Lattice Vibrations and Structure of Rare-Earth Fluorides

The Raman spectra of La${\mathrm{F}}_{3}$, Ce${\mathrm{F}}_{3}$, Pr${\mathrm{F}}_{3}$, and Nd${\mathrm{F}}_{3}$ have been obtained with an argon-ion laser at room temperature and low temperatures. On the basis of polarizations and intensities of the lines an assignment is made of the Raman-active modes. The infrared-reflectance spectrum was measured with electric vector parallel to the crystal axis. The spectra are consistent with a hexamolecular unit cell, of ${{D}_{3d}}^{4}$ symmetry, with small distortions from the more symmetrical hexamolecular ${{D}_{6\mathrm{h}}}^{3}$ and bimolecular ${{D}_{6\mathrm{h}}}^{4}$ unit cells proposed earlier. The ${{D}_{3d}}^{4}$ symmetry appears to offer an explanation of apparent disagreements among earlier studies of these crystals.

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