OH DEFECTS IN NON-METAMICT XENOTIME

Due to its high capacity for actinide elements and tolerance against radiation damage, xenotime has been considered as a candidate for nuclear waste storage form (Lumpkin, 2006; Oelkers and Montel, 2008). Natural metamict xenotime contains considerable amounts of hydrogen (370 ppm-1.7 wt%; Talla et al., 2011) compared to non-metamict one (5-10 ppm; Talla et al., 2011). The position and charge compensation mechanism for hydrogen atoms in the xenotime crystal lattice can be useful reference for understanding the radiation-damaged structure. In this study, we examine the polarized IR spectra of synthetic (= non-metamict) YPO₄xenotime and propose the possible position and incorporation mechanism of hydrogen. Besides, we compare the polarized IR spectra between natural and synthetic specimens and discuss the difference of hydrogen incorporation mechanism.

The samples were synthesized using a piston-cylinder apparatus at 1.0 GPa under H₂O-saturated conditions. By lowering the temperature from 1200 to 800°C at a rate of 10°C/h, xenotime single crystals of up to ca. 300 µm in length were obtained. A sharp absorption band was observed at 3302 cm^-1 and its infrared absorption was strongly polarized perpendicular to the c-axis. The H₂O concentration was determined to be 20 ± 10 ppm from the molar absorption coefficient of 111,217 L mol^-1 cm^-2. On the basis of the pleochroism and the configuration of the oxygen atoms in the xenotime structure (proposed by Ni et al., 1995), the orientation of the OH dipole was determined to be in either the [100] or [110] direction and the hydrogen incorporation controlled by Y³⁺vacancy was considered.

The OH band around 3302 cm^-1 was also reported in the polarized IR study on natural gem-quality (= apparently non-metamict) xenotime, though the OH band appeared only after the heat-treatment above 1000°C and disappeared above 1400°C. The natural pristine xenotime showed the OH band at 3480 cm^-1 and its IR absorption was strongly perpendicular to the c-axis. This band was related to substituted P⁵⁺-site by a charge compensating trace element (e.g., Si⁴⁺). Therefore, the majority of hydrogen in natural non-metamicted xenotime may be controlled by couple substitution with charge compensating trace elements.