OH defects in synthetic xenotime (YPO₄)

The polarized infrared spectra of hydrogen-containing xenotime (YPO₄) were measured to obtain information on its OH defects. Sample syntheses were conducted at 1.0 GPa using a piston-cylinder apparatus under water-saturated conditions. By lowering the temperature at a rate of 10°C/h from 1200 to 800 °C, xenotime single crystals up to ~300 μm in length were obtained. A sharp OH stretching vibrational band was observed at 3302 cm⁻¹. Using a molar absorption coefficient of 111 217 L mol⁻¹ cm⁻², the H₂O concentration in the sample was estimated as 20±10 ppm. The OH band showed strong infrared absorption perpendicular to the c-axis and weak absorption parallel to the c-axis. According to electrostatic considerations and assuming the presence of Y³⁺ or P⁵⁺ vacancy, possible orientations of the O–H bonds were estimated. We propose that the OH incorporation mechanism in synthetic xenotime is controlled by Y³⁺ vacancy $(YY×↔V‴Y+3Hi•)$⁠.