Observation of an incommensurate structure by optical spectroscopy with a 5f ion as an internal probe. U 4+ –ThCl 4 System

The second order phase transition in β‐ThCl 4 ( T c = 70 K) leads to an incommensurate tetragonal structure modulated along the fourfold symmetry axis. This is due to the freezing of an optical soft mode which induces a modulation of the chlorine ion positions in a plan perpendicular to this axis. Consequently in neighbouring unit cells the crystal field strength acting on the U 4+ ion is slightly different. Then the line shape in the optical spectra shows a continuous variation which reflects the shift of the energy as the symmetry changes. Selective excitation of the resulting different classes of sites, S 4 , D 2 , and various C 2 p ermits one to establish a correlation between absorption and emission profiles of U 4+ ions. The green and the red observed fluorescences of U 4+ in ThCl 4 are assigned in the different symmetries and the energy level mapped out as a function of a phase angle φ l related to the distortion according to the Delamoye and Currat model.