Magnetic ordering and Raman scattering from electronic excitations and phonons in uranium trichalcogenides

The Raman spectra of US 3 , USe 3 and UTe 3 have been studied as a function of temperature and polarisation. Eight or nine of the twelve Raman-active phonons expected for the ZrSe 3 -type structure have been identified. Low-lying electronic transitions associated with the U 4+ ions have been observed in US 3 and USe 3 , and possibly in UTe 3 . The excitons found at low temperatures in US 3 and USe 3 are consistent with two-dimensional antiferromagnetic ordering. Long-range ordering occurs below the temperature where there is a maximum in the magnetic susceptibility. The exchange interaction in these singlet-ground-state magnets is weak compared with the crystal field.