Low-temperature X-ray diffraction experiments were performed on 4,4''-difluoro-p-terphenyl (DFTP) and
4,4'''-Difluoro-p-quaterphenyl (DFQP). The structure analyses were
performed for the high-temperature phases and the librational motion of
the benzene rings was analyzed. The super-lattice reflections were found in
DFQP below about 200 K on the same position as the low-temperature phase of
p-quaterphenyl. In DFTP, the satellite reflections with small
wavevector component dc (about 1/6) along the c*-axis
were found below the transition temperature.
(Solid State Commun., 91, 89-92 (1994))
Phonon dispersion relations of crystalline bis(4-chlorophenyl)sulfone were calculated assumind various degree of flexibility of the molecule. A dip in the lowest branch appeared around the location of the satellite reflection observed experimentally in the incommensurate phase, irrespective of the flexibility. An internal degree of freedom of the molecule, which breaks the two-fold axis of the molecule, couples with the lowest branch and strongly enhances the dip.
(Solid State Commun., 81, 241-243 (1992))
See also Displacive Transition or Order-Disorder Transition?
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