M. Genet

Synthesis and crystal growth of some thorium and uranium tetrahalides: ThCl4, ThBr4, UCl4 and UBr4

A kilogram scale production of thorium and uranium tetrachloride and tetrabromide in polycrystalline form has been developed. Crystal growth of single crystals of several cm3 in size using the Bridgman method is also described. Some physical properties of these tetrahalides published elsewhere, such as crystalline structure, refractive index, luminescence, vibrations modes, and phase transition are briefly mentioned. The optical properties of ThBr4 and ThCl4 and the use of these materials as a host matrix for spectroscopic studies of tetravalent actinide ions is also reported.

SPECTRUM AND ENERGY LEVELS OF Pr3+ IN ThBr4

The strong features in the absorption spectrum and the laser excited fluorescence spectrum have been interpreted as arising from levels of Pr3+ in the D2d symmetry site of ThBr4. 43 energy levels have been fitted to the parameters with an rms deviation of 61 cm−1. The values of the crystal field parameters are B20=260.0, B40=−644.2, B44=929.2, B60=1089.0 , and B64=240.6 cm−1. The presence of other crystal symmetry sites is observed.

Solubility of thorium phosphate-diphosphate

The solubility of the pure thorium phosphate-diphosphate, Th4(PO4)4P2O7, has been measured in 0.1 M NaClO4 solutions by PERALS spectrometry (photon electron rejecting alpha liquid scintillation), ICP-MS (inductively coupled plasma-mass spectrometry) and PIXE (particle induced X-ray emission). The results obtained by the three methods are in good agreement and show that the total concentration of thorium in solution is mainly controlled by the precipitation of two compounds: Th(HPO4)2 in acidic media (pH⩽4.5) and Th(OH)4 in basic and near-neutral media.

Synthesis of some new thorium phosphates

Abstract Some thorium phosphates have been synthesised: caesium–thorium phosphate, barium–thorium phosphate, thorium fluoride phosphate, thorium sulphate phosphate, and thorium silicate phosphate, starting from aqueous solutions. Their chemical composition was established by Electron Probe Microanalysis (EPMA) and/or by Particle Induced X-ray Emission (PIXE). The X-rays diffraction patterns and/or the infrared spectra are given for these series of thorium phosphate.

Optical spectra of Pa4+ diluted in ThCl4 single crystals

Absorption and emission spectra of Pa4+ in ThCl4 were recorded at different temperatures varying from 4.2 to 300 K. The seven‐crystal field states were measured experimentally and fitted to the spin‐orbit and crystal field parameters resulting from D2d symmetry. The obtained parameter values (in cm−1) are F0 = 3658, ξ = 1525, B20 = −1373, B40 = 1725, B44 = −2487, B60 = −2290, and B64 = −53. Emission occuring at 4915 and 4078 cm−1 was used to determine the 2F5/2 low lying levels.

Correlation between 31P NMR and X-ray diffraction data in the refinement of the atomic positions of the thorium phosphate-diphosphate crystal structure

Abstract In the framework of the long term repository of radwastes, the thorium phosphate-diphosphate, Th 4 (PO 4 ) 4 P 2 O 7 , appears as a promising material for the immobilization of tetravalent actinides. In the way of a better characterization of this matrix, the average crystal structure was reconsidered on the basis of both 31 P NMR and X-ray diffraction experiments. The NMR 1D and 2D results obtained suggested the existence of two local configurations in the same proportion for the diphosphate group. In correlation with this position disorder of the P 2 O 7 groups, the electronic environments of the phosphorus atoms in the PO 4 tetrahedra are different, involving the splitting of each resonance line observed by NMR. The local crystal structure was refined and the results obtained are in good agreement with the structural data of phosphate compounds.

Transparent Gel and Xerogel of Thorium Phosphate: Optical Spectroscopy With: Nd3+,Eu3+,Cr3+ and Rhodamine 6G,

Chemical conditions for thorium phosphate gel preparation have been determined. The transparency is of good optical quality and the gel is very stable for a long time. Under drying condition, this gel can give rise to the xerogel which is still transparent. We can also prepare this xerogel by simple evaporation at room temperature of a very concentrated solution of thorium phosphate. From this viscous medium, the xerogel can be obtained in various kinds of shapes : threads, slabs and blocks. Solidification time depends on the final volume desired and spreads from few minutes to several weeks. Absorption spectrum of pure gel and xerogel have been recorded. Gel and xerogel doped with very well known probes like Nd3 and Er3+ were examined to compare their optical properties with aqueous medium of the same chemical composition. Eu3+ doped gel and xerogel were also studied using their fluorescence properties. The optical properties of Cr3 in doped gel and xerogel allowed us to determine the kinetics of hydration sphere modification during the drying period. Finally, as xerogel synthesis takes place at room temperature, fragile organic dye can be used as dopant, so Rhodamine 60 absorption and emission spectra have been studied in these conditions.When, at that time, the xerogel is doped with Coumarin 460 and Th3+,an energy transfer is observed between dye and Th3+ ions, which contributes to enhance the fluorescence of Th3+ ions. Eu3+ behaves similarly. In conclusion, gel and xerogel of thorium phosphate tested with usual probes such as 3d, 4f ions and dyes seem to be very promissing matrices.

Infra-red emission spectrum of ThBr4:U+4

Abstract Emission spectrum of ThBr4:U+4 single crystals has been investigated up to the infra-red region (4000–11 000 cm−1) excitation being done at 4 K with 5145 A argon laser line. Vibrational analysis has been used to find zero phonon lines, according to previous results obtained on absorption spectrum.

Intrinsic photoluminescence of pure β-ThBr4 single crystal

Abstract The investigation of the intense blue photoluminescence of β-ThBr4 single crystal has been carried out by optical studies. Absorption and emission spectra versus temperature are presented. The thermal quenching of the blue photoluminescence has been performed by measuring the temperature dependence of the fluorescence lifetime. The analysis of these data yields activation energy value of 3622 cm-1. Using the time-dependent theory, the experimental low temperature emission profile has been simulated. The data are interpreted in terms of a charge transfer process between the bromine 4p levels and the thorium 5f-6d levels. A single configuration-coordinate, quadratic coupling model is proposed to explain the observed decay time thermal dependence.

Synthesis and characterization of uranium bromide phosphate UBrPO4.2H2O and uranium hydroxide phosphate U(OH)PO4

Abstract Uranium chloride phosphate tetrahydrate UClPO 4 ·4H 2 O was obtained by mixing uranium (IV) hydrochloric solution and concentrated phosphoric acid [1]. From crystal structure studies its formula was determined as dihydrate [2]. Using the same method, i.e. starting from uranium (IV) hydrobromic solution and H 3 PO 4 , two crystal forms of a new compound, uranium bromide phosphate UBrPO 4 ·2H 2 O were synthesized. Their XRD patterns, UV-visible and infrared spectra are presented in this paper. The hydrolysis process of the chloride and bromide phosphates leads to the amorphous uranium hydroxide phosphate U(OH)PO 4 ·6H 2 O.