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Inorganic Chemistry
Solid State Inorganic Chemistry
Dr Ray Withers
http://rsc.anu.edu.au/research/withers.php
This research group seeks to
understand and exploit the factors (strain, composition, electronic
band structure etc.) that determine structure and function in
the crystalline solid state. Our interest is in the balance between
local crystal chemistry and longer range order in a wide range of
compositionally and/or displacively flexible crystalline solids. The
principal experimental research tools used are synthesis,
transmission electron microscopy and scanning electron microscopy in
combination with powder and single crystal X-ray diffraction.
Systems investigated include wide range non-stoichiometric solid
solutions, displacively flexible framework structures, ferroic
phases, solid electrolytes, and incommensurately modulated
structures. Achievements over recent years include the first
coherent view of the crystal chemistry underlying the phenomenon of
ferroelectricity within the Aurivillius family of displacive
ferroelectrics, the use of compositely modulated structure formalism
to describe, refine, and interpret "infinitely adaptive",
non-stoichiometric solid solution fields, and the discovery and
subsequent modelling of displacive flexibility and its consequences
in a range of flexible framework structures.
Ordering Phenomena in Doped
Ni1+xMyTe2
(M = Ag, Cu, In)
A detailed TEM and XRD study has been carried out on B8 related ternary
solid solution phases in the
Ni1+xMyTe2
(M = Ag, Cu, In) systems. Despite previous reports of large B8
ternary compositional homogeneity ranges in the
Ni1+xMyTe2
(M = Ag, Cu) systems, no evidence for incorporation of Ag into
Ni1+xTe2 could be found while Cu could
be incorporated but only if the Cu content was at much lower
concentrations than previously suggested. In the case of M = In, two
roughly determined solid solution fields were observed, an expected
one based on the Ni1+xTe2 solid solution
(SS1) and a second unexpected, quite distinct and compositionally
broad solid solution (SS2) centred on the nominal composition
Ni2.80In0.80Te2. The latter SS2
solid solution was shown to be incommensurately modulated with a
composition-dependent primary modulation wavevector. The origin of
the incommensurate modulation in the case of the SS2 solid solution
is attributed to the regular alternation of In-rich and Te-rich
layers along the c direction. (with F. Brink,
L. Norén, and H. Rundlof [U. Uppsala, Sweden])
Flexible Microporous Framework Structures
The
inherent displacive flexibility of many framework structures plays a
crucial role in many of their useful physical properties (low thermal
expansion, easy ion exchange, catalytic activity, ..) and arises as a
result of the existence of essentially zero frequency, Rigid Unit
Modes (RUM's) entailing rotations but not distortions of their
constituent polyhedral units. Such materials are frequently
polymorphic and often undergo several displacive structural phase
transitions, due to the condensation of RUM modes, as a function of
temperature, pressure etc. A program has begun to
systematically investigate a range of flexible, microporous framework
structures in order to identify the appropriate RUM modes (via
temperature-dependent electron diffraction), to analyze the
displacive structural phase transitions therein, and to understand
the physical properties associated with this inherent displacive
flexibility. (with Y. Liu, L. Norén, Li Ying, and
T.J.
White [Inst. Environmental Science and Engineering, Nanyang Tech. U.
Singapore])
Modulated Structures in Metal Oxyfluoride Systems
The
compositional range of existence of the simultaneously ferroelectric
and ferroelastic A2BMVIO3F3
(A, B = alkali metal, MVI = Mo,W) family of
elpasolite-related oxyfluoride phases has been widened by the
successful synthesis of elpasolite-related
Tl3MoO3F3.
DSC showed two major polymorphic phase transitions at 42 °C
and 130 °C, respectively. Electron diffraction and XRD
studies of the complex room temperature polymorphic form of this
material has revealed a long period monoclinic superstructure
coexisting with a highly structured, three-dimensional, continuous
diffuse intensity distribution (arising from local O/F ordering and
associated structural relaxation). (with F. Brink, L. Norén)
The Effects of Local Strain on the Crystal Chemistry of Solid Solutions
Planning
for an ARC funded project to characterize substitutional strain in
several wide range non-stoichiometric solid solution systems is under
way. Several different but complementary techniques (vibrational
spectral line broadening, diffuse intensity measurement and modelling
and charge density analysis) will be used to investigate if it is
possible to obtain direct measures of the enthalpy of mixing in such
systems. It is planned to investigate Fe/Zn ordering in sphalerites,
A/B ordering in AB2S4 thiospinels and
oxygen/vacancy ordering and coupled cation ordering in
anion-deficient "defect perovskites". (with T.R.
Welberry, and A. Pring [South Australian Museum], N. Ishizawa [Tokyo
Inst. Tech., Japan])
The Refinement of Partially Overlapped Reflections from Twinned Crystals
Automatic
procedures often result in the identification of a unit cell for just
the major of two twin related components of an X-ray diffraction
pattern. We have developed a technique to cope with such data
collected on our Nonius CAD4 CCD diffractometer by modifying the twin
parameters ai in the model Y(h)
= Σi ai
|F (hi)|2
to become aipj,
where the integer j defines the refinable process used for
evaluating pj. This procedure
has now been applied to a number of structures with considerable
success. (with A.D. Rae, A.J. Edwards, A.C. Willis)
Polytypic Behaviour in Crystals in Terms of 1:1
Disordered Parent Structures
The
1:1 disordered parent structure is the Fourier transform of
reflections of a particular index condition. Ordering the structure
necessarily removes some symmetry elements and the choice of these
symmetry elements is not necessarily unique. In the last year we
have found four different crystal systems where different orderings
correspond to options that include different space groups as well as
different orientations of the same structure. We have studied two
polymorphs of 9,10-phenanthrene quinone, both showing polytypic
behaviour. The low temperature form has a 1:1 disordered
P21/c
parent structure and ordering requires the doubling of the a
and b axes to form layers of C-1 symmetry. Stacking
layers c/2 apart results in either Cc or F-1
symmetry. We found and refined twinned crystals of each space
group. Stacking faults were a feature of both. We also found and
refined a crystal that could only be described in terms of the
coexistence of uncorrelated blocks with different space groups. The
averaging of intensities from different mosaic blocks with different
stacking fault populations affords a workable model. (with A.D.
Rae, A.C. Willis)
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