On the presence of OH defects in the zircon-type phosphate mineral xenotime, (Y,REE)PO4

The infrared (IR) spectra of gem-quality xenotime crystals containing considerable amounts of rare earth elements (REEs), are characterized by sharp and strongly pleochroic absorption bands in the 3650-3350 cm(-1) region. In contrast, the spectra of partially metamict samples are dominated by a broad band centered at around 3450 cm(-1). Xenotime presents the interesting case of a nominally anhydrous mineral, where the OH stretching frequency region of weakly hydrogen-bonded OH groups is overlapped by absorption bands due to low-energetic f-f electron transitions of REEs, especially of dysprosium. In polarized spectra measured parallel to the c-axis, Dy shows a prominent sharp band at 3519 cm(-1). The assignment of the REE bands is based on the polarized IR spectra of REE doped xenotime single crystals, which have been synthesized by the flux method. A single band at 3480 cm(-1), strongly polarized perpendicular to the c-axis, is assigned to the stretching vibration of an OH group. Deuteration experiments performed at 950 degrees C prove the assignment of this band and the presence of additional structural OH groups, appearing at annealing temperatures above 500 degrees C. Models of the OH point defect incorporation into the crystal structure of xenotime can be derived on the basis of fully occupied cation sites and under the assumption of Y- and P-site vacancies. The water content of the gem-quality samples ranges from 5 to 10 wt ppm and for the partially metamict samples from 370 wt ppm to 1.7 wt% H2O.