OPTICAL VIBRATIONS OF HYDROGEN IN SOME RARE-EARTH MONOHALIDE HYDRIDES

We report an investigation of the local modes of hydrogen and deuteriu m atoms by means of inelastic neutron scattering at 10 K and 100 K in some hydride and deuteride monohalides of the rare-earth elements, REX H(x), with RE = Y and Th, X = Cl and Br and x congruent-to 0.8. These compounds contain hydrogen/deuterium atoms in distorted tetrahedral me tal-atom voids. The hydride spectra exhibit two modes centred at about 125 meV and 90 meV for YBrD(x). For the deuterium compounds YBrD(x) ( x congruent-to 0.8) second-harmonic transitions are observed. The ener gy splitting of the two modes is of the order of 6-10 meV. The low-ene rgy mode is about twice as intense as the high-energy mode. The splitt ing of the modes is consistent With local C3v symmetry and analysed in terms of a model suggested recently by Bennington et al . Broadenings of the modes are observed and attributed to a weak splitting of the d oubly degenerate low-energy mode due to a reduction of local symmetry. To relate the spectra to crystallographic properties a powder neutron -diffraction experiment and a refinement of the crystal structure of Y BrD0.79 have also been performed.