X-Ray Powder Diffraction Beamline at CAMD
The Louisiana Board of Regents funded the proposal “Synchrotron Powder X-Ray Diffractometry” to CAMD in 2004 to build a powder diffraction beamline. The beamline produced its first diffraction pattern in June 2006. Though funding to build this powder diffraction facility came from the Board of Regents’ Geological and Environmental Sciences category, it can be used by researchers from many disciplines.
Powder X-ray diffractometry (PXRD) is a widely-used characterization tool. It is typically performed with a sealed tube X-ray source having a restricted energy (or wavelength) spectrum consisting of a few intense peaks (characteristic lines) superimposed on a low-intensity broad-continuum background. This type of source also has a high angular divergence limiting the angular resolution. A synchrotron, in contrast, produces very bright X-rays over a wide wavelength range with very low divergence. Thus many types of experiments can be done with a synchrotron powder X-ray diffractometer which cannot simply be replicated with a laboratory diffractometer.
The mature field of PXRD received a boost when it was coupled with synchrotron radiation in the 1980s. Many important problems in materials science, chemistry, and mineralogy such as crystal structure determination and refinement of commercially important synthetic zeolites, early crystallization processes in cements, precise location of trace elements in minerals, etc., have been solved with synchrotron powder X-ray diffraction.
The advantages of synchrotron X-ray powder diffraction over conventional powder diffraction are:
• A range of wavelengths over a wide energy range can be used; with the laboratory source, it is usually fixed
• Energy dispersive X-ray diffraction is possible
• High intensities
• Narrower peak width, thus reducing overlaps between peaks
• Easier modeling – peak shapes are simpler compared to laboratory source
• Resonant diffraction - using incident X-ray energy below and above the absorption edge of a particular element (resonant diffraction), its location in a crystal structure can be more precisely defined.
Figure 1 shows a photograph of the powder diffraction end station. The four-circle Huber diffractometer is in the center. For powder diffraction operation only two circles are being used. The beam is monitored and the diffraction pattern normalized by an ionization chamber on the left. The powder sample is placed in the middle on the holder which can be rotated ate various speeds. The solid state detector is on the right. The source and receiving slits are 300 and 200 μm, respectively. The radius of the goniometer circle is 17 cm.
The diffraction pattern of lanthanum hexaboride (LaB6), a National Institute of Science and Technology (NIST) standard commonly used to assess performance of powder diffractometers, is shown in Figure 2. The same standard, run at the Stanford Synchrotron Radiation Laboratory (SSRL) when it was a second generation source operated at 2.5 GeV, is also shown for comparison. The lower flux at the CAMD bending magnet produces lower peak to background ratio. The resolution is moderate.
Rietveld refinement of crystal structure, whole pattern fitting, and quantitative phase analysis using these techniques are commonly performed with powder diffraction data. The absence of overlapping Ka1 and Ka2 peaks and simple peak shapes make Rietveld refinement with synchrotron radiation simpler. Figure 3 shows Rietveld refinement for the same novaculite (obtained from The Gem Dugout) with GSAS. The synchrotron data show a lower residual error.