Solid state physics and actinide spectroscopy with ThBr4 and ThCl4

From the solid state physics to the optical spectroscopy of 5 f elements, we have described in details how ThBr4 and ThCl4 are acting an important role in the applied and basic research field. The possible applications are based on the scintillation properties of these crystals while the fundamental aspect is related to their special incommensurate structure which modulates the host properties of these matrices at low temperature. This very small perturbation of bromine atom positions have optical consequences on the absorption and emission of the actinide studied ions which are interpreted in function of the modulated structure parameters determined by inelastic and elastic neutron scattering and by Raman spectroscopy. The structural model proposed shows, for the first time, how the optical properties of the investigated ion are influenced by the sinusoidal displacements of the bromine atoms, leading to an energy continuum in the absorption and reducing the site symmetry from Dsub(2d) to a multisite of D2 symmetry