Elastic anomalies across phase transitions of praseodymium to 12 GPa

The elastic properties of praseodymium (Pr) were investigated at pressure up to 12 GPa at room temperature using the ultrasonic interferometry technique. Softening and discontinuities in compressional (P) and shear (S) wave velocities were observed at around 2.5, 6.5, and 10.5 GPa, which are interpreted as indications of the phase transitions from Pr-I (dhcp) to Pr-II (fcc), Pr-II (fcc) to Pr-III (d-fcc), and Pr-III (d-fcc) to Pr-VII (monoclinic or orthorhombic), respectively. Discontinuities in compressibility have been observed across the phase transitions of I-II and II-III of Pr by an analysis of pressure derivatives of the elastic moduli. A comparison with the elastic behaviors of other lanthanide elements (e.g., Ce and La) suggests that the elastic anomalies associated with these phase transitions are likely to be caused by the 6s-5d electron transfer and the softening of acoustic phonon mode facilitated by decreased atomic distances under pressure.