A. Bytchkov

X-ray transfocators: focusing devices based on compound refractive lenses.

A tunable X-ray focusing and/or monochromating device, called a transfocator, is described. Examples of its implementation on ID11 at the ESRF are given.

Levitation apparatus for neutron diffraction investigations on high temperature liquids

We describe a new high temperature environment based on aerodynamic levitation and laser heating designed for neutron scattering experiments up to 3000°C. The sample is heated to the desired temperature with three CO2 lasers from different directions in order to obtain a homogeneous temperature distribution. The apparent temperature of the sample is measured with an optical pyrometer, and two video cameras are employed to monitor the sample behavior during heating. The levitation setup is enclosed in a vacuum-tight chamber, enabling a high degree of gas purity and a reproducible sample environment for structural investigations on both oxide and metallic melts. High-quality neutron diffraction data have been obtained on liquid Y3Al5O12 and ZrNi alloy for relatively short counting times (1.5h).

The structure of liquid calcium aluminates as investigated using neutron and high energy x-ray diffraction in combination with molecular dynamics simulation methods

The structure of laser heated aerodynamically levitated (CaO)(x)(Al₂O₃)(1-x) high temperature liquids, with x = 0.33, 0.5, 0.75, was measured by using neutron and high energy x-ray diffraction. The partial structure factors for the liquids at 2500 K were also determined using molecular dynamics computer simulations. The simulation results are in excellent agreement with the diffraction measurements. The results show a predominant tetrahedral Al coordination with approximately 20% of fivefold coordinated Al at x = 0.33 which reduces with increasing CaO concentration. The Ca atoms occupy a broad range of coordination environments but with a predominance of sixfold distorted octahedra. The simulations confirm the presence of 13, 7 and 0.6% OAl₃ triclusters connecting AlO₄ tetrahedra in the structure of CA2 (x = 0.33), CA (x = 0.5) and C3A (x = 0.75) liquids, respectively.

Development of structural order during supercooling of a fragile oxide melt.

The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.

Unraveling the atomic structure of Ge-rich sulfide glasses

In contrast to the well-established structure of glassy GeS2, consisting of corner- and edge-sharing GeS(4/2) tetrahedra, the structural features of Ge-rich sulfide alloys remain essentially unknown. Two contrasting points of view: (1) a tetrahedral model, and (2) a distorted NaCl approach were neither confirmed nor excluded mostly because of missing advanced structural studies. Using high-energy X-ray scattering and neutron diffraction, we show the complexity of the short and intermediate range order in Ge(x)S(1-x) glasses, ⅓ ≤ x ≤ 0.47, formed by corner- and edge-sharing tetrahedra with two-fold coordinated sulfur species and a variable number of Ge-Ge bonds, and Ge-S units with three-fold coordinated sulfur at x ≥ 0.36.

31P NMR studies of short –range order in phosphorus-selenium glasses

The local structure of P-rich and Se-rich phosphorus-selenium glasses was studied using high-resolution (31)P solid-state MAS NMR. Two-dimensional (31)P homonuclear through-bond correlation MAS experiments and 2D homonuclear J-resolved MAS measurements were performed at high spinning frequency to probe P-P and P-Se-P connectivities between the different P sites for the compounds in two glass-forming regions, P(2.5)Se(97.5)-P(50)Se(50) and P(67)Se(33)-P(84)Se(16). Amorphous phosphorus and crystalline alpha-P(4)Se(3) and beta-P(4)Se(3) were also studied as reference materials. Glasses from the Se-rich region contain mainly three- and four-coordinated P sites linked together by Se(n) chains, whereas P-rich glasses contain a mixture of P(4)Se(3) molecular units and possibly other structural units embedded in a red-phosphorus-like polymeric network.

Longitudinal excitations in Mg–Al–O refractory oxide melts studied by inelastic x-ray scattering

The dynamic structure factor S(Q,omega) of the refractory oxide melts MgAl2O4 and MgAl4O7 is studied by inelastic x-ray scattering with aerodynamic levitation and laser heating. This technique allows the authors to measure simultaneously the elastic response and transport properties of melts under extreme temperatures. Over the wave vector Q range of 1-8 nm-1 the data can be fitted with a generalized hydrodynamic model that incorporates a slow component described by a single relaxation time and an effectively instantaneous fast component. Their study provides estimates of high-frequency sound velocities and viscosities of the Mg-Al-O melts. In contrast to liquid metals, the dispersion of the high-frequency sound mode is found to be linear, and the generalized viscosity to be Q independent. Both experiment and simulation show a weak viscosity maximum around the MgAl4O7 composition.

Structure and dynamics of levitated liquid materials

Aerodynamic levitation is a simple way to suspend samples which can be heated with CO2 lasers. The advantages of this technique are the simplicity and compactness of the device, making it possible to integrate the device easily into different kinds of experiments. In addition, all types of sample can be used, including metals and oxides. The integration of this technique at synchrotron and neutron sources provides powerful tools to study molten materials.

Versatile Levitation Facility for Structural Investigations of Liquid Metals

A versatile levitation facility for structural investigations of liquid metals was developed. It consists of a vacuum-tight process chamber containing a nozzle for aerodynamic levitation and a coil for inductive heating. The chamber is designed for structural investigations by x-ray diffraction and by x-ray absorption. The levitation facility has been used on beamline ID15 at ESRF in Grenoble. High Energy X-Ray diffraction measurements of liquid Al-based alloys using an image plate detector were performed. This paper discusses the experimental set-up for diffraction measurements, and presents first results obtained on liquid Al25at%Ni. The short range order was studied as a function of temperature and composition. Nearest neighbour distances as well as coordination numbers were determined. A pronounced prepeak in the scattering function S(Q) was found.

Structural properties of molten dilute aluminium–transition metal alloys

The short-range order in liquid binary Al-rich alloys (Al-Fe, Al-Ti) was studied by x-ray diffraction. The measurements were performed using a novel containerless technique which combines aerodynamic levitation with inductive heating. The average structure factors, S(Q), have been determined for various temperatures and compositions in the stable liquid state. From S(Q), the pair correlation functions, g(r), have been calculated. The first interatomic distance is nearly temperature-independent, whereas the first-shell coordination number decreases with increasing temperature for all the alloys investigated. For the Al-Fe alloys, room-temperature scanning electron microscropy (SEM) studies show the formation of a microstructure, namely the existence of Al(13)Fe(4) inclusions in the Al matrix.