Prof. Ray Withers
Research School of Chemistry
Canberra, ACT, 0200
The prime research interest of the Solid State Chemistry Group is the interplay between local crystal chemistry and longer range macroscopic order. We are particularly interested in the study of compositionally and/or displacively flexible materials (see e.g. Progress in Solid State Chemistry 26, 1-96, 1998) - wide range non-stoichiometric solid solutions, displacively flexible framework structures, incommensurately modulated structures, ferroic phases, tough ceramics, solid electrolytes ....Some specific areas of recent research interest include:
- Compositionally flexible, long range ordered (albeit in higher than 3-dimensions), compositely modulated solid solution phases in which two (or more) parent sub-structures (each individually characterized by their own 3-d space group symmetries and unit cell dimensions) intergrow.
The parent sub-structures are in general mutually incommensurable along at least one common direction with the relative periodicity along this direction being variable, directly related to the composition of the solid solution and acting as a "chemical ruler" (see e.g. Progress in Solid State Chemistry 26, 1-96, 1998)
- Inherently displacively flexible framework structures composed of essentially rigid corner-connected polyhedral units e.g. the mineral Fresnoite (Ba2TiSi2O8 - see below) composed of tetrahedral SiO4 and square pyramidal TiO5 polyhedral units. The flexibility of such materials arises from the surprisingly numerous different ways in which it is often possible to change the relative orientation and positioning of neighbouring polyhedral units without distorting either the shape or size of the individual polyhedral units (see e.g. Solid State Sciences 5, 115-123, 2003).
Their inherent flexibility means that they are often associated with useful physical properties such as low, or even negative, thermal expansion and frequently undergo complex sequences of displacive structural phase transitions on lowering of temperature as a result of the "freezing in" of essentially zero frequency co-operative polyhedral rotation modes (so-called Rigid Unit Modes or RUM's - see e.g. Physics and Chemistry of Minerals 23, 55-61, 1996). The modulation wave-vectors associated with these RUM's often give rise to highly structured diffuse intensity distributions which can be observed via temperature-dependent electron diffraction (see e.g. Solid State Sciences 5, 427-434, 2003).
- Compositionally flexible solid solution phases exhibiting highly structured diffuse intensity distributions falling on characteristic locii in reciprocal space and the local crystal chemical rules underlying such behaviour. Examples include some wide range non-stoichiometric T1+xB (T a transition metal and B from Group 13-16) B8 type solid solutions, sub-stoichiometric transition metal carbides and nitrides and compositionally flexible mineral solid solution phases such as wüstite (Fe1-xO, 0.85<x<0.97) or mullite (Al2(Al2+2xSi2-2x)O10-x,0.17<x<0.59).
Recent examples that we have investigated include O/F ordering in a range of transition metal oxyfluoride phases (see e.g. J.Solid State Chem. 170, 211-220, 2003) and Bi/Zn ordering in the (Bi1.5-aZn0.5-b)(Zn0.5-gNb1.5-d)O(7.5-1.5a-b-g-2.5d) pyrochlore (see e.g. J. Solid State Chem. 177, 231-244, 2004).
Other areas of recent research interest include solid electrolyte phases such as the anion-deficient, zirconia-based CSZ's or solid solutions based upon doping of Bi2O3 e.g(1-x)Bi2O3.xMO, M = Ca, Sr, Ba, ferroic materials of all sorts e.g the Aurivillius family of displacive ferroelectrics, local strain and its crystal chemical consequences, ordering in doped rare earth cobaltates etc...We have an extensive solid state synthesis capability for growth of single crystal and powder specimens. Our principal experimental research tools are Transmission Electron Microscopy in combination with single crystal and powder diffraction.
Current Members of the Group
- Dave Rae
- Yun Liu
- Lasse Norén
- Frank Brink
- Valeska Ting
Some Past Members
- Yasu Tabira
- John Thompson
- Siggi Schmid
- Chris Ling
- Ankie Larsson
- Klaus Fütterer
The group has a wide range of research interests and capacities.
For more details download the latest RSC Annual Report in pdf format.
A good way to see what the RSC and our group are up to!
Updated 13th April 2004