Application of Space-Group Theory to the Vibrational Problem of di-Tetramethyl Ammonium Uranium Hexachloride

The absorption spectrum of [(CH3)4N]2UCl6 (abbr. TMA) is discussed. The spectrum is vibronic in nature, i.e., the electronic transitions between levels belonging to the 5f2 configuration of the U4+ ion are coupled with the vibrations of the TMA lattice. The number and symmetries of the normal modes of vibration of a unit cell are calculated in the factor group approximation. The Winston—Halford decomposition formula is used to reduce the representation of motions among the irreducible representations of the symmetry group of TMA. Calculations are compared with the experimental results. The gross features of the spectrum are found to be in agreement with the theoretical predictions. The method of obtaining the character tables for finite symmorphic space groups is presented in summary form. A general vibrational problem in crystals is discussed.