Investigation of phonon dispersion and crystal field splitting in cerium deuteride

The phonon dispersion relations in the three symmetry directions of CeDsub(2.12) have been measured using triple-axis spectrometers. A sixth-neighbour Born-von-Karman tensor force model (21 parameters) describes satisfactorily the experimental characteristic features. Such an agreement could not be obtained in a previous work, where a third-neighbour central force model (8 parameters) was used; the improvement is mainly due to the introduction of D-D tensor forces of longer range. Complete dispersion relations for the acoustic branches in CeDsub(2.72) have also been obtained. With respect to the corresponding data for CeDsub(2.12) one gets a remarkable increase in the frequencies of the longitudinal modes belonging to the Σ1A and Λ1A branches. Using a time-of-flight spectrometer the crystal field splitting of the 2Fsub(5/2) ground state of the Ce3+ ion in CeDsub(x) has been measured for polycrystalline samples with 1.94<=x<=2.75. A consistent description of the results could only be obtained assuming that, over the investigated concentration range, the samples have tetrahedral vacancies. For CeDsub(1.94) an effective charge of - 0.30 abs (e) was derived, both for octahedral and tetrahedral deuteriums. For CeDsub(2.42), however, an effective charge of - 1.35 abs (e) had to be attributed to the octahedral deuteriums. (author)