Modeling the atomic structure of an amorphous Ni71Nb29 alloy produced by mechanical alloying using reverse Monte Carlo simulations

The local atomic structure of an amorphous Ni₇₁Nb₂₉ alloy produced by mechanical alloying technique was determined using only one X-ray total structure factor S(K) as input data for reverse Monte Carlo (RMC) simulations. The results showed that the amorphous alloy has a local atomic structure similar to that predicted by the additive hard sphere (AHS) model for a Ni and Nb mixture with same composition of the alloy, and quite different of that found in the rhombohedral NiNb crystal. The obtained coordination numbers for the first neighbors showed that the amorphous alloy has a preference to form homopolar pairs.     

Reverse Monte Carlo simulations of an amorphous Cr25Nb75 alloy produced by mechanical alloying

The local atomic structure of an amorphous Cr₂₅Nb₇₅ alloy produced by mechanical alloying was determined using only one X-ray total structure factor S(K) as input data for reverse Monte Carlo simulations. The results showed that the amorphous alloy has a local atomic structure similar to that predicted by the additive hard sphere model for a Cr and Nb mixture with same composition of the alloy, and quite different of those found in the cubic and hexagonal Cr₂Nb crystals.     

Structure characterization of CVD amorphous Si3N4 by pulsed neutron total scattering

The structure factor S(Q) of chemically vapor-deposited (CVD) amorphous Si₃N₄ has been measured by pulsed neutron diffraction over the range of the scattering vector Q from 1–330 nm^?1. The oscillatory behavior in the S(Q) persists up to Q = 300 nm^?1 and there is appreciable small angle scattering intensity. The SiN bond length is l_SiN = 0.1729 nm, and its coordination numbers n_SiN and n_NSi are 3.70 and 2.78 respectively. The bond angles around a Si and a N atom are found to be 109.8 and 121°. Analysis of the small angle scattering intensity shows the existence of voids with an average diameter of about 1 nm and a volume fraction of about 4%, which may stabilize the amorphous structure of Si₃N₄ having rigid covalent bonds due to relaxing the strain energy accumulated in the matrix.     

Crystal structures of K-and Cs-exchanged forms of zorite

The crystal structures of K-and Cs-exchanged forms of zorite were studied by X-ray diffraction and IR spectroscopy: K_4.75Na_1.82[Ti(Ti_0.79Nb_0.20)₄Si₁₂O₃₄(O,OH)_5.2] × 10.62 H₂O (sp. gr. Cmmm, R= 0.0481 for 516 independent reflections) and Cs_4.34Na_1.90[Ti(Ti_0.80Nb_0.18)₄Si₁₂O₃₄(O,OH)₅] × 5.37 H₂O (sp. gr. Cmmm, R = 0.0285 for 621 independent reflections). Both structures retain the mixed polyhedral framework of zorite: Na₆Ti(Ti,Nb)₄(Si₆O₁₇)₂(O,OH)₅ × nH₂O, where n ～ 11. It is shown that the positions of the atoms located in the cavities of the frameworks of these compounds differ from those in the structures of zorite and its synthetic analogs.     

The structural properties of the liquid CdTe based on Reverse Monte Carlo modelling

The Reverse Monte Carlo (RMC) method only provides information on structure, not on dynamics, and so has restricted applicability. We have only attempted to describe the trends in structural terms. The static structure of liquid Cadmium Telluride is performed using RMC modelling technique fitting to neutron diffraction data. In order to retain its tetrahedral environment with the coordination number about four in its liquid state, we have used some plausible physical constraints. In this way, it is possible to derive static structure of liquid CdTe with analysing the experimental data. We have shown the results for the partial static structure factors S_ij(q) and the partial pair distribution functions g_ij(r). We have also determined the distribution of the number of neighbors within the first coordination shell and calculated the bond angle distribution. A good agreement with the experiment is observed for the structural properties of liquid CdTe.     

Crystal structure of Pb-exchanged form of zorite

The crystal structure of a Pb-exchanged form of zorite is studied by X-ray diffraction: Pb_3.95(Ca_0.1Sr_0.05)[Ti(Ti_0.80Nb_0.20)₄Si₁₂O₃₈(OH)] · 9.52H₂O (sp. gr. Cmmm, R = 0.0530 for 680 independent reflections). The structure retains the mixed polyhedral framework of zorite, Na₆[Ti(Ti,Nb)₄(Si₆O₁₇)₂(O,OH)₅] · 11H₂O. This framework is composed of xonotlite-like [Si₆O₁₇] ribbons linked to each other by columns of vertex-sharing (Ti,Nb)O₆ octahedra and isolated TiO₅ half-octahedra. Lead atoms in the Pb-exchanged form occupy one site, unlike Cs cations in the Cs-exchanged form of zorite, which are strongly disordered and partially occupy eight positions. The position of Pb²⁺ cations corresponds to the Na(2) position in the zorite structure, the Sr position in the Sr-exchanged form of ETS-4, and the K position in the K-exchanged form and is similar to the position of the water molecule W(3) in the structure of the Cs-exchanged form of zorite.     

Bulk glassy (Fe0.474Co0.474Nb0.052)100 − x(B0.8Si0.2)x alloys prepared using commercial raw materials

The alloy compositions have been optimized by modifying the B and Si contents in (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x alloy system with commercial materials. The thermal stability of the supercooled liquid improves with the increased B and Si contents from x = 22 to 28. The composition of the alloy with x = 26 is close to a eutectic point. By copper mold casting, (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x bulk glassy alloys with diameters up to 5 mm were synthesized for the composition range of x from 22 to 28. In addition to high glass-forming ability (GFA), the (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x glassy alloys exhibit good soft magnetic properties as well, i.e., rather high saturation magnetization of 0.84–1.07 T, low coercive field of 1.8–3.2 A/m, high initial permeability of 10100–24100 at 1 kHz under a field of 1 A/m and Curie temperature of 620–730 K.     

Determination of partial pair distribution functions in amorphous Ge15Te85 by simultaneous RMC simulation of diffraction and EXAFS data

The atomic structure of amorphous Ge₁₅Te₈₅ has been studied by X-ray absorption spectroscopy (EXAFS) at Ge K-edge. EXAFS data were modeled simultaneously with existing X-ray and neutron diffraction measurements by the reverse Monte Carlo (RMC) simulation technique. Combination of the three independent measurements allowed separation of the partial pair correlation functions g_GeTe(r) and g_TeTe(r) and estimation of the corresponding coordination numbers.     

Optimization of the microstructure and corrosion resistance of Finemet type alloys in KOH solutions

The effect of Cr replacement for Fe on the kinetics of Cu clustering and on the electrochemical behavior of Fe_73.5−XCu₁Nb₃Si_13.5B₉Cr_X amorphous alloys has been studied for improving the corrosion resistance of Fe–Si–B–Nb–Cu Finemet type alloys. The corrosion testing of these materials was performed in 0.5 M KOH solutions using AC impedance measurements. These experimental results are discussed from the thermodynamical point of view, and the optimized microstructure to achieve a high corrosion resistance has been found.     

Analysis of the structure of an amorphous sediment obtained upon decomposition of potassium oxofluroniobate in water

An amorphous sediment was prepared by the decomposition of potassium oxofluoroniobate K_{2 ? x} Nb₄O₃(O, F)₃F in water. For this sediment, the atomic radial distribution function was analyzed with the use of a fragment model, which allows one to interpret expanded coordination spheres as being formed by first several strongest diffusion maxima. These spheres carry the most reliable information on the structure. It is shown that the amorphous sediment consists of the randomly packed Nb(O,F)₆-octahedra with interatomic distances close to those observed in the ordered region in the N-Nb₂O₅ structure. During decomposition in water, the initial K_{2 ? x} Nb₄O₃(O, F)₃F phase loses KF-layers, whereas Nb₆ clusters with metal bonds are destroyed.     

Structure of Fe–Ni and Fe–Ni–S molten alloys by neutron diffraction

The static S(Q) liquid structure factors for binary Fe_xNi_100−x, x = 90 and 85 and ternary Fe₈₅Ni₅S₁₀ molten alloys are investigated by means of the concurrent use of neutron diffraction and Reverse Monte Carlo simulations. The measured g(r) radial distributions reveal atomic orderings varying from that present in the Fe₉₀Ni₁₀ alloy, reminiscent of the ideal b.c.c. structure of solid Fe, to a far more open structure found in Fe₈₅Ni₅S₁₀. From data at hand no clear evidence of clustering of sulphur within the Fe–Ni matrix is found but, in contrast, its addition leads to significant changes in structural properties manifested by the loss of the intermediate range order.     

Partial correlations in Ni60Nb40 metallic glass

Combined X-ray and neutron diffraction experiments were performed on the Ni₆₀Nb₄₀ metallic glass samples prepared by rapid quenching from the melt with natural Ni^nat and isotope ⁵⁸Ni, respectively. The partial structure factors were separated for the three kinds of atomic pairs: NiNi, NiNb and NbNb. The partial distribution functions were calculated by means of Fourier transformation and the following atomic distances were obtained: $\text{r}{}_{\mathrm{<mtext>NiNi</mtext>}}\text{=2.52}\text{A}\text{?}\text{, r}{}_{\mathrm{<mtext>NiNb</mtext>}}\text{=2.72}\text{A}\text{?}$ and $\text{r}{}_{\mathrm{<mtext>NbNb</mtext>}}\text{=2.70}\text{A}\text{?}$. The values n_NiNi=7.3, n_NiNb=4.5, n_NbNi=6.8 and n_NbNb=5.4 were obtained for the number of nearest neighbours.     

Observation of the collective vibrational modes in Ni42Nb58 metallic glass

The dynamic structure factors, S(Q, E), for Ni₄₂Nb₅₈ metallic glass have been measured using neutron inelastic scattering on the IN4 spectrometer (ILL, Grenoble, France), over a range of momentum transfer, 0.7 < ℏQ/Å⁻¹ < 8, and energy transfer, 2 < E/meV < 50. Three samples with the same chemical composition and different Ni isotope content were investigated. The constant-E and constant-Q cuts through the S(Q, E) functions have been obtained. They reveal the E-and Q-dependencies due to the existence of the collective vibrational excitations in Ni₄₂Nb₅₈ glass. At the same time the difference of Ni and Nb atoms dynamics was observed by isotope contrast in the scattered spectra.     

Structure of potassium germanophosphate glasses by X-ray and neutron diffraction: 3. Reverse Monte Carlo simulations

Reverse Monte Carlo (RMC) is used to investigate the origin of the first sharp diffraction peaks (FSDP) found for K₂O–GeO₂–P₂O₅ glasses at very small scattering vector Q = ～7.5 nm^?1. Structures of the ternary glass with the greatest intensity of FSDP (KGeP5 – 25/50/25 mol% K₂O/GeO₂/P₂O₅), of the binary combinations of the three oxides and of vitreous GeO₂ are modeled. Results are deduced from comparisons of the partial structure factors and inspections of model sections. The P sites are uniformly distributed in the structure of KGeP5. The K⁺ ions interact more with the PO₄ units (via O_T-corners) than with Ge-centered units. Main component of the FSDP comes from the S_GeGe(Q) factor. The FSDP is due to separations of ～1 nm between the longish Ge-rich clusters which are visible in the corresponding models. Different to our tentative structural models reported before, the PO₄ tetrahedra possess a broad distribution of numbers of O_T corners. The FSDP’s of the binary K₂O–GeO₂ and K₂O–P₂O₅ glasses (～10 nm^?1) are due to a chemical order between network former and network modifier regions. The MRO of a mixed GeO₂–P₂O₅ glass of small P₂O₅ content (FSDP at ～16 nm^?1) shows great similarity to the MRO of vitreous GeO₂.     

Energetics of Cd-based binary liquid alloys

The thermodynamic properties such as, free energy of mixing (G_M), heat of mixing (H_M), entropy of mixing (S_M) and microscopic properties such as, concentration fluctuation in long wavelength limit (S_CC(0)), Warren–Cowely short range ordering parameter (α₁), ratio of mutual and intrinsic diffusion coefficients (D_M/D_id) of two Cd-based liquid alloys have been studied by using two different models. The properties of Cd–Hg liquid alloy at 600 K have been studied on the basis of regular solution model, and the properties of Cd–Na liquid alloy at 673 K have been studied on the basis of quasi-chemical expression for weakly interacting system.     

Diffusion Paths for Interstitial Impurities in Different Polymorphic Modifications of Niobium Silicide Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

Structural models of the α, β, and γ modifications of Nb₅Si₃ silicides, which are used as a reinforcing phase in composites obtained in situ based on the Nb?Si system, have been constructed by computer simulation methods. A geometric analysis of unit cells is performed using the H-poisk program to estimate the voids existing in the structures. The results of measuring the number of voids and their sizes are reported. A conclusion about possible diffusion paths of interstitial boron, carbon, nitrogen and oxygen atoms is drawn based on the calculation results, and the solubility of these impurities in the structure of each Nb₅Si₃ modification is estimated.     

Structural variation of expanded fluid mercury during M–NM transition: A Reverse Monte Carlo study

The structure of fluid Hg during the metal–non-metal transition has been modelled by Reverse Monte Carlo simulation in terms of the structural factors S(Q) measured by X-ray diffraction at SPring-8. The full region of S(Q) including S(0) was employed in the simulation so as to take the effect of fluctuation phenomenon into account. The modelling reveals that as the M–NM transition is approached, the distribution of coordination numbers, P(N), of the first-neighbor shell broadens obviously and shifts to lower values, supporting the speculation of Franz’s model. Furthermore, the coordination number P_S(N), denoted for atoms with a shorter bond, changes markedly in width and average value, while much smaller is found for that of longer bond as the density decreases. The variation of P_S(N) of the atoms with the shorter bond are directly related to the M–NM transition of fluid Hg.     

Structural analysis of Na2OGa2O3SiO2 glasses by the X-ray diffraction method

The structure of Na₂OGa₂O₃SiO₂ glasses of four different compositions containing up to 45 mol. % Ga₂O₃ has been determined by the X-ray diffraction method. The radial distribution function D(r) obtained indicates that Ga³⁺ ions are in fourfold coordination and GaO₄ tetrahedra are formed in these glasses. The reduced intensity function S·i(S) and radial distribution function D(r) which were calculated based on a structural model for A-25, 0.25(Ga₂O₃)·0.75(Na₂O·2SiO₂)/3 glass agreed well with the observed ones.     

Synthesis and crystal structure of low ferrialuminosilicate sanidine

Iron-containing potassium feldspar crystals are prepared using the hydrothermal synthesis in an alkaline medium at temperatures ranging from 500 to 52°C. The crystal structure of the synthetic potassium feldspar is refined [Ital Structures diffractometer, MoKαradiation, 1327 unique reflections with Fs>σ(F), anisotropic approximation, R(F) = 0.044]. It is established that, under the given preparation conditions, the synthesis leads to the formation of the monoclinic modification with the following unit-cell parameters: a = 8.655(7) Å, b = 13.101(9) Å, c = 7.250(8) Å, β = 116.02(2)°, space group C2/m, and Z = 4. The cation distribution over crystallographically inequivalent tetrahedral positions T(1) and T(2) is determined and justified using X-ray diffraction data. According to this distribution, the iron-containing potassium feldspar is assigned to the low ferrialuminosilicate sanidine. The proposed structural formula K _{A = 0.99}(Si_1.2Fe_0.5Al_0.3)_{ΣT(1) = 2}(Si_1.81Al_0.19)_{ΣT(2) = 2}O₈ agrees well with the data of the electron microprobe analysis. It is revealed that iron occupies the T(1) position and manifests itself as a majority rather than minority impurity element with respect to aluminum.     

Stochastic theory of scattering from idealized spinodal structures: II. Scattering in general and for the basic late stage model

Certain model late stage spinodal structures can be generated mathematically by the relation ΔC_L(r) = G[ΔC₀9r], where ΔC_L(r) is the composition deviation from the mean of the late stage structure and ΔC₀(r) is a random superposition of waves. Then the autocorrelation function (ACF) of ΔC_L(r) depends only on the ACF of ΔC₀)r), and the latter is related simply to the distribution of wavelengths present. Coherent scattering (light, small-angle X-ray, etc.) can be computed from the ACF. The “basic late stage model” (BLSM) is defined by G(ΔC) = 2H(ΔC)?1, where H(x) is the Heaviside step function. This converts the smooth, wavy ΔC₀(r) into a late stage structure having the same morphology but equilibrium compositions and sharp interfaces.The scattering theory formalism is developed generally and then applied to the BLSM using illustrative wavelength distributions. Scattering as a function of angle and wavelength is computed. It is also found that the BLSM turbidity $\text{≌ Ck}{}^4\text{w}{}^3$, where $\text{w}$ is the mean phase width and C depends only weakly upon the wavelength distribution of ΔC₀(r).