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Physical and Theoretical Chemistry
Disordered Materials
Professor Richard Welberry
http://rsc.anu.edu.au/research/welberry.php
This group combines diffuse X-ray
scattering methods with computer simulation to deduce the arrangement
of atoms and molecules in disordered crystals. Conventional crystal
structure determination reveals only averaged arrangements,
inadequate to explain some of the basic properties of many minerals,
inorganic compounds, organic compounds and alloys that exhibit
crystalline disorder. Diffuse scattering gives information on how
neighbouring atoms or molecules interact with each other.
Quantitative studies of diffuse scattering are, however, still rare
because of the intrinsically very low intensities involved.
The
group uses dedicated diffuse-scattering diffractometer systems based
on curved position-sensitive wire detectors. These allow high
quality diffuse scattering data to be efficiently recorded over large
regions of diffraction space and provide a unique facility for
tackling a whole range of complex structural problems.
The group's interests span a wide range of different fields, each
presenting problems for which this specialised technique can give
unique information. Areas in which we have applied the techniques
include: disordered molecular crystals, guest/host systems such
as urea inclusion compounds, non-stoichiometric inorganic materials
and minerals (for example, the cubic stabilised zirconias, mullite
and wüstite), flexible framework structures such as silica
polymorphs and their analogues, alloys, and quasicrystal phases.
Much of the current effort of the group is concerned with the further
development and exploitation of the least-squares method for directly
fitting a Monte Carlo (MC) simulation to observed X-ray diffraction
data, proposed for the first time in 1997. An ARC Discovery Grant bid
to support this work was successful in the 2002 round. A second ARC
Discovery Grant bid (jointly with the Withers Group) to support work
to investigate the rôle of strain in disordered systems was
also successful.
Refinement of Monte Carlo Models of Disordered Molecular Crystals
The automatic refinement method has been extended to investigations of
various derivatives of benzil (reported last year), which contain
additional static disorder and/or more internal degrees of
flexibility. The main emphasis of the work is to develop the
techniques whereby the diffuse scattering from a molecular crystal
can be measured and analysed on a routine basis. Such an analysis
provides information on how the molecules interact with each other
that is not available from conventional crystal structure
determinations. Studies have commenced during the year on three
benzil derivative compounds. These are 4,4'-dimethoxybenzil,
4,4'-dimethylbenzil and 4-Cl,4'-methylbenzil. The figure below shows
the quantitative agreement that has been obtained for observed and
calculated diffraction patterns of the 4,4'-dimethoxybenzil compound.
Further
work has been been carried out on the adduct of azelaic acid,
C9H16O4, and hexamethylenetetramine,
C6H12N4. The interest in this
compound relates to the fact that the orientational disorder in the
acid molecules appears to be transmitted via the intermediary HMT
molecules, which act like switches.
Work
has also continued on the optically anomalous material
1,5-dichloro-2,3-dinitrobenzene. The disorder in this material
represents a considerable step-up in complexity from previous studies
since it involves two types of static disorder. Not only do the
molecules crystallise in two basically different orientations with Cl
and NO2 interchanging position, but each of the NO2
groups is also orientationally disordered. As a result of the static
disorder, the whole molecule undergoes large local relaxational
displacements. The automatic refinement methodology has been an
invaluable tool in investigating this system. Although substantial
progress has been made in building a model of this disordered system,
a completely satisfactory model is still some way off. (with A.J.
Edwards,
D.J. Goossens, A.P. Heerdegen, and W.I.F. David [ISIS,
Oxon, UK], M. Gardon,
K. Schenk [U. Lausanne, Switzerland])
Observed and calculated diffraction patterns for the system
4,4'-dimethoxybenzil
'Size-effect'-like Distortions in Quasicrystalline Structures
We
have carried out a study in which we have shown that by applying
'size-effect' distortions to a perfect Penrose tiling, on the basis
that rhomb-edges which connect different types of vertices assume
different lengths, we can obtain a diffraction pattern which shows
remarkable similarity to the zero-level (h5=0) section of
the observed diffraction pattern of decagonal
Al71Co13Ni16
quasicrystals. In particular a central clearly-delineated decagon is
observed, on the inside of which there is reduced intensity, and on
the outside of which there is enhanced intensity. Such a transfer of
intensity is characteristic of size-effect distortions in crystals
but for these systems it is necessary to have disorder involving (at
least) two types of atoms. In the present case the effect is observed
with all vertices of the tiling pattern occupied by a single
scatterer and the system remains topologically equivalent to the
Penrose pattern with long range quasicrystallinity (see below). (with
M. Honal)
The distorted Penrose tiling pattern with its corresponding diffraction
pattern
High-pressure X-ray Scattering of
Na1/2Bi1/2TiO3
Many
of the outstanding properties in oxides are related to materials with
an intrinsic nano-scaled local structure, where the different regions
are characterized by competing chemical, structural and/or physical
properties. One of the major challenges
in the analysis of the nano-scaled oxides is experimental access to
the local properties, which is often at best a difficult task, a fact
that often inhibits the understanding of properties such as colossal
magneto-resistance, giant piezo-electricity and high-temperature
superconductivity. We have carried out an investigation of the
relaxor ferroelectric Na1/2Bi1/2TiO3,
considered as a model type of nano-structured oxide, by combining the
parameter high-pressure with X-ray diffuse scattering (XRDS). We show
that nano-scaled characteristics can be investigated to exceptional
detail by combining such measurements with simulation of different
diffuse scattering models. We observe two distinct structural
disorders, one of which is characterized by the observation of
asymmetric 'L-shaped' diffuse scattering features which are caused
by planar polar defects in the perovskite structure. (with J.
Kreisel, [Laboratoire Matériaux et Génie Physique, ENS
de Physique de Grenoble, France], P.A.Thomas, [Dept. Physics, U.
Warwick, UK], M.A. Glazer, [Clarendon Lab., U. Oxford, UK])
A model of the disordered structure of
Na1/2Bi1/2TiO3
and its calculated diffraction pattern
Oxygen/Fluorine Ordering and Diffuse Scattering in
K3MoO3F3
Bond valence sum calculations have been used to investigate the
crystal chemistry of the elpasolite-related oxyfluoride
K3MoO3F3
in order to obtain insight into the type/s of structural distortion
(away from an ideal, high symmetry, elpasolite type parent structure)
responsible for a characteristic, highly structured,
three-dimensional diffuse intensity distribution. The first required
type of local structural distortion corresponds to large amplitude
MoO3F3 octahedral rotations while the second is
associated with O/F ordering and associated induced Mo ion shifts.
Monte Carlo modelling has been used to show how the latter, when
coupled with an appropriate local crystal chemical constraint, can
give rise to the observed structured diffuse scattering. Examples of
different sections of the calculated diffraction pattern are shown
below. The study is part of a wider search for diffraction evidence
of oxygen/fluorine ordering in metal oxyfluoride systems. (with
R.L. Withers, L. Norén, F.J. Brink)
Three example reciprocal sections (100, 113, 331) of the
K3MoO3F3
system, calculated from a Monte Carlo model based on a simple
chemical constraint
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