Structural characterisation of metastable Tb- and Dy-monazites

Abstract 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 Ln PO 4 monazites ( Ln =  La–Gd). Infrared spectroscopic investigations on TbPO 4 and DyPO 4 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 TbPO 4 , continuing the general linear correlation between the Raman shifts and Ln 3+ radii observed for the lighter Ln PO 4 monazites. In contrast, the Raman spectrum of DyPO 4 was severely biased by fluorescence effects.