Raman spectrometric studies of selected lanthanide tribromides and trichlorides

Laser Raman spectroscopy has been used in our laboratory to identify the crystal structures of lanthanide and actinide compounds. The phonon Raman spectrum of a crystalline sample is characteristic of the particular crystal structure(s) exhibited by the sample. GdCl ₃ exhibits two crystal structures, the UCl₃ -type hexagonal and the PuBr ₃ -type orthorhombic. In the literature it is reported that the low temperature form is orthorhombic; results of experiments in our laboratory suggest that it is hexagonal. Interconversion between these two forms can be accomplished with temperature and/or pressure. In the present work laser Raman spectrometry was used to monitor crystal structure changes in GdCl₃ as a function of temperature or pressure to determine the temperature or pressure at which the hexagonal-to-orthorhombic transformation occurs.

Raman spectroscopy has also been used to determine the symmetry assignments for the Raman-active bands of a single crystal. Raman spectra of polycrystalline NdBr ₃ have been recorded at room temperature and pressure and at approximately 100 K. In addition, polarized Raman spectra of a single crystal NdBr ₃ have been measured. Based on these polarization measurements, symmetry assignments of eight Raman-active modes were made. These assignments are useful in interpreting the phonon Raman spectrum of any compound exhibiting the PuBr ₃ s-type orthorhombic structure.