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Changes in the crystal structure of 4, 4'-dibromobenzophenone as seen by vibrational spectroscopy and by X-ray diffraction

4, 4'-dibromobenzophenone (DBrBP) crystal exhibits temperature induced phase transition depending on the thermal history of the sample. It was stated that the temperature dependent vibrationnal spectra show interesting behavior in the low frequency part. For this molecule several molecular modes are expected in the region of lattice vibrations, similarly as for the DFBP and DClBP. As for these two crystals, the vibrational frequencies of the molecular modes in DBrBP are also expected to be significantly infuenced by the crystal field. In order to elucidate the nature of the phase transition we undertook parallel investigations of the DBrBP single crystal by polarized Raman spectroscopy and by X-ray diffraction. X-ray structural study reveals the Ccc2 orthorombic structure with four molecules in the unit cell, and with the binary axes of the crystal coinciding with the molecular binary axes. These facts enabled us to separate the molecular from the lattice modes in the vibrational spectra of oriented single crystal, and to assign the vibrations particularly sensitive to the temperature changes. The more complete understanding of the low-frequency vibratinal spectrum gives us additional information about the nature of the phase transition involved.

vibrational spectroscopy ; X-ray diffraction ; dibromobenzophenone

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