2006 Dickson, Jared

							                                Dickson, Jared
          X-ray Powder-diffraction Simulations of Large-scale
                      Defect-structure Models

                 Faculty Mentor: Dr. Branton Campbell, Physics and Astronomy


Open-framework compounds, most notably the aluminosilicate zeolites, play a critical role in
modern technology. Their robust crystalline structures contain large cavities and channels of
molecular dimensions which make them useful as molecular sieves (desiccants, membrane
filters, gas-separators), ion-exchangers (soaps, detergents, water softeners, radioactive waste
sequestering agents), chemical catalysts (e.g. petroleum refinement), and nanomaterial growth
templates. The many uses of open-framework materials arise from having size and shape-
selective pore systems that determine which molecules can enter and what happens to them while
inside. Once their structure-property relationships are well characterized, researchers commonly
tune the useful properties of open-framework materials by modifying their atomic structures in a
controlled way.


Single-crystal x-ray diffuse scattering has recently been used to characterize complex nanoscale
defect structures in zeolite mordenite (B.J. Campbell et al., J. Appl. Cryst. 37, 187-192, 2004),
which may prove to be a useful means of modifying their adsorptive and catalytic properties.
However, most industrial and academic researchers interested in exploring these novel defect
structures do not have ready access to sophisticated diffuse scattering tools.


My research efforts were aimed at simulating the effects of these newly-discovered defect
structures in common x-ray powder diffraction (PXD) data so that standard laboratory techniques
can be used to identify and quantify the framework defects in their materials. The ability to
generate powder patterns from defect-structure models is a new feature of the DISCUS software
package (Th. Proffen and R.B. Neder, J. Appl. Cryst. 32, 838, 1999). After receiving a beta
version of this feature from Professor Reinhard Neder at the Institut fuer Mineralogie at the
University of Wurzberg in Germany and verified that it works on small-scale models, we
developed an algorithm that employs both the Discus and Mathematica software packages to
create large-scale three-dimensional defect models models in mordenite. Because this effort
required the resources of the Marylou supercomputers provided by the BYU College of
Engineering, we compiled and configured DISCUS to work on maryloux.


To achieve sufficient reciprocal-space resolution to observe defect-induced broadening in
mordenite, spherical models containing about several million atoms were needed. Using the
standard Fourier Transform option provided by DISCUS, we found that this would take about
four months on a single maryloux node. Prof. Neder then pointed out that the columnar shape of
the mordenite defects should permit us to greatly simplify the calculations by taking advantage
of the translational periodicity parallel to the defect columns. This reduces the model size to
about 250,000 atoms, so that simulations can be completed in under an hour. However, this
clever "trick" employs an alternative routine that has not been fully coded yet to allow for
sufficient flexibility in the output -- an issue that requires modifications to the freely-available
DISCUS source code. Further work should be aimed at completing these modifications and
completing a family of simulations with varying defect concentrations.


The opportunities provided by this research were very rich and rewarding. I especially enjoyed
the ability to solve problems that were not in a textbook and as such had been solved countless
times before. Our research has not yet produced our desired results, we have eliminated one
pathway to solving our problem and our other method looks extremely promising with more
development. With continued work I am excited to continue the research to completion, as the
results will have an actual use and will not just be a report on a shelf.