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Program Leader
Ian Swainson
Phone: 613-584-8811x43995
Fax: 613-584-4040
Email: Ian.Swainson@nrc-cnrc.gc.ca
Structures and Dynamics
Structural Chemistry
The experimental program in structural chemistry focuses on the structure determination and characterization of complex systems. Much of the work is carried out on polycrystalline samples and utilizes Rietveld refinement of data from the high resolution powder diffractometer (C2) of DUALSPEC. By its nature, such a program tends to be rather diverse. This technique is particularly powerful for measuring small changes in structures as a function of some parameter such as temperature. Thus, the investigation of structural phase transitions is a significant component of the program.
Among other structural types that have been studied are ammonium salts, usually in deuterated form. Using data from DUALSPEC, it has been possible to refine both the orientation of the ammonium ion and the anisotropic motion of the ion in the lattice. Several extended framework structures such as clathrates, zeolites and inclusion compounds have been investigated. Clathrates are a type of inclusion compound in which the host forms the cage structure capable of holding guests. The intermolecular forces between the host and the guest usually are weak in comparison with chemical bond. Currently, we are interested in a new type of clathrate, semiconductor clathrates, MxN46 where M is a metal and N is Si, Ge or Sn. These materials can exhibit a variety of electronic behaviour from semiconductor to superconductor by trapping different proportions of metal guest. The thermal conductivities of semiconductor clathrates are much lower than their diamond phases. They are promising candidates for applications such as thermoelectric cooling and power generation. We have determined the lattice parameters and thermal factors of semiconductor clathrates by neutron powder diffraction. Combined with other thermal properties including heat capacity and compressibility we found that Grüneisen parameters of these materials are abnormally high at very low temperature which is due to the strong scattering of heat-carrying acoustic phonons by the rattling of guests.
The structural chemistry of geological materials is another component of the program. A major interest in these materials is the statistical ordering of similar atoms on specific sites (e.g. Si and Al) and the phase transitions that result from such ordering. Results have been obtained recently for a model spinel system formed by a solid solution. Molecular and ionic motions are investigated by inelastic scattering methods with the triple-axis spectrometers. Recent measurements of the molecular excitations in several mineral systems have been used to learn about the interatomic forces in these materials, in particular the strange inelastic scattering from calcite.
We have recently examined the structure of ikaite CaCO3.6D2O, a "rare" mineral that forms in freezing water and its geological importance in terms of its pseudomorphs used as paleothermometers. We are also working on some aspects of gas hydrates, formed under similar conditions to ikaite.
