OH DEFECTS IN NON-METAMICT XENOTIME
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  or  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.