Crystal structure and Raman spectrum of hydroxyl-bästnasite-(Ce), CeCO₃(OH)

Hydroxyl-bästnasite-(Ce), ideally CeCO₃(OH), had been regarded isostructural with bästnasite-(Ce), CeCO₃F, the dominant member of the bästnasite family that produces ∼70% of the world's supply of rare-earth elements. Using single-crystal X-ray diffraction and Raman spectroscopy, our structural analysis on hydroxyl-bästnasite-(Ce) shows that the previous assumption is incorrect. The crystal structure of hydroxyl-bästnasite-(Ce) possesses P6̄5 symmetry with unit-cell parameters a = 12.4112(2), c = 9.851](3) Å, and V= 1314.2(1) ų, in contrast to the space group A6̄2c and a ≈ 7.10, c ≈ 9.76 Å, and V ≈ 430 Å₃ for bästnasite-(Ce). Moreover, there are 6, 3, and 5 symmetrically-distinct CO₃ groups, Ce cations, and (OH/F) ions, respectively, in hydroxyl-bästnasite-(Ce), but 1, 1, and 2 in bästnasite-(Ce). The two structures, nevertheless, are similarly characterized by the layers of CO₃ groups alternating with the Ce-(OH/F) layers along the c direction. The Raman spectrum of hydroxyl-bästnasite-(Ce) is dominated by three strong bands at 1080, 1087, and 1098 cm^-1 in the CO₃ symmetrical stretching region, along with at least four bands in the OH stretching region. Our study further suggests that natural hydroxyl-bästnasite-(Nd) is most likely isotypic with hydroxyl-bästnasite-(Ce), rather than with bästnasite-(Ce), as previously proposed.