A modified diamond anvil cell (DAC) is used for structural studies of amorphous materials at high pressures using a monochromatic synchrotron x-ray beam. The DAC modification includes (1) the use of x-ray transparent seats for a large angular opening for x-ray scattering, and (2) the introduction of a boron gasket insert to increase the sample thickness and to minimize the gasket-hole deformation. A procedure for absorption correction and background subtraction in DAC experiments is described, together with an optimization process for obtaining accurate data of the structure factor and the corresponding pair distribution function. Data for amorphous iron at 67 GPa are presented for demonstration. It is shown that quantitative structural data can be determined for amorphous materials at very high pressures using the DAC. The apparatus should be also useful for structural studies of liquids at high pressures.

Structural investigation of amorphous materials at high pressures using the diamond anvil cell

A new diffraction technique for combined angle- and energy-dispersive structural analysis and refinement (CAESAR), by collecting angle-dispersive data using a solid-state detector (SSD) and white synchrotron radiation, is introduced. By step scanning a well calibrated SSD over a limited 2θ range, a series of one-dimensional energy-dispersive data (intensity versus energy) are obtained as a function of 2θ. The entire intensity (Int) data set consists of several thousand channels covering a range of photon energies, E (up to ∼150 keV), at each of the ∼1000 2θ steps, forming a 2–4 mega-element two-dimensional array, Int(E, 2θ). These intensity data are then regrouped according to photon energies, which are defined in the multichannel SSD as individual channels, yielding a large number of intensity versus 2θ (angle-dispersive) data sets, Int(E = const., 2θ), each of which corresponds to a given photon energy or wavelength. The entire data set, selected subsets or composite scans can be used for multiple data set Rietveld refinement. Data collected both on α-Al2O3 (a NIST diffraction standard) at ambient conditions and on a mixture of MgO and Au at high pressure were analyzed using the Rietveld technique, with varying schemes of data treatment. Furthermore, it is demonstrated that data within certain energy bands (ΔE/E = ±10%) may be binned together to improve counting statistics in a composite angle-dispersive scan, even when collected with much coarser scan steps of 0.1 or 0.2°. This technique is useful for high-pressure as well as general purpose powder diffraction studies that have limited X-ray access to the sample using synchrotron radiation. Several advantages are discussed.

/pdf/a-new-technique-for-angle-dispersive-powder-diffraction-3fecilsrtz.pdf

A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiation.

As the degree of difficulty in synthesizing single crystals at high pressure and temperature increases, powder X-ray diffraction becomes a primary technique for determination of crystal structure, unit cell parameters as a function of pressure and temperature, and phase transformations. The use of intense synchrotron radiation has revolutionized high-pressure and high-temperature research. Increases in X-ray intensity by several orders of magnitude have enabled researchers to study very small samples and collect diffraction data in time scale on the order of a minute. Such advances have opened new possibilities for studying materials under extreme high pressure and temperature conditions and time-dependent kinetic problems.

Diamond-anvil cell and large-volume multi-anvil apparatus are the primary high-pressure tools used to simulate pressure-temperature conditions of Earth and planetary interiors and to study material properties and phase transformations at high pressure and temperature. The coupling of these techniques with synchrotron X radiation has made in situ diffraction measurements possible at simultaneous high pressure and temperature. Increasingly fast accumulation of in situ data has dramatically increased our knowledge of material behavior at high pressure and temperature. In this paper, we review high-pressure and high-temperature techniques used at synchrotron facilities. Each technique has its own advantages and disadvantages that determine its own unique capability and applications. The large-volume apparatus, capable of generating high temperatures (>2000°C) at moderate pressures ( 100 GPa) at moderate temperatures (<900°C), has been extensively used for in situ measurements of P-V-T equations of state and non-quenchable phase transitions in metals, oxides, and sulfides. We also review synchrotron powder X-ray diffraction techniques and optical arrangements used for data acquisition at high pressure and temperature. Although the energy-dispersive diffraction technique has been the …

High-Pressure and High-Temperature Powder Diffraction

Experimental model for alkalic chloride-rich liquids in the upper mantle

Molten NaCl was analyzed by high-pressure X-ray diffraction experiments using synchrotron radiation up to 5 GPa and 1600 degrees C along the melting curve. The interference function Qi(Q) and the correlation function g(r) were derived from the diffraction data. The first-neighbor distance r 1 is about 2.7 Aa and the second-neighbor distance appears around 1.4r 1 -1.5r 1 . The coordination number, CN, of the nearest neighbor ions increases with pressure from 3.5 at 0.1 MPa to 4.5 at 5 GPa. This is the evidence that the NaCl melt has a B1-like structure with large vacancies over this pressure range and becomes densified by an increase in CN as a result of second neighbor compaction.

/pdf/high-pressure-x-ray-diffraction-study-on-the-structure-of-33ntqrjrut.pdf

High-pressure X-ray diffraction study on the structure of NaCl melt using synchrotron radiation

Energy-dispersive X-ray diffraction experiments of molten KCl under high pressure have been carried out by using synchrotron radiation. The diffraction profiles of molten KCl were acquired just above the melting temperature of KCl up to 4 GPa. The reduced structure factor S(Q)'s for molten KCl do not show any change in their primary features, except for a gradual increase in the first peak intensity with increasing pressure. This implies that molten KCl does not show a first-order phase transition, such as the B1-B2 transition, found in solid KCl, but that the local structure in molten KCl must be changed by compression. According to a molecular-dynamics simulation, this change of S(Q) can be explained by a continuous increase in the coordination number of the nearest-neighbor ions in molten KCl with pressure.

Pressure-induced structure change of molten KCl

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Pressure-induced structure change of molten KCl