Raman and Birefringence Studies on the Incommensurate Phase Transition of Bis(4-chlorophenyl)sulfone Crystal

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Temperature dependences of polarized Raman spectra and birefringence of bis(4-chlorophenyl)sulfone (BCPS) crystal were measured. Gas-phase Raman spectra of BCPS were also measured. Raman bands observed in the low-frequency region at room temperature were shown to have partial nature of intramolecular vibrations. The lowest-frequency Raman band observed at low temperatures softened as the temperature was raised up to T₁=150 K. A gradual change of birefringence was also observed below T₁. These are in harmony with previous reports on the normal-incommensurate phase transition of this crystal at T₁. The critical exponents for the changes in the soft-mode frequency and the birefringence were compared with that for the previously reported intensity change of the satellite X-ray diffraction. These data were explained by assuming an incommensurate orientational modulation of the phenyl groups of BCPS molecules. However, the incommensurate-commensurate transition which has been expected to take place around 115 K was not detected by the present measurements.