Pressure-induced valence instability in praseodymium metal and dilute LaPr and YPr alloys: a counterpart of the cerium case

An intriguing phase transformation occurs in praseodymium metal under a pressure of approximately 200kbar. The transition is accompanied by dramatic resistance anomalies similar to those at the γ/α transition in cerium. The atomic volume decreases by 19%. This volume collapse is therefore quite comparable to the one at the Ce γ/α transition. The lattice symmetry changes from orthorhombic to hexagonal in (slight) contrast to the isostructural transition in Ce. The depression of the superconducting Tsub(c)(P) of lanthanum due to the presence of Pr impurities shows a resonance curve - like maximum in the same pressure range in close analogy to Ce-doped systems at lower P. Recent superconductivity data for very dilute YPr alloys reveal a similar dramatic increase in pair-breaking power above 200 kbar. This latter result is particularly interesting since it proves that the phenomenon occurs in single Pr impurity atoms even if they are imbedded in a non-f-level host matrix. The increased direct overlap of the 4f2 wave functions in pure Pr under pressure may therefore not be the decisive cause for the electronic phase transition. These experiments provide convincing evidence for a pressure-induced 4f2 shell instability in pure Pr as well as in dilutely dissolved Pr atoms. This new 4f-shell instability has remarkably common features with the well-known valence instability of Ce at lower P. One can therefore hope that the present findings will help to settle the still puzzling Ce problem. Finally, the discovery opens the perspective that several other light rare earth metals (or perhaps even all rare earth metals) may undergo similar transitions if they are subjected to pressures up into the megabar range. (Auth.)