Site-selective time resolved laser fluorescence spectroscopy of Eu and Cm doped LaPO₄

Abstract

Samples of LaPO 4 doped with Eu 3+ or Cm 3+ were synthesized by a hydrothermal process which resulted in a solid solution at temperatures less than conventional processing. Time resolved laser fluorescence spectroscopy was used to probe the incorporated Eu 3+ or Cm 3+ in order to gain structural information on its local environment. This revealed that Eu 3+ and Cm 3+ incorporate on the La site as expected. The emission spectrum of Eu 3+ resolves the fully degenerate 5-fold splitting of the peaks in the F 2 transition due to the low symmetry of the site, confirming previous calculations. A minor site in the Eu 3+ doped sample is identified as coordinated with hydroxide contamination. Direct excitation of Cm 3+ doped samples show the presence of "satellite" species. Although these spectral features have been observed in Cm 3+ doped LuPO 4 and YPO 4 , this is the first time that these satellites are resolved into their individual species. These are hypothesized to be due to a disturbance in the ideal structure which creates a break in the equivalence of the four lanthanum sites within a unit cell. The 4-fold ground state splitting of all species is identical, although slightly shifted, indicating similar environments. The fluorescence lifetimes were long (1.2 ms for Cm and 3.6 ms for Eu) indicating an absence of water in the immediate coordination sphere due to the incorporation of the doping ion.

New insights into phosphate based materials for the immobilisation of actinides

“…In contrast, studies on actinide-bearing monazite materials are rather rare. As expected based on the surrogate studies, Pu - [57,58], Am - [59][60][61], Cm - [62][63][64] as well as Cf-, Bk-, and Es-orthophosphates [65] were reported to crystallise in a monoclinic monazite structure because the ionic radii of these actinides fit into the range between La and Gd [36]. The formation of Pu III PO 4 [61,66,67] monazite gained particular interest because it was synthesised in absence of additional reactants to reduce the oxidation state of Pu IV to Pu III during reaction.…”

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4)

“…The safe disposal of high-level radioactive wastes (HLW) originating from various sources such as nuclear power plants, reprocessing facilities, or dismantled nuclear weapons will require durable matrices for their immobilization over long time periods. Especially for the immobilization of specific waste streams containing minor actinides (Cm 3+ , Am 3+ ) or plutonium (Pu 3+,4+ ), crystalline ceramic materials have been envisioned as host matrices due to their high radiation tolerance, − high dopant loading, − and chemical durability. − Such ceramics include lanthanide phosphates (LnPO 4 ) crystallizing in the monazite structure, which has been shown to incorporate varying compositions of several trivalent actinide and lanthanide elements in well-defined atomic positions within the crystal lattice. − Furthermore, the incorporation of tetravalent actinides in coupled substitution reactions preserving charge neutrality within the phosphate ceramic structure have been demonstrated, extending the range of radionuclides present in specific radioactive waste streams that could be incorporated within the monazite matrix. − The existence of very old natural analogues containing high concentrations of uranium and thorium show that the crystalline phosphate structure is very tolerant toward self-irradiation damages as well as chemical weathering. ,,, This high radiation tolerance and low solubility in aqueous media have been further confirmed in several studies using both pristine monazites , as well as monazites codoped with various inactive lanthanide analogues or actinide elements, − speaking for their use as host materials for the immobilization of radioactive waste streams.…”

“…For rhabdophane matrices, there are no studies addressing actinide incorporation within the solid structure from the dopant point of view. Thus, our study aims at generating new data on actinide incorporation in rhabdophane solid-solutions and to complement already existing data for lanthanide-doped monazite phases ,, with data obtained for an actual actinide element. Here, the use of Cm 3+ , with outstanding luminescence properties, can provide new insight into the number of species in the studied solid-solutions and strong indications for the order or crystallinity of the solid phase under investigation, which are properties that are not accessible to the same extent with the more commonly used luminescent lanthanide Eu 3+ .…”

Combining luminescence spectroscopy, parallel factor analysis and quantum chemistry to reveal metal speciation – a case study of uranyl(vi) hydrolysis

“…It was successfully applied to Cm( iii ) and Eu( iii ) cryo spectroscopy. 23 , 24 However, inhomogeneous broadening leads to a smearing of excitation spectra at room temperature and interpretation becomes difficult.…”