Comparison of the pressure dependences of Tc in the trivalent d-electron superconductors Sc, Y, La, and Lu up to megabar pressures

Whereas double hcp (dhcp) La superconducts at ambient pressure with T-c similar or equal to 5 K, the other trivalent d-clectron 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 second 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 with 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.