Porträt Prof. Dr. Kvashnina, Kristina; FWOS

Prof. Dr. Kristina Kvashnina

Head of Department "Molecular Structures"
Responsible for the BM20 (ROBL) beamline at ESRF
Phone: +33 476 88 2367

Restart News
September 29, 2020
ROBL-II is fully operational again!

Gerber, E. et al., Nanoscale 2020, 12, 18039-18048.

The structure of PuO2 nanoparticles: a multimethod approach

Plutonium-containing colloids can be transported with groundwater across km-long distances, and are hence a potential risk for the safety of radioactive waste repositories.

The structure and oxidation state of PuO2-like nanoparticles have been extremely controversially discussed in the past.

We shown now by a combination of different synchrotron methods that very different synthesis routes lead to plutonium nanoparticles that have a uniform particle size distribution of about 2 nm, contain only tetravalent Pu, and are structurally identical to macroscopic cubic PuO2.

The structure of PuO2 nanoparticles ©Copyright: Dr. habil. Scheinost, Andreas  

Kvashnina, K. et al. Angewandte Chemie 2019, 58, 17558-17562.

A novel pentavalent plutonium solid phase on the pathway from aqueous Pu(VI) to PuO2 nanoparticles

We provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4PuO2CO3, which is stable over a period of several months. For the first time, state-of-the-art experiments at Pu M4 and at L3 absorption edges combined
with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.

Kvashnina_2019_Pu(V)carbonate ©Copyright: Dr. habil. Scheinost, Andreas

T. Dumas et al., ACS Earth Space Chem. 2019, 3, 2197-2206.

Plutonium retention by coprecipitating magnetite: Kinetic entrapment versus sorption

Pu(III) can be incorporated by magnetite, by creating a pyrochlore-like local cluster within the magnetite structure. The maximum amount of incorporated Pu is 50%; and with time/recrystallization, this amount decreases to 33%. Surface complexation seems to be thermodynamically more import. Incorporation seems to proceed through kinetic entrapment

Plutonium retention by coprecipitating magnetite ©Copyright: Dr. habil. Scheinost, Andreas

H. Rojo, A. C. Scheinost, B. Lothenbach, A. Laube, E. Wieland and J. Tits, Dalton Transactions 2018, 47, 4209-4218.

Retention of selenium by concrete barriers

Structure and sorption of an elusive anion, HSe-

HSe- is barely sorbed by most minerals, posing a significant risk for the safe disposal of this long-lived fission product. The cement barrier foreseen for intermediate-level waste repositories, however, contains AFm phases, which due to their anion exchanging properties may be able to retain HSe-1.

Retention of selenium by concrete barriers ©Copyright: HZDR/Scheinost

Here we could show that HSe- is in fact significantly sorbed in the interlayers of AFm-HC.

HSe- is lesser sorbed by a similar phase, AFm-MC, due to the lesser accessibility of its interlayer space.

A. Amon, A. Ormeci, M. Bobnar, L. G. Akselrud, M. Avdeev, R. Gumeniuk, U. Burkhardt, Y. Prots, C. Hennig, A. Leithe-Jasper and Y. Grin, Accounts of Chemical Research 2018, 51, 214-222. IF 20.3

Cluster formation in the superconducting intermetallic Be21Pt5

Solving the crystal structure of this new superconductor (Tc = 2.1 K)

X-ray diffraction: localization of the heavier Pt atoms

Neutron diffraction: localization of the light Be-atoms (differential Fourier syntheses)

Analysis of chemical bonding in Be21Pt5 by electron localizability indicator (ELI)

Cluster formation in the superconducting intermetallic Be21Pt5 ©Copyright: HZDR/Scheinost

Gregson, M., Lu, E., Mills, D.P., Tuna, F., McInnes, E.J.L., Hennig, C., Scheinost, A.C., McMaster, J., Lewis, W., Blake, A.J., Kerridge, A. and Liddle, S.T. (2017) The inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes. Nature Communications 8, 14137. IF 12.1

Principles of chemical bonding:
Trans-configuration a general rule for lanthanides and actinides?

Principles of chemical bonding:Trans-configuration a general rule for lanthanides and actinides? ©Copyright: HZDR/Scheinost

Butorin, S.M, Kvashnina, K.O., Prieur, D., Rivenet, M., Martin, P.M. (2017) Chem. Comm. 53, 115.

Towards the safe disposal of spent nuclear fuel:
Charge compensation mechanisms in Ln-doped UO2

Ln-doped UO2 as model for spent nuclear fuel with lanthanide fission products

High-resolution M4-edge spectroscopy crucial to resolve electronic structure of actinides:

  • Charge of LnIII is compensated for by equimolar formation of UV
  • Significant change of bond covalency, from Mott-Hubbard in undoped UO2 to U 5f-O 2p hybridization in Ln-doped UO2
Towards the safe disposal of spent nuclear fuel: Charge compensation mechanisms in Ln-doped UO2 ©Copyright: HZDR/Scheinost

Fröhlich, D.R., et al., Inorganic Chemistry, 2017, 56, 6820-6829.

Deciphering Am speciation in presence of formate

Deciphering Am speciation in presence of formate ©Copyright: HZDR/Scheinost

Challenge for EXAFS, since small spectroscopic variations (< 9%) in pH-range 2 to 4

Combined DFT and advanced statistical analysis of EXAFS (ITFA):

  • confirmed thermodynamic constants
  • deciphered structure of the two (monodentate) 1:1 and 1:2 Am-formate complexes

C. Hennig, S.Weiss, W. Kraus, J. Kretzschmar, A. C. Scheinost, Inorg. Chem. 2017, 56, 2473-2480

Solution species and crystal structure of Zr(IV) acetate

Solution species and crystal structure of Zr(IV) acetate ©Copyright: HZDR/Scheinost

The complex formation of zirconium with acetic acid was investigated with Zr K-edge EXAFS

spectroscopy and single crystal diffraction. Zr K edge EXAFS spectra show that a stepwise

increase of acetic acid in aqueous solution with 0.1 M Zr(IV) leads to a structural rearrangement

from initial tetranuclear hydrolysis species [Zr4(OH)8(OH2)16]8+ to a hexanuclear

acetate species Zr6(O)4(OH)4(CH3COO)12.