Phase transitions, in which a structure changes from one atomic arrangement to another without a change in composition, provide the most rigorous arena for testing ideas about the forces and balances of forces that stabilize particular atomic configurations in crystal structures. The study of phase transitions has direct applications to energy-related materials; most modern materials used in devices (gmr materials, piezo- and ferro-electrics and multi-ferroics) derive their technological properties as a result of structural phase transitions. The Transactions Symposium of the 2009 ACA meeting in
Toronto
covered all aspects of phase transitions, including presentations on theory, experimental methodologies and quantitative analysis techniques in systems ranging from minerals and inorganic compounds through to proteins and biological compounds. The symposium comprised 11 speakers over two oral sessions that totaled 5½ hours, along with a very-well attended poster session. The papers presented by nine of the speakers at the symposium have been compiled and will shortly be published on-line via the ACA website.
Three foreign keynote speakers were supported by the SE grant to a total of $4200; Diego Gatta (Dept. Mineralogy, University of Milan, Italy), Michel Dusek (Institute of Physics, Praha, Czech Republic) and Manuel Perez-Mato (University of the Basque Country, Spain). Their participation not only greatly enhanced the Transactions Symposium itself, but all three played a very active role in the entire meeting, with Dr. Dusek also giving a short course in advance of the main meeting.
Diego Gatta (
Milan
) presented a wide-ranging review, much based on his own work, of the behaviour of zeolites at high pressures as studied by single-crystal diffraction. Symmetry arguments show that such microporous crystalline framework materials currently used as catalysts should, in principle, be riddled with phase transitions which would be expected to close up the micro-pores and thus affect their catalytic potential. However, Gatta’s work shows that structural phase transitions are relatively rare in zeolites because of the inherent flexability of the structures that allow them to respond to changes in pressure or temperature by continuous rotations of the tetrahedral units comprising the framework. Pressure then drives increased, and temperature decreased, tetrahedral rotations, and changes in the external variables are the simple drivers of the response of such materials. The same conclusion was drawn in the talk by Simon Parsons (Edinburgh), who used the example of his work on the high-pressure polymorphism of salicylaldoxime and of serine to demonstrate that while phase transitions sometimes appear to be driven by bonding forces, such as H-bonds being forced to become short, the dominant factor in determining structural stability at high pressures is the packing efficiency. The pressures involved, of several GPa, mean that the PV term becomes the dominant component of the free energy of a molecular crystal. The essential message was summarized towards the end of the day by I. David Brown (Hamilton, ON) who, in the context of an exploration of bonding forces through the concept of bond valence once again emphasized that chemistry does not change between phases or across phase boundaries, but that phase transitions are driven by changes in the intensive external thermodynamic variables such as temperature or pressure.
Structural phase transitions are a special class of transitions in which the symmetries of the two phases have a group-subgroup relationship, and the lower-symmetry structure can be regarded as distorted version of the high-symmetry form. Prof. Perez-Mato (
Bilbao
) coordinates the worlds’ leading group in the symmetry analysis of structural phase transitions. The Bilbao Crystallographic Server that has been developed under his direction provides software tools that enable other researchers to perform symmetry analyses of phase transitions. Prof. Perez-Mato’s talk illustrated the applications of these tools to developing an understanding of the transitions in several energy-application materials. He used BaTiO3 as an example to show how the changing of polarization directions leads to domain formation and several ferroelectric phases at low temperatures, and how the symmetry controls the strain that develops spontaneously at ferro-electric transitions couples with the polarization and thus how ferroelectrics can be “switched” by the application of external stress.
The power of analyzing structural phase transitions, and the resulting structures, from a group-theoretical point was also emphasized by Branton Campbell (
Provo, UT
). He showed that the apparently complex structural evolution of the distorted low-symmetry phase below a transition can often be more simply and naturally explained in terms of “symmetry-adapted distortion modes” which he called “Nature's basis for parameterizing structural phase transitions”. The essence of a distortion can often be captured by a relatively small number of the available modes, while the other modes have amplitudes that cluster near zero. As a consequence, the symmetry-adapted description reduces the effective complexity, leading to a clearer understanding and more stable refinement of crystal structures to diffraction data. Close to high-temperature phase transitions such modes can represent the dynamic fluctuations of the structure. Thomas Proeffen (
Los Alamos
) showed that, by analyzing the total scattering in terms of a probability density function, one can show that many displacive transitions in oxides involve no change in local structure but merely a change in the long-range correlations.
Michel Dusek (Praha) lecture focussed on the analysis of incommensurate phase transitions with changes in temperature. Such transitions give rise to modulated structures with different local geometry and thus site reactivities from the parent structures. Dr. Dusek is one of the primary authors of the widely-used program package JANA, and he demonstrated the power of the JANA software for determining and refining both twinned and modulated structures. His talk was complimented by presentations from two representatives from companies that build the diffractometers for collecting the primary diffraction data. Mathias Meyer (Oxford Diffraction) demonstrated that indexing and integrating diffraction patterns from crystals that have become twinned as a result of a phase transition is straightforward with the tools now available and Michel Ruf (Bruker-AXS) showed the same is true for the integration of diffraction patterns from incommensurately modulated crystals.
