May 1, 2007

Update on the X16C Powder Diffraction Beamline

Penn State students Nadia Martinez-Villegas and Ekaterina Bazilevskaya aligning a sample onto the X16C diffractometer. They are using powder x-ray diffraction to study the kinetics of solid-phase precipitation of iron and aluminum oxides.

Beamline X16C has operated for two years as a Participating Research Team emphasizing high-resolution powder diffraction. That program originated at X3, when it was a multipurpose beamline operated by the State University of New York, but moved in order to make room for development of the X9 undulator beamline for small angle x-ray scattering. X16C was originally built by Bell Labs for x-ray spectroscopy, and was refurbished by the NSLS to receive the scientific program from X3.

X16C is equipped with a flat, channel-cut monochromator and no mirror. In its usual operating mode, the diffractometer is equipped with a crystal analyzer, so the intrinsic diffraction profile is a few thousandths of a degree wide. Consequently, the flux on the sample and the total count rate are rather limited, but diffraction lineshapes can be modeled accurately. This makes the beamline optimal for high-resolution powder diffraction studies of samples of at least a few milligrams.

One of the major uses of X16C is for ab initio determination of crystal structures from powder diffraction data. Crystallography is currently a routine procedure for small molecules, generally up to about 100 atoms in the irreducible cell, as long as one has suitable single crystals of the sample. However, many interesting materials are available only as powders, and the determination of their structure from x-ray diffraction data is much more challenging. This is because different diffraction peaks overlap, and so it is not possible to measure intensities of a large number of independent reflections. Two factors contribute to overcoming this difficulty: availability of high-resolution data to maximize the information content of a powder diffraction pattern, and data analysis tools that can deal with the imperfect information from such a data set. These techniques are applicable to problems in solid-state chemistry and physics, catalysis, pharmaceutical sciences, and many other areas of materials science.

An example of the work performed at beamline X16C is shown in Figure 1, which is a plot of a portion of the powder x-ray diffraction pattern of a magnesium boron hydride, a promising hydrogen storage material, synthesized by a team from GE Global Research. Based on this powder diffraction pattern, Jae-Hyuk Her and Peter Stephens of Stony Brook University solved the crystal structure, which is shown in Figure 2. This work has been submitted for publication, where all details will be presented (J.-H. Her, P. Stephens, Y. Gao, G. Soloveichik, J. Rijssenbeek, M. Andrus, and J.-C. Zhao, submitted to Acta Cryst. B).

Figure 1. A small portion of the powder x-ray diffraction pattern of a magnesium boron hydride, collected at beamline X16C.

Figure 2. Illustration of the crystal structure of magnesium boron hydride, determined from the powder diffraction pattern of Figure 1.

ARTICLE BY: Peter Stephens