Hydrostatic high-pressure studies to 17 GPa were carried out on superconducting YBa 2 Cu 3 O x over the full range of oxygen content x. The observed bell-shaped T c ( P) dependences are found to depend markedly on the temperature at which the pressure is changed. The time-dependent relaxation processes are studied using both T c and the electrical resistivity at 298 K as probes. The activation volume for oxygen diffusion is determined from the pressure-dependent activation energy and compared with estimates from a simple hard-sphere model.

Whereas dhcp La superconducts at ambient pressure with T c ≃ 5 K, the other trivalent d-electron metals Sc, Y, and Lu only superconduct if high pressures are applied. Earlier measurements of the pressure dependence of T c for Sc and Lu metal are here extended to much higher pressures. Whereas T c for Lu increases monotonically with pressure to 12.4 K at 174 GPa (1.74 Mbar). T c for Sc reaches 19.6 K at 107 GPa, the 2nd highest value observed for any elemental superconductor. At higher pressures a phase transition occurs whereupon T c drops to 8.31 K at 111 GPa. The T c (P ) dependences for Sc and Lu are compared to those of Y and La. An interesting correlation is pointed out between the value of T c and the fractional free volume available to the conduction electrons outside the ion cores, a quantity which is directly related to the number of d electrons in the conduction band.

The dependence of T c on nearly hydrostatic pressure has been measured for an isotopically pure ( 11 B) MgB 2 sample in a helium-loaded diamond-anvil-cell to nearly 20 GPa. T c decreases monotonically with pressure from 39.1 K at ambient pressure to 20.9 K at 19.2 GPa. The initial dependence is the same as that obtained earlier (dT c /dP ≃ −1.11(2) K/GPa) on the same sample in a He-gas apparatus to 0.7 GPa. The observed pressure dependence T c (P ) to 20 GPa can be readily described in terms of simple lattice stiffening within standard phonon-mediated BCS superconductivity.

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