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

¹ Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, U.S.A.
² Jet Propulsion Laboratory, MS-183-301, 4800 Oak Grove Drive, Pasadena, California 91109-8099, U.S.A.

Correspondence: ^* E-mail: hyang{at}u.arizona.edu

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 P symmetry with unit-cell parameters a = 12.4112(2), c = 9.8511(3) Å, and V = 1314.2(1) ų, in contrast to the space group P 2c and a 7.10, c 9.76 A, and V 430 ų 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.

Key Words: Bästnasite • hydroxyl-bästnasite-(Ce) • single-crystal X-ray diffraction • crystal structure • Raman spectra