Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs

Rare-earth phosphates with the general formula REEPO4·nH2O belong to four distinct structural types: monazite, rhabdophane, churchite, and xenotime. We report herein the first direct comparison between vibrational spectra of these compounds for the same metal cation i.e. gadolinium. The four GdPO4·nH2O samples were prepared through wet chemistry methods and first characterized by X-ray diffraction. Three distinct spectral domains, associated to the deformation and stretching modes of phosphate tetrahedra (PO4) and to water molecules vibrations were then analyzed from FTIR and Raman data, and discussed regarding the structural characteristics of each sample. The most obvious differences between the spectra were associated to δ(H2O) and δs(PO4) modes and led to propose a simple method to rapidly and unambiguously discriminate the four polymorphs.