and the glass forming network melts ZnCl 2 and GeSe 2 . To the extent that these systems can be regarded as ionic melts a FSDP in S cc (k) implies a non-uniformity in the charge distribution on the scale of the intermediate-range order (IRO). The structure of molten GeSe 2 is compared with the structures of molten ZnCl 2 , glassy GeS 2 and glassy Si0 2 . Although the GeSe 2 and ZnCl 2 melts have different short-range order, there are similarities in the observed IRO which can be attributed to the arrangement of the electropositive species M. The essential features of the measured total structure factor for glassy GeS 2 can be reproduced by using the molten GeSe 2 S zB (k). This result lends support to the notion that the S zB (k) for liquid GeSe 2 (and ZnCl 2 ) are characteristic of both the liquid and glassy states of other network glass forming systems. The structures of molten GeSe 2 (or ZnCl 2 ) and glassy Si0 2 are, however, found to be different. The observed discrepancies are largest in the region of the FSDP which signifies pronounced differences in the nature of the IRO for these systems.

Ordering potential and the structural properties of binary Yukawa mixtures

The authors present the results of numerical calculations, in the mean spherical approximation (MSA), for model liquid binary alloys interacting via Yukawa potentials, namely phi ij(r)=Aijexp(- lambda r)/r. They have chosen parameters which allow the description of the structural properties of liquids Li-Na and Li-Pb at different concentrations and Li4Pb at three temperatures. They show: (i) that once the ordering potential, nu (r)=1/2( phi 11(r)+ phi 22(r)-2 phi 12(r)), is specified the total structure factor S(k), the concentration-concentration partial structure factor Scc(k) and the number-number partial structure factor SNN(k) are insensitive to changes in the pair interactions phi ij(r); and (ii) that the thermodynamic properties-specifically Scc(0) and the correlation energy-are sensitive to changes in phi ij(r).