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John C. RaichProfessor 
Structural Phase TransitionsStructural phase transitions occur when materials change their crystallographic structure as the temperature and pressure are varied. In the case of molecular crystals, a change of the crystal structure implies either a change in center-of-mass positions or orientations of molecules in the crystal, or both. Phase transitions in three-dimensional and two-dimensional (molecules absorbed on surfaces) systems are currently being studied at Colorado State.For some structural phase transitions the positions or orientations of molecules in the distorted phase are found to be displaced from their high temperature values. Such transitions occur because the crystal is unstable with respect to a particular normal mode of vibration, translational or librational, of the crystal in the high temperature phase. These so-called "soft modes" are characteristic of displacive transitions. There are other structural changes where the phase transition is associated with a localization of molecular orientations which are disordered in the high temperature phase. This type of behavior is called "order disorder." The reorientation of molecules involves relatively large displacements and cannot be described in terms of conventional lattice dynamics. The first description of structural phase transitions was Landau's phenomenological theory. During the last 10 to 20 years dramatic advances in the theory of phase transitions have occurred. Our understanding of structural phase transitions has continued to progress rapidly also because of a variety of experimental techniques. In particular, neutron and light scattering enable us to probe the dynamics of structural transitions. Present theoretical efforts at Colorado State are directed towards understanding structural phase transitions in organic and inorganic molecular crystals. Of special interest are the interactions of molecular reorientations with translational displacements. This coupling allows us to observe the effects of phase transition via the behavior of the acoustic phonons by using Brillouin scattering. Hence the modeling of the microscopic dynamics of light and neutron scattering of molecular crystals undergoing structural transitions is an important tool in comparing theory with experiment. |