Abstract
Crystal-field transitions associated with U4+ impurities diluted in ThBr4 give rise to broad absorption bands characterized by edge singularities. We show that the experimental spectra are consistent with the known occurrence of a sinusoidal distortion which modulates the Br- ion equilibrium positions, thus reducing the actinide site-symmetry from D2d to D2. The observation of spectral singularities corresponding to D2d-sites is interpreted as resulting from the partial pinning of the incommensurate modulation by the U4+ impurities

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