Observation of a phase transition in ThBr4 and ThCl4 single crystals by far-infrared and Raman spectroscopy study

At 4 K the visible and infrared absorption and emission spectra of U⁴⁺ in ThBr₄ and ThCl₄ single crystals are not very consistent with what is predicted by the selection rules for the room temperature structure. Thus we investigated Raman scattering in the temperature range 10–300 K to look for a structure change and obtain a better understanding of the spectroscopy of U⁴⁺ in ThBr₄ and ThCl₄. At room temperature, the observed Raman lines have been assigned on the basis of a D_4h factor group analysis. The study of the temperature dependence of the Raman spectra permitted us to discover phase transitions of ThBr₄ and ThCl₄ at 95 and 70 K, respectively. The splitting observed for the strongest E_g symmetry mode shows a lowering of the symmetry below the transition point. Powder X-ray diffraction at 77 K of hygroscopic ThBr₄ is being carried out to determine the low-temperature structure.