Chemistry Reference
In-Depth Information
carry out a Rietveld refinement calculation using the experimental powder XRD
data recorded for the synthesized material and using the crystal structure of the
known material as the initial structural model in the refinement. As discussed in
Sect.
2.7
, Rietveld refinement gives rigorous consideration to fitting the peak
profiles, the peak positions, the peak intensities (including refinement of parameters
describing the effects of preferred orientation), and the presence of any known
impurity phases (by including such materials as a “second phase” in the refine-
ment). This protocol allows the question of whether a synthesized material either
matches or does not match the structure of a known material to be answered with
confidence, and represents a substantially more rigorous approach than visual
comparison of powder XRD patterns.
6 Examples of Structure Determination from Powder
XRD Data
In this section we present illustrative applications of structure determination of
molecular materials directly from powder XRD data, illustrating the current scope
of methodologies in this field and highlighting some of the specific issues (and
challenges) discussed above.
For many molecular materials, crystallization from solution does not yield single
crystals of suitable size and quality for single-crystal XRD, and instead produces
only microcrystalline powders. In such cases, structure determination from powder
XRD data provides a viable route for establishing structural understanding of the
resultant materials.
Certain solid phases, on the other hand, cannot be obtained (even as microcrys-
talline powders) by crystallization experiments, but instead can be generated only
by other types of preparation procedure. Some types of preparation processes
commonly (or in some cases inherently) yield microcrystalline products, including
(1) preparation of materials directly from solid-state chemical reactions (see
Sect.
6.6
), (2) preparation of materials by solid-state desolvation processes (see
Sect.
6.4
), (3) preparation of materials by solid-state grinding (mechanochemical)
processes (see Sect.
6.2
), and (4) preparation of materials directly by rapid precipi-
tation from solution (as opposed to crystallization) (see Sect.
6.7
). Again, structure
determination from powder XRD data may represent the only opportunity for
determining the structural properties of new solid phases obtained by such
processes.
6.1 Structure Determination of Oligopeptides
A number of oligopeptide structures have been determined from powder XRD data
using the direct-space strategy for structure solution. The first example concerns
Phe-Gly-Gly-Phe [
100
], for which the structure solution calculation (employing
