Chemistry Reference
In-Depth Information
new strategies and methodologies for carrying out structure determination directly
from powder XRD data, and as many important materials can be prepared only as
microcrystalline powders, progress in advancing the scope and power of
methodologies in this field has the potential to make considerable impact in several
areas of structural sciences.
Although single-crystal XRD and powder XRD patterns contain essentially the
same information, the diffraction data are distributed in three-dimensional space in
the single-crystal XRD pattern, whereas the diffraction data are “compressed” into
one dimension in the powder XRD pattern (Fig.
1
). As a consequence, there is
usually considerable overlap of peaks in the powder XRD pattern. Such peak
overlap obscures information on the positions and intensities of the diffraction
maxima, and the intrinsic difficulty of obtaining reliable information of this type
from the powder XRD pattern can impede (or in some cases prohibit) the process of
carrying out crystal structure determination. Molecular solids typically have large
unit cells and low symmetry, which result in a high density of peaks in the powder
XRD pattern. Thus, the problem of peak overlap is often particularly severe for
such materials, and the task of carrying out structure determination from powder
XRD data is rendered more challenging. Nevertheless, in the case of organic
Measure 3-Dimensional
Diffraction Data
Single-Crystal X-ray Diffraction
X-ray beam
Single Crystal Sample
Detector
Measure 1-Dimensional
Diffraction Data
(Peak Overlap)
Powder X-ray Diffraction
X-ray beam
θ
Detector
2
Powder Sample
(Randomly Oriented Crystals)
2
θ
Fig. 1 Comparison of single-crystal and powder XRD measurements. In powder XRD, the
diffraction phenomenon for each individual crystallite in the powder is the same as the diffraction
phenomenon in single-crystal XRD. However, the powder comprises a large collection of
crystallites with (in principle) a random distribution of crystallite orientations, and as a conse-
quence, the three-dimensional diffraction data are effectively compressed into one dimension
(intensity vs diffraction angle 2
y
) in the powder XRD measurement
