EXAFS Studies of the Am3+ AND Cm3+ AQUO IONS


EXAFS Studies of the Am3+ AND Cm3+ AQUO IONS

Funke, H.; Stumpf, T.; Rossberg, A.; Hennig, C.; Reich, T.

Besides plutonium, the long-lived isotopes of americium and curium play a crucial role for long-term performance assessment of nuclear waste repositories. In order to predict the mobilization and retardation of these nuclides, it is necessary to know the structures of the inorganic and organic actinide species that are involved in the process of migration. Thereby, the characterization of the Am3+ and Cm3+ aquo ions forms the base for further investigations.
In the literature only few measurements can be found of co-ordination numbers of trivalent actinides in aqueous solutions. By using UV/Vis spectroscopy, a co-ordination number of 9 was calculated for Am3+ aq [1]. The complexation behavior of curium is often investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS) [2,3]. In analogy to trivalent lanthanides, a co-ordination number of nine is assumed for Cm3+aq. An extended X-ray absorption fine structure (EXAFS) spectroscopy investigations of Am3+ and of Cm3+ in 0.25 M HCl obtained co-ordination numbers of 10.3(0.3) and 10.2(0.3), respectively [4].
In order to determine a reliable reference value for the co-ordination number of Cm3+ and Am3+ aquo ions, the samples used in our EXAFS measurements were prepared in non-complexing 1.0 M perchloric acid. High actinide concentrations (1 x 10-3 mol/L) were used in order to achieve low-noise EXAFS spectra, which enable a precise determination of bond lengths and co-ordination numbers of the Am3+ and Cm3+ aquo ions.
The experiments were performed at the Rossendorf beamline (ROBL) at the ESRF in Grenoble [5]. The radiochemistry experimental station at ROBL has been designed to perform X-ray absorption fine structure (XAFS) spectroscopy of solid and liquid non-sealed radioactive samples [6]. EXAFS spectra were collected in transmission mode at the americium LIII edge at 18504 eV (E0=18520 eV) and at the curium LIII-edge at 18970 eV (E0=18990 eV), respectively. Zr and Nb metal foils were measured and the first inflection point of the X-ray absorption spectra (Zr K edge at 17998 eV, Nb K edge at 18986 eV) were used for energy calibration. EXAFS data treatment and fit were done using the EXAFSPAK program package [7]. Parameters for the backscattering phase and amplitude, the mean free path, and the reduction factor were calculated using the FEFF8 code [8]. Since no suitable Am or Cm reference compound was available for the measurements, the EXAFS spectra were fitted with constant S02 equal to 0.9 and 1.0. Two different sets of atomic clusters were taken as the basis for the phase and amplitude calculations. On the one hand the solid state structure of the curium dioxides was used [9] with a co-ordination number of eight and on the other hand the structure of the tetraaquatrinitratoeuropium(III)-dihydrate [10]. This compound shows the curium-homologous lanthanide Eu3+ as central atom, which is surrounded by ten oxygen atoms. Both clusters lead to nearly the same results. Dirac-Fock potentials were used for the core of the central atoms in the FEFF calculations. The EXAFS spectra of the Am3+ and Cm3+ aquo ions using the Eu cluster [10] are shown in figure 1. One shell is uniquely defined in both cases. The corresponding EXAFS structural parameters are collocated in table 1.
Figure 1. Raw Am3+ (upper graph) and Cm3+ (lower graph) LIII-edge k3-weighted EXAFS spectra and corresponding Fourier transforms. (Solid line - experiment; dashed
line - theoretical fit.)

Table 1. EXAFS structural parameters for the Am3+ and Cm3+ aquo ions. The standard deviations as estimated by EXAFSPAK are given in parenthesis.
Sample Shell N (S02=0.9) N (S02=1.0) R [Å] s2·103 [Å2] ?E [eV]
Am3+ Am-O 10.4(0.3) 9.3(0.3) 2.47(0.02) 9.3(0.3) -12.6(0.3)
Cm3+ Cm-O 9.3(0.3) 8.4(0.3) 2.46(0.02) 7.5(0.4) -22.3(0.4)

The measured bond distances, R±0.02 Å, are the same for the Am3+ and Cm3+ aquo ions and agree with the results of [4]. The co-ordination number, N±1, is somewhat higher for ...

  • Lecture (Conference)
    32. Journées des Actinides, 2002, Ein-Gedi, Israel, 15.3.-23.3.2002

Permalink: https://www.hzdr.de/publications/Publ-4391
Publ.-Id: 4391