Observation of an incommensurate structure by optical spectroscopy with a 5f ion as an internal probe

The second order phase transition in β-ThCl4 (T/sub 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 U4+ 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, S4, D2, and various C2 permits one to establish a correlation between absorption and emission profiles of U4+ ions. The green and the red observed fluorescences of U4+ in ThCl4 are assigned in the different symmetries and the energy level mapped out as a function of a phase angle phi/sub l/ related to the distortion according to the Delamoye and Currat model. (author)