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.

The crystal structure of the equiatomic TiZr alloy is studied in the pressure range up to 57 GPa using the diamond anvils and synchrotron radiation. It is found that this alloy follows the ␣→→␤ transition series earlier observed on Zr and Hf. Superconductivity measurements on the same alloy to 47 GPa show that the transition to the bcc ␤ phase is connected with an increase in the superconducting transition temperature to 15 K. The pressure dependence of the atomic volume is fitted to an equation of state. Low values of the pressure derivative of the bulk modulus as well as the increase in the superconducting transition temperature are discussed in terms of a pressure-induced s -d electron transfer.

Measurements of the superconducting transition temperature T c have been performed by the inductive method on MgB 2 at pressures up to 28 GPa. T c decreases with applied pressure, exhibiting a cusp at about 9 GPa. We interpret the appearance of this anomaly in the T c (P) dependence as a result of the pressure-induced electronic transition. Recent band-structure calculations for MgB 2 give some support for such an explanation.

The effect of pressure on amorphous Ge was probed by Raman spectroscopy combined with moleculardynamics simulations. A large jump occurs in the principal peak position due to nearest-neighbor Ge-Ge vibrations at 11-12 GPa and 7-5.5 GPa, respectively, during increasing/decreasing pressure due to a polyamorphic transition occurring between the low-density amorphous semiconductor and a metallic high-density polyamorph ͑HDA͒. We measured the superconducting transition temperature ͑T c ͒ using magnetic susceptibility measurements in the diamond anvil cell and determined that T c for the high-density HDA polyamorph was higher than that for the ␤-Sn structured Ge-II crystalline phase.

No abstract