Solid-state absorption and Raman phonon spectroscopic investigations of PrCl<sub>3</sub>, PrBr<sub>3</sub>, and NdBr<sub>3</sub> under high pressure have been carried out at room temperature. These studies showed that the UCl<sub>3</sub>-type hexagonal form of PrCl<sub>3</sub> and PrBr<sub>3</sub> converts with the application of pressure to the PuBr<sub>3</sub>-type orthorhombic crystal structure. The absorption and Raman spectra obtained from PrBr<sub>3</sub> and NdBr<sub>3</sub> at pressures above ~260 kbar exhibit differences from spectra obtained at lower pressures, where they exhibit the PuBr<sub>3</sub>-type orthorhombic structure. These spectral differences suggest that a phase transformation might have taken place to an as-yet-unidentified crystal structure.
W. R. Wilmarth, G. M. Begun, R. G. Haire, J. P. Young, and J. R. Peterson, "The Effect of Pressure on the Raman and Absorption Spectra of PrCl3, PrBr3, and NdBr3," Appl. Spectrosc. 43, 1038-1045 (1989)