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European Commission

Short description of the work
The aim of this study is to investigate UxZr(1-x)SiO4 and ThxZr(1-x)SiO4 solid solutions regarding their mixing behavior. Complete solid solutions between USiO4 and ThSiO4 have been shown to be possible [1-3] and lattice parameters in the UxTh(1-x)SiO4 system exhibit behaviour according to Vegard's Law. ZrxHf(1-x)SiO4 solid solutions have also been described in detail recently [4] and exhibit a negative excess volume of mixing, indicator for non-ideal mixing behaviour. To the best of our knowledge neither has the successful synthesis of UxZr(1-x)SiO4 solid solutions nor that of ThxZr(1-x)SiO4 been reported so far. First principle calculations by Ferriss et al. [5] suggest that there is a miscibility gap between USiO4 and ZrSiO4 and ThSiO4 and ZrSiO4 (calculated for 500 K and 1000 K). For the HfSiO4 and ZrSiO4 system their paper proposes near ideal mixing with a weak tendency for phase separation. The latter was later confirmed by Cota et al [4].

Stoichiometric solid ThxZr(1-x)SiO4 and UxZr(1-x)SiO4 samples were prepared from sol-gel synthesis through a hydrothermal procedure with the aim to obtain well defined material for XRD characterization and subsequent solubility experiments. The ThxZr(1-x)SiO4 (x=0.1, 0.2,…,0.9) solid solution series as well as ZrSiO4 was successfully synthesized and characterized prior to measurement by XRD, IR, and Raman. The data show signs for complete miscibility. Refinement of the lattice parameters from XRD data showes that the ThxZr(1-x)SiO4 solid solutions follow Vegard’s rule but contain a positive deviation in excess volume of mixing.

EXAFS was used to study the coordination of Zr(IV) and Th(IV) to investigate the local structure. Results received from bulk material will be compared to the findings from colloidal samples [1]. The near-order structure in ThxZr(1-x)SiO4 was investigated by using the Zr K-edge (E0 = 17996 eV) and Th L3-edge (E0 = 16300 eV) for EXAFS spectroscopy. From the Th L3 edge Th-Zr and Th-Th distances and at the Zr K edge Zr-Th and Zr-Zr distances were obtained for the solid solutions. The Th/Zr ratio obtained from the XANES edge-step was used to determine the mole fraction x. The obtained values are confirmed by SEM-EDS analysis. Reference EXAFS spectra of ZrO2(mkl), ZrO2(tetr) and ZrO2(cub) were measured to indicate the ZrO2 content quantitatively and a sample series with well-defined content of ZrSiO4 and ZrO2(mkl) was included to verify the capacity of quantitative analysis.

References
[1] L.H. Fuchs, E. Gebert, Amer. Min. 43, (1958), 243-245.
[2] D. Costin et al., Inorg. Chem. 50 (21), (2011), 11117-11126.
[3] S. Labs et al., Envir. Sci. Tech. 48 (1), (2014), 854-860.
[4] A. Cota et al., J. Phys. Chem. C 113 (2013), 10013 – 10019.
[5] E.D.A. Ferris et al., Amer. Min. 95 no. 2-3 (2010), 229-241.

Main visitor contact data
Name: Sabrina Labs
Organisation: FZJ

JRP Identification
JRP nr: TALI-C02-05
JRP title: Investigation of UxZr(1-x)SiO4 and ThxZr(1-x)SiO4 Solid Solutions: colloid formation and solid state characteristics
JRP scope: Scope 2: Actinide in the geological environment

Visited Associated Pooled Facility
Visited APF during the stay: HZDR - ROBL
Name of the APF Contact Person: Andreas Scheinost

Other APF and organisation involved in the JRP
Other organisations involved:
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Description of the work done at the associated pooled facility
Start date of the stay: 8/28/2014
End date of the stay: 9/2/2014
Quantity of access: 4
Access Unit: Days
Misc.: Work during week-ends

Other APF visitors of the JRP during the stay
Visitor 2: Stephan Weiss (HZDR)
Visitor 3:
Visitor 4:
Visitor 5: