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Powder x-ray diffraction

A 12-position autosampler is attached to this x-ray powder diffractometer. The round green disk in the center of the photo is a sample in position for diffraction. The diffractometer can also be fitted with an airtight environmental chamber (inset). X-rays enter and exit through a beryllium window at the top of the chamber. The environmental chamber can be removed and loaded in a glovebox or hood.

The powder diffraction capability in Los Alamos' Chemistry Division has two diffractometers, which are capable of running a wide variety of samples containing radioactive isotopes. This experimental technique can be used for phase identification of polycrystalline samples. In addition, a new capability is being developed to determine full crystal structures from high-quality polycrystalline samples. Thorium- and uranium-containing samples can be run with a polymeric containment procedure on a 12-position autosampling attachment. Transuranic samples can be run in an airtight environmental chamber, which protects workers and the environment from exposure.

The plot of an x-ray diffraction pattern of polycrystalline Cs2UO2Cl4 is shown in red. The molecular structure corresponding to the diffraction pattern is depicted in the background. Marianne Wilkerson of Los Alamos' Chemistry Division synthesized the sample.



Single-crystal x-ray diffraction

The single-crystal x-ray diffraction capability in Los Alamos' Chemistry Division is a world leader in crystal structure determination of small molecules containing actinide elements. A crystal structure is determined when a crystal is placed in an x-ray beam and the diffraction pattern is measured and analyzed, resulting in a three-dimensional picture of the molecules comprising the crystal. Researchers have determined 41 transuranic-containing structures in the past 9 years, including plutonium, neptunium, americium, and curium. In addition, more than 300 thorium- and uranium-containing crystal structures have been determined. These structures build on the results of Los Alamos scientist William Zachariasen, who in the 1950s and 1960s determined more than 180 actinide structures of extended solids and alloys. These molecular structures are the foundation for understanding materials and molecular properties and have appeared in scores of papers, including a recent issue of Nature (Nov. 21, 2002) and the cover of the International Edition of Angewandte Chemie (1997).

Crystallographer Brian Scott of the Chemistry Division adjusts a crystal on the single-crystal diffractometer.

To mitigate the associated health hazards of working with transuranic elements, researchers have developed a triple-containment technique for the crystals used in these experiments. The crystal is first coated in epoxy and placed on a glass fiber (left), the crystal and fiber are then placed in a capillary and sealed (middle), and the capillary is coated with acrylic nail polish (right). The acrylic coating keeps the capillary and contents in place in the event of breakage.

The molecular structure of NpO2(18-crown-6).

This close-up of a broken capillary containing a crystal shows how the triple-containment technique maintains the integrity of the sample and prevents sample loss in the event of breakage.


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