X-ray spectroscopic study of the electronic structure of monazite- and xenotime-type rare-earth phosphates

Crystalline solid-state oxides that exhibit resistance to radiation-induced structural damage are being considered as materials for the immobilization of waste actinide elements. In this regard, compounds adopting the monazite (REPO₄; RE = La to Gd) or xenotime (RE'PO₄; RE' = Tb to Lu and Y) structures are being investigated for this application. Both of these structures coexist in natural samples which leads to the formation of solid solutions. In this study, multiple series of solid solutions (La _1-xYb_xPO₄, La_1-xY_xPO ₄, and Sm_1-xHo_xPO₄) were synthesized and investigated by powder X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS). XANES and XPS were performed to examine the influence of crystal structure on spectra. Examination of P K- and P L_2,3-edge XANES spectra indicated that a positive shift in absorption energy and change in line shape occur with an increase in the concentration of xenotime structure. Analysis of Sm and Ho M₅-edge XANES spectra showed that the spectral intensity changes because of a charge-transfer mechanism. The XPS spectra exhibited only a minor shift in binding energy depending on the structure. It has been established in this study that XANES can distinguish between samples that adopt the monazite or xenotime structure.