X-ray diffraction study of amorphous alloys Al–Ni–Ce–Sc with using Ehrenfest’s formula

The effect of replacement of Ce by Sc in the amorphous Al₈₅Ni₁₀Ce₅ alloy on its structure has been studied. The replacement was shown to result in increasing the coordination number in the first coordination sphere, changing the type of short-range order. A splitting the first peak of the function of radial atom distribution into two subpeaks made it possible to determine separately the coordination number for the atom pair groups Ce–Al, Sc–Al, Al–Al and Ni–Al, Al–Al. The structure of the amorphous alloys Al₈₅Ni₁₀Ce_5−xSc_x (x = 0, 1, 5) was determined using X-ray diffractometry combined with application of Ehrenfest’s formula, which allowed us to directly determine the first coordination sphere radius from data on the structure factor and some cluster parameters obtained from the prepeak on the structure factor curve.     

Theoretical calculation of dense random packings of equal and non-equal sized hard spheres applications to amorphous metallic alloys

A model describing the structure of amorphous metallic alloys is proposed using a packing of non-equal-sized hard spheres. Models containing up to 1000 spheres were generated by computation. Hypotheses guiding the calculation are chosen in order to obtain a model as compact as possible and to take into account affinity between metal and metalloids: therefore two small spheres are not allowed to be in hard contact. This method of calculation is first tested by building up heaps of equal-sized spheres. Both the radial distribution function and the interference function are calculated for each model and the variations of these curves with the number of spheres are studied. In the case of two sizes of spheres heaps calculations are devoted to the study of the so-called ‘shoulder interference function’ which characterizes many amorphous alloys such as NiP, PdSi, etc. It is shown that the existence of such a shoulder depends on two parameters: the ratio of sphere diameters and the relative concentration of small and large spheres. These results are in good agreement with some recent experimental observations. This shoulder is well-marked when the diameter ratio reaches 10% and when the concentration in small spheres lies in the range 10–15%. In such a structure (e.g. amorphous NiP and PdSi alloys) it is observed that small spheres are always surrounded by nine large spheres such as P or Si atoms in crystalline Ni₃P or Pd₃Si. Large spheres, i.e. metallic atoms, form distorted icosahedra, the distortion of which varies from one to the other. The short-range order has a five-fold symmetry which is not compatible with any long-range order.     

Structural properties and evolution in the solidification process of Cu–Ag alloys

The atomic structures of liquid Cu–Ag alloys (Cu₈₀Ag₂₀, Cu₆₀Ag₄₀, Cu₄₀Ag₆₀ and Cu₂₀Ag₈₀) have been investigated in the solidification process by means of X-ray diffraction. The results show that the liquid structures of Cu–Ag alloys, which are micro-homogeneous and similar with their solid state, display quite stable changes on a large-scale range indicated by the correlation radius r_c, although the short-range order structures change obviously around their liquidus (liquid–solid transformation). This is different from the results of the In₃₀Sn₇₀ alloy we previously reported, which shows an abrupt structure changes around its liquidus. In addition, the Gaussian peaks decomposition of the radial distribution functions (RDF(r)) was also performed to shed light on the element distribution and structure evolution in the whole solidification process especially around the liquidus.     

Structure of liquid Al-Cu-Co alloys near the quasicrystal-forming range

A local short-to-intermediate range order in liquid Al_63.9Cu_19.4Co_16.7, Al₇₁Cu₆Co₂₃, and Al_6oCu₂₉Co₁₁ alloys was investigated by X-ray diffraction technique and the reverse Monte Carlo modeling. A prepeak at Q ~ 17 nm^− 1originating from the unique bonding between the TM-TM pair (TM = Co, Cu) is observed in the structure factors of all investigated melts. The Voronoi-Delaunay analysis of RMC models indicates that a medium-range ordering of TM atoms in dense non-crystalline polytetrahedral clusters is associated with a chemical short-range order. The icosahedral short-range order is also closely related to the dense packing polytetrahedral clusters. A decrease of temperature leads to an enhancement of both chemical short-range order and icosahedral short-range order.     

Role of concentration fluctuations in metallic glass formation

The glass forming composition range in several binary metallic systems is correlated with the composition dependence of S_cc(0), the concentration-concentration fluctuation structure factor at its long wavelength limit. The magnitude of S_cc(0) has been evaluated for the liquid phase from available thermodynamic data. It has been observed that S_cc(0) exhibits a maximum and tends to the ideal value in the glass forming composition range. Significant and systematic negative deviations from ideal values or the tending to zero of S_cc(0) are observed at the stoichiometry corresponding to complexes in the liquid state in the case of compound forming or associated systems. These observations are discussed in terms of the chemical short-range order in the liquid state. It is concluded that while a reasonable degree of order may exist for the compound forming compositions, in the glass forming region itself the liquids are nearly random mixtures of complexes and unassociated component atoms.     

Analysis of diffuse background on the X-ray diffraction pattern of fullerite C<Subscript>60</Subscript>

Diffuse scattering on the X-ray diffraction pattern of fullerite C₆₀ is analyzed. The characteristics of the short-range order of the amorphous component of fullerite are determined by the Finbak-Warren method. It is established that the short-range order of the amorphous component is similar to the short-range order of lonsdaleite.     

Short-Range Order in FeCrNi Alloys

The short-range atomie order in Fe_82−xCr₁₈Ni_−x (x = 15, 20, and 25 wt%) alloy single crystals has been studied by the x-ray diffuse scattering technique. Diffuse ring around the central node was found for all alloys. Analysis of the intensity distribution allowed to determine the short-range order partial parameters. Correlations in atom arrangements in the regions of about 10–12 Ångstrom in diameter were found to be characteristic for the alloys studied. It is assumed, that regions of the structural short-range order become magnetic clusters at the sufficiently low temperatures.     

Amorphous vanadium pentoxide

Structure and semiconducting properties of amorphous vanadium pentoxide obtained by splat cooling have been studied. Long-range order appears suddenly when the oxide is heated to about 200°C. The short-range order of the glass seems to be almost identical to that encountered in orthorhombic V₂O₅. ESR spectra are typical of localized electrons, they show that the electronic mobility is lower in the amorphous state. The hopping frequency of the unpaired 3d electron from a V⁴⁺ to a V⁵⁺ ion increases when long-range order is established. This phenomenon is related to the electrical resistivity which is about one order of magnitude higher in the glass.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Simulation of ion transport in layered cuprates La<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4 − <Emphasis Type="Italic">δ</Emphasis></Subscript>

The processes of oxygen diffusion in La_{2 ? x} Sr _x CuO_{4 ? δ} phases have been simulated for the first time by the molecular-dynamics method. Calculations were performed for the temperature range 300–2500 K. The behavior of the radial pair correlation functions, which characterize the degree of order of O1 ions in CuO₂ layers, indicates that O^2? anions form a weakly correlated subsystem within a CuO₂ layer. To quantitatively confirm the conclusions about the predominantly two-dimensional character of ion transport and different mobilities of O1 and O2 particles in the cuprates under study, the pair oxygen diffusion coefficients in the La_{2 ? x} Sr _x CuO_{4 ? δ} lattice were calculated. It is shown that oxygen diffusion occurs through the conventional hopping mechanism mainly in CuO₂ layers; correspondingly, the diffusion coefficient for equatorial ions (O1) exceeds that for apical oxygen anions (O2) by an order of magnitude. The motion of oxygen anions was traced at the microscopic level through analysis of the particle transport trajectories. It has been proven for the first time that diffusion of O1 ions in the ab plane in a nonstoichiometric LaSrCuO_3.61 sample occurs through jumps to the nearest position or along CuO₂ layers; in a more complicated way, it may occur through unoccupied O2 lattice sites.     

Medium range order in sodium silicate glasses: role of the network modifier

The effect of different concentrations of the network modifier Na₂O on the position of the Boson peak, observed in the low frequency Raman scattering experiments, is analyzed. Glasses containing 25% Na₂O (Na₂O · 4SiO₂) and 33% Na₂O (Na₂O · 2SiO₂) are compared with pure SiO₂ glass. The observed shift of the Boson peak is interpreted in terms of a change of a correlation length, ℓ, corresponding to some medium range order. This length, ℓ, decreases when the amount of sodium oxide increases.     

Short-range order and atomic clusters in liquid Cu–Sn alloys

We report an investigation of the short-range order of liquid Cu–Sn alloys carried out by means of X-ray and neutron diffraction. The diffracted X-ray intensity was evaluated with a model assuming micro-inhomogeneities in the material. Furthermore, the reverse Monte Carlo modelling technique was applied to derive partial correlation functions. The alloys under investigation are found to be micro-heterogeneous with clusters of Cu₃Sn stoichiometry distributed among the remaining Sn atoms.     

Photochromic behavior of Li-stabilized MoO3 sol–gels

Photochromic behavior of Li-doped MoO₃ sol–gels prepared by the peroxo sol–gel method was studied in details using various experimental techniques, including UV–vis, XRD, Raman, EPR, and XPS. The lithium doping has drastically enhanced the stability of the MoO₃ sol–gels. Upon UV-light irradiation from a high-pressure mercury lamp, the Li-doped MoO₃ sol turned from yellowish into deep blue, and the sol in ethanol exhibited much more intense color change than the sol in water. The UV-light exposure has the effect of re-arranging the structural units of MoO₃ and building up short-range order inside the solid. The formation of a blue colored bronze in the photo-irradiated sols because of the reduction of Mo⁶⁺ to Mo⁵⁺ is also evidenced by the experimental data. Based on these data, a mechanism for the photochromic behavior is proposed.     

Structure and dynamics of levitated liquid aluminates

We used the aerodynamic levitation technique combined with CO₂ laser heating to study the structures of liquid CaAl₂O₄ and MgAl₂O₄ above their melting point with X-ray and neutron diffraction. We determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. The combination of the two scattering techniques makes it possible to derive information not accessible with a single measurement. We also obtained information on the dynamics of liquid MgAl₂O₄ with inelastic X-ray scattering. In particular, we determined the sound velocity and the longitudinal viscosity.     

Structural aspects of volume nucleation in silicate glasses

An interrelationship between parameters of short and intermediate range order in silicate glasses and the tendency to nucleate homogeneously in the volume is tested. Changes in the average coordination number and metal-oxygen distance of network modifying cations as well as changes in the concentration of constitutive silica tetrahedra accompanied with the crystallization of 18 stoichiometric glass compositions into their crystalline analogs are determined. The intermediate range structure of the glasses is investigated by configurational entropy and flow birefringence. The changes in structural parameters are analyzed in terms of the reduced glass transition temperature T_rg, which is negatively correlated with the maximal rate of volume nucleation. The results indicate that the short-range structure in stoichiometric glasses is, in general, very similar to the corresponding crystal structure but independent of the T_rg-scale and for this reason independent of nucleation properties. In contrast to the short range of the glass structure, birefringence induced by a forced flow above the glass transition temperature and configurational entropy are positively correlated with increasing T_rg. The results indicate increased structural order in the intermediate range for melts with a high supercool limit (T_rg < 0.58). It is concluded that this order phenomena may promote nucleation events and may help to explain the tendency to volume nucleation of silicate glasses with T_rg < 0.58.     

Structure and dynamics of liquid Al1−xSix alloys by ab initio molecular dynamics simulations

First-principles molecular dynamics (MD) simulations are performed to study the structure and dynamics of liquid Al_1−xSi_x (x = 0.0, 0.12, 0.2, 0.4, 0.6, 0.8) at the temperature of 1573 K. The composition dependence of static structure factors, pair correlation functions, and diffusion constants are investigated. We found that the structure of the liquid Al_1−xSi_x alloys is strongly dependent on the composition. From our simulation and analysis, we can see that although liquid Al_1−xSi_x is metallic, there are some degrees of covalent tetrahedral short-range order in the liquid. The degree of tetrahedral short-range order increases linearly as the Si concentration in the liquid increased. The diffusion coefficients of both Al and Si atoms in liquid Al_1−xSi_x alloys at 1573 K are not very sensitive to the composition.     

Change of short-range order with temperature and composition in liquid GexSe1−x as shown by density measurements

Density measurements were performed on melts of the binary chalcogenide system Ge_xSe_1?x (0 ? x ? 0.5) up to 1000°C. The isotherm of molar volumes V_m at 750°C shows a relative maximum near GeSe₂. Molar volumes of the system behave linearly between GeSe and Se at 1000°C. V_m's of melts between x = 0.30 and x = 0.35 decrease at high temperatures on heating. The anomalous density behaviour of the melts clearly shows a change of short-range order from a less to a more densely packed structure, caused by the development of a pσ-bonding system. Within the composition range $\text{0 ? x ?}\text{1}\text{3}$ the short-range order at lower temperatures is determined mainly by $\text{GeSe}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}$ tetrahedra linked directly corner-to-corner or via Se atoms. At higher temperatures pσ bonds arise more and more, even in melts rich in selenium. Within the composition range $\text{1}\text{3}\text{? x ? 0.5}$ the short-range order is mainly determined by a distorted octahedral configuration, even at lower temperatures. From the short-range orders of melts and from crystalline structures of GeSe₂ and GeSe, the tendency of glass formation from the melt is discussed in detail.     

Molecular Ordering in Liquid Crystals and the Effect of End-Chains on the Even-Odd Effect

The second and fourth orientational order parameters [Pbar]₂, [Pbar]₄ of the homologous series of 4-n-alkyl-4′-cyanobiphenyls (nCB, n = 5 ～ 8) in the nematic liquid crystalline state have been determined from the Raman depolarization ratios by the resonance Raman probe method. It was found out that not only [Pbar]₂ but also [Pbar]₄ exhibits evident even-odd effect when they are compared at the same reduced temperature. For understanding the mechanism of the even-odd effect, a theoretical calculation has been carried out on the basis of the mean field theory of Marcelja, in which the effect of the end alkyl-chain on the liquid crystalline state is taken explicitly into consideration. The observed trend in [Pbar]₂, [Pbar]₄, nematic-isotropic transition temperature were well reproduced. Detailed inspection of the results shows that the presence of anisotropic molecular field plays an important role in the appearance of the even-odd effect. The anisotropic molecular field produces redistribution of the statistical weight of various conformers in such a way that the end chains of odd-members align better along the direction of the rigid core part than those of even-members.     

Structural study of levitated liquid Y2O3 using neutron scattering

The local structure of liquid Y₂O₃ at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.     

Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.