Reply to ``Comment on `Light scattering study of the phase transition in sym-triazine' ''
Rae’s1 comments on the author’s paper concerning the phase transition in sym‐triazine2 are answered. (AIP)
The structural phase change in s-triazine. I. The crystal structure of the low-temperature phase
The crystal structure of s-triazine at 150K has been determined by X-ray diffraction. The unit cell is monoclinic with a=0.6884 nm, b=0.9569 nm, c=0.7093 nm, beta =126.61 degrees . The low-temperature structure can be generated from the high-temperature hexagonal form by shearing the crystal in the a*c plane and rotating the molecules about the b axis. The shear and rotation angles have been measured at a number of temperatures between 110K and the transition temperature of 198.07K. They are found to be proportional to each other and their variation with temperature is in good agreement with that expected from the order parameter in the case of a quasi-continuous transition.
Brillouin scattering spectroscopy of ferroelectric and ferroelastic phase transitions
Light scattering spectroscopy has played a crucial role in the study of numerous phase transitions and critical phenomena. In the case of structural phase transitions in crystalline solids, high resolution Raman and Brillouin scattering experiments have provided detailed information on the ‘soft modes’, the highly temperature-dependent lattice vibrations whose frequencies fall towards zero at the transition temperature. The condensation of the soft modes precipitates the spontaneous symmetry breaking which inevitably accompanies these transitions. Ferroelectrics comprise a particular class of crystals exhibiting structural phase transitions which have been investigated extensively by light scattering techniques. In addition to numerous soft mode studies, other subtle aspects of these transitions have been investigated by light scattering, including mode interactions and central peaks. In this presentation, the fundamental aspects of structural phase transitions and their relation to light scattering will be reviewed. A number of light scattering studies of ferroelectric crystals spanning the past decade will be summarized and used to illustrate the current level of understanding of these phenomena.