Structural characterisation of metastable Tb- and Dy-monazites

Metastable Ln-orthophosphates (Ln = Tb, Dy) with monoclinic monazite-type structure were obtained via a precipitation route in aqueous solutions and subsequent thermal treatment. The lattice parameters and fractional coordinates of the atoms in the crystal structures were determined by Rietveld refinement of X-ray diffraction data. The lattice parameters of the metastable monazites obtained here fit well into the linear trends observed for stable LnPO(4) monazites (Ln = La-Gd). Infrared spectroscopic investigations on TbPO4 and DyPO4 with monazite structure confirmed the expected shift of the modes to higher wavenumbers with increasing atomic number and decreasing cation radius. Raman spectra of the metastable Tb- and Dy-monazites were obtained for the first time, revealing a typical monazite spectrum for monoclinic TbPO4, continuing the general linear correlation between the Raman shifts and Ln(3+) radii observed for the lighter LnPO(4) monazites. In contrast, the Raman spectrum of DyPO4 was severely biased by fluorescence effects.