X-ray Spectroscopic Study of the Electronic Structure of Monazite- and Xenotime-Type Rare-Earth Phosphates

Abstract

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 (La1–x Yb x PO4, La1–x Y x PO4, and Sm1–x Ho x PO4) 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 L2,3-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.

Cerous phosphate gels: Synthesis, thermal decomposition and hydrothermal crystallization paths

“…Solution 1 containing only 85% H 3 PO 4 was heated at 100°C for 1 h. Solution 2 was prepared by mixing 85% H 3 PO 4 and 65% HNO 3 (the H 3 PO 4 :HNO 3 molar ratio was chosen to be equal to described in [29]) followed by heating at 100°C for 1 h. Nitric acid was added to solution 2 to initiate the transformation of oligophosphoric acids, which could exist in the solution, into orthophosphoric acid. A comparative analysis of the orthophosphate anion contents in solutions 1 and 2 showed that solution 1 contained 77.5 ± 0.3 wt.% PO 4 3− and solution 2 contained 82.4 ± 0.4 wt.% PO 4 3− (confidence intervals are given for P = 95%); the latter value corresponded to the PO 4 3− content in pure 85% H 3 PO 4 (82.4%). The difference between these two solutions was 4.9%, which indicated the presence of oligophosphoric acids in the 85% solution of phosphoric acid heated to 100°C for 1 h. These data allowed us to conclude that a comparatively low content of oligophosphate anions can form in hot orthophosphoric acid during the dissolution of cerium dioxide.…”

“…[21,22]), cerous phosphate gels contained uncontrolled amounts of orthophosphoric acid and inorganic anions and cations (e.g. NO 3 − , SO 4 2− and NH 4 + ), which inevitably affected the results of respective studies of their properties and crystallization behaviour. The synthesis and purification method described in this paper provided a means of specifying the composition of cerous phosphate gels in a more flexible manner.…”

“…Trivalent REE orthophosphates are among the most extensively studied REE compounds [2][3][4][5][6][7]. They are of interest as X-ray and gamma-ray scintillators, thermophosphors for remote temperature measurements, biomaterial hosts, and so on [8][9][10][11].…”

“…Larsen and Cilley also found that Ce +4 phosphates are cation-exchange materials that can exchange hydroxonium ions for alkali metal ions (Li + , Na + , K + ). The systematic studies of PO 4 3 − / Ce +4 system [15] allowed the following four different Ce +4 orthophosphates to be separated: amorphous cerium orthophosphate with the molar ratio P:Ce~1.7, cerium orthophosphate with P:Ce~1.15 containing up to~7% of sulfate ions, cerium orthophosphate with P:Ce = 1.5, and fibrous cerium orthophosphate (suggested composition is Ce(HPO 4 ) 2 ·H 2 O). The last compound was then investigated thoroughly by Barboux et al [16].…”

Theoretical and Experimental Study of the Crystal Structures, Lattice Vibrations, and Band Structures of Monazite-Type PbCrO4, PbSeO4, SrCrO4, and SrSeO4

“…1,2,6,7,12 For other compounds, like SrCrO 4 , PbSeO 4 , and SrSeO 4 , their crystal structures have been experimentally determined, but few of their Raman-active modes have been measured. In the case of SrCrO 4 , attempts have been made to determine the electronic band gap, but values ranging from 2.4 to 2.9 eV can be found in the literature. 13 −15 Ab initio calculation has shown to be an accurate method to study the physical and chemical properties of CaSeO 4 and compounds related to the monazites.…”