Because of its apparent simplicity, diffusion of hydrogen in solids can be regarded as a general model system for diffusion. However, only rudimentary knowledge exists for the dynamics of hydrogen in complex hydrides. Insight into the specific diffusion process is given by hydrogen-deuterium exchange experiments. Thermogravimetry and Raman spectroscopy are used to measure the hydrogen-deuterium exchange during the decomposition of LiBH4. At a temperature of 523 K the self-diffusion constant of deuterium in LiBH4 is estimated to be D approximately 7 x 10(-14) m(2) s(-1). A careful analysis of the Raman spectra shows that hydrogen is statistically exchanged by deuterium in LiBH4; i.e., the diffusing species is assumed to be the single hydrogen atom.

We have performed micro Raman measurements on LiBH4 and LiBD4 powders for temperatures between 5 and 300 K. At the lowest temperature, the peak energies agree very well with the results of a calculation within the density functional theory for the orthorhombic structure. The spectra are dominated by three separated bands: the external modes, the internal bending, and the internal stretching vibrations. Internal refers to vibrations within the BH 4 tetrahedrons, whereas external modes imply motions of Li and BH 4. The temperature dependence of the observed phonons corroborates the strong anharmonicity of the system. Due to the anharmonicity, Fermi resonances occur between the first order stretching modes and the second order bending modes of LiBH4. Moreover, the linewidths of the observed modes in LiBH4 have an Arrhenius-like component, with activation energies ranging from 250 to 500 K.

We provide direct evidence for successive exchange of D atoms in the isotopically pure BH 4 − units of the crystalline solid lithium borohydride. We prove the coexistence of all BH 4−n D n − ͑0 Յ n Յ 4͒ units in the bulk by deconvolution of the D-stretching vibrations band of Raman spectra at 83 and 5 K in partially D-exchanged LiBH 4 and comparison with first-principles Raman intensity calculations. The measured distribution of BH 4−n D n − units is in good agreement with a binomial distribution biased by H-D zero-point motions. This implies breaking the strong D 2 covalent bond and transport of hydrogen in LiBH 4 below the melting temperature.

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The phonons and the crystal structures of the ␤-pyrochlore superconductors KOs 2 O 6 and RbOs 2 O 6 are studied on single crystals by using micro-Raman spectroscopy. The symmetry of the modes is determined by polarization dependent measurements for various scattering geometries. It is concluded that the crystal structures are centrosymmetric ͑Fd3m͒. The vibrational frequencies have been computed from first principles and are in good agreement with the experimental data. The Raman-active rattling mode of the alkali ion is identified at 75 and 60 cm −1 for the K and Rb compounds, respectively.