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The effect of pressure on the superconducting transition temperature T c of La was studied up to 50 GPa. T c (P) shows a rather complicated variation with a discontinuous increase in T c at about 2.2 GPa due to the first-order phase transition from dhcp to fcc structure. At about 5.4 GPa a sharp peak is observed due to the soft-mode phase transition from fcc to the distorted fcc structure and two broad maxima are found within the stability region of the distorted fcc structure around 12 and 39 GPa. Some differences between these and previous low-pressure data for metastable fcc La are noticed. The results are discussed in connection with pressure-induced structural phase transitions found in earlier x-ray-diffraction experiments and band-structure calculations giving evidences for van Hove singularities in the density of states.

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Hi& molution neutmn scattering studies of vibrationel spectra (2-800 meV) were carried out on the e-, 6'and 6-phof titanium hydride, obtained after high pressure treatment. The observed splitting of the fundamental optic peak in the &-phase spectrum can be explained by a strong H-H interaction along the D axis. Well pronounced features are seen on the low energy side of the tw-and three phonon optic bands. Thex are assigned to bound bi-and tri-phonon excitations. This is the first observation of biand tri-phonons in metal hydrides.

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