Chemistry Reference
In-Depth Information
4 Experimental Considerations
4.1 Synchrotron vs Laboratory Powder XRD Data
We now consider the relative merits of using synchrotron powder XRD data vs
conventional laboratory powder XRD data in structure determination from powder
XRD data, recognizing that the use of synchrotron radiation generally gives rise to
powder XRD data of higher resolution and improved signal/noise ratio. With high
resolution, problems due to peak overlap can be alleviated, at least to some extent,
thus increasing the reliability in determining accurate peak positions (which is
particularly advantageous for unit cell determination) and increasing the reliability
in extracting the intensities of individual diffraction maxima from the powder XRD
pattern. In this regard, synchrotron radiation can be particularly advantageous when
the traditional approach (or a direct-space technique that uses a figure-of-merit
based on extracted peak intensities) is to be used for structure solution. Thus, the
success of traditional techniques for structure solution is generally enhanced by
using data recorded on an instrument with the highest possible resolution. However,
for direct-space structure solution techniques that employ a figure-of-merit based
on a profile R-factor (such as
R
wp
), the most important requirement is not high
resolution itself, but rather that the peak profiles are well-defined and accurately
described by the peak shape and peak width functions used in the structure solution
calculation. In such cases, the use of laboratory powder XRD data can be just as
effective as the use of synchrotron data, and many examples (including the majority
of those presented in Sect.
6
) demonstrate that the use of a good-quality, well-
optimized laboratory powder X-ray diffractometer is usually perfectly adequate for
research in this field. Within the context of Rietveld refinement, we note that the use
of synchrotron data generally leads to structural results of greater accuracy, as a
consequence of the fact that the data in the high 2
region of the powder XRD
pattern are usually of higher quality for synchrotron data than laboratory data.
y
4.2 Preferred Orientation
In general, structure solution from powder XRD data has a good chance of success
only if the experimental powder XRD pattern contains reliable information on the
intrinsic relative intensities of the diffraction maxima, which requires that there is
no “preferred orientation” in the powder sample. Preferred orientation arises when
the crystallites in the powder sample have a nonrandom distribution of orientations,
and this effect can be particularly severe when the crystal morphology is strongly
anisotropic (e.g. long needles or flat plates). When a powder sample exhibits
preferred orientation, the measured relative peak intensities differ from the intrinsic
relative diffraction intensities, limiting the prospects for determining reliable struc-
tural information from the powder XRD pattern. In order to circumvent this
