Pressure-induced valence change in the rare earth metals:The case of Praseodymium

The rare earth metal praseodymium (Pr) transforms from the d-fcc crystal structure (Pr-III) to {$\alpha$}-U one (Pr-IV) at 20 GPa with a large volume collapse (${\rm\Delta} V/V$ = 0.16), which is associated with the valence change of the Pr ion. The two 4{\it f} electrons in the Pr ion is supposed to be itinerant in the Pr-IV phase. In order to investigate the electronic state of the phase IV, we performed the high pressure electrical resistance measurement using the diamond anvil cell up to 32 GPa. In the Pr-IV phase, the temperature dependence of the resistance shows an upward negative curvature, which is similar to the itinerant 5{\it f} electron system in actinide metals and compounds. This suggests the narrow quasiparticle band of the 4{\it f} electrons near the Fermi energy. A new phase boundary is found at $T_{0}$ in the Pr-IV phase. From the temperature and magnetic field dependences of the resistance at 26 GPa, the ground state of the Pr-IV phase is suggested to be magnetic. Several possibilities for the origin of $T_{0}$ are discussed.