G. Baldinozzi, D. Simeone, D. Gosset, L. Lunéville, S. Surblé

1. Irradiation induced structural transformations in normal spinels, potential materials for nuclear waste management G. Baldinozzi, D. Simeone, D. Gosset, L. Lunéville, S. Surblé Matériaux fonctionnels pour l’énergie SPMS, CNRS - École Centrale Paris & DEN/DMN/SRMA, CEA Saclay Coll. Synchrotron Soleil & ESRF ISCSA TEM JANNUS Orsay & Saclay GANIL Caen • OUTLINE: • Scientific Context • Spinel structure • Experimental & Results • Model & Conclusion

2. Ceramics are complex materials • Understand their behavior at equilibrium (length scales) • Characterize the equilibrium properties • Nanostructured ceramics: mechanical properties • … and out of thermodynamic equilibrium (time frames) • Material in working conditions(massive production of defects, ...) • Modeling of the elementary mechanisms Defect engineering to control the material properties at different length scales and for different time frames ZrO2, HfO2, UO2+x TiC, ZrC, SiC Irradiation

3. Scientific aim • Why do we study the behavior of spinel structures under irradiation ? • To understand and to model the properties of ceramics kept far from their thermodynamic equilibrium • To study the elementary mechanisms and to forecast the ceramics behavior under irradiation • Test models, impact of chemical bonds • Driven alloys like in metals? Bragg William models ? • Industrial context • Radiation tolerant materials • Disposal and transmutation of high level waste

4. AB2O4 spinels A cation is tetrahedrally coordinated (divalent) B cation is octahedrally coordinated (trivalent) • Filled octahedra form criss-cross rows, with alternating layers of parallel rows offset as shown on the right side of the picture. The square holes enclosed by the rows of octahedra are filled with tetrahedra

5. Spinel Chemistry: chemical bond & site selectivity Charge density in slabs ELF = 0.6 in glyphs • Chemical bonds and site selectivity • Hybridization • Crystal Field • Then charge and size considerations kick in … MgAl2O4 ZnAl2O4 MgCr2O4

6. Order & disorder in AB2O4 spinels • “Normal” spinel structure: (A)[B2]O4MgAl2O4, ZnAl2O4, FeCr2O4, FeAl2O4, MgCr2O4, … • Violates Pauling’s rules as larger cation (Mg) in tetrahedral site A. Controlled by crystal field stabilization energies rather than simple packing geometry. Results in lower free energy configuration. • “Inverse” spinel structure: (B)[AB]O4(Fe3+)[Fe2+Fe3+]O4, (Fe2+)[Fe2+Ti4+]O4 • More standard, but still violates Pauling’s rules. Half of octahedral sites filled by larger A cation. • Annealing: nonconvergent cation disordering (Navrotsky & al, J Inorg Nucl Chem 1961 – O’Neill & al, J Phys Chem Minerals 1994) a spectrum of disordered spinels that ranges from normal to inverse

7. GIXRD • Above the critical angle, the instrumental broadening is reduced • SRIM calculations of ion implantation and damage profile allow to optimize the grazing angle • Microstructural information (lineshapes): • strain fields • size of diffracting domain • Structural information : • Phase identification, amorphous fraction • LRO - crystalline phases (atomic positions, site occupancies) • SRO - amorphous phases (bond distances, coordination number) MgCr2O4 • Radiation damage of a PWR simulated by ion irradiation (JANNUS) • The irradiated layer is rather thin (0.2 µm – GIXRD)

8. PSD Detector Equatorial Sollers Slits Monochromatoror parabolic mirror Beam 50µm*5mm Sample holder± 1µm, ± 0,02° Spinel irradiation at room temperature Simulation of neutron irradiation by low energy ions (cascades) and of fission products by swift heavy ions XRD X-ray diffraction: Asymmetric reflection setup(fixed, grazing impinging beam) Au @ 4 MeV

9. II) Structural evolution of spinels under irradiation Behavior of spinel under irradiation: Modification of Diffraction patterns

10. XRD before & after irradiation at room T • Structural information: • The (ooo) peaks depend on the cation distributions • In ZnAl2O4 no symmetry change • In the other two compounds the (ooo) peaks have nearly or totally vanished • The structural refinements provide the localization of the charge density in real space MgAl2O4 MgCr2O4 ZnAl2O4

11. Electron density from X-ray diffraction • Fourier syntheses derived from the observed diffracted intensities indexed in the Fd3m space group MgAl2O4 MgCr2O4 ZnAl2O4

12. Comparison between the three spinel structures • In ZnAl2O4 no symmetry change is detected and an increase of the inversion parameter occurs as a function of the ion fluence • Irradiation induces a cations exchanges between the tetrahedral (8a) and octahedral (16d) sites : isostructural phase transition similar to the phase transition observed in spinel out of irradiation • The charge density distribution is different in magnesium compounds • A and B occupy 8a, 8b, 16c and 16d and 48f in MgAl2O4 • A and B occupy mostly 16c and 16d MgCr2O4 • The charge density distributions in Mg spinels can be described in the Fm-3m space group (a’=a/2): cations occupy the 4a and 8c Wyckoff sites in Fm-3m

13. FFT Diffraction from a 500 nm region 512 pixels = 23 nm Local structure: TEM in MgCr2O4 • Odd Bragg reflections always exist but they are broadened • The structure is Fd-3m at the local scale (20 nm) • The average structure is Fm3m over 500 nm (domain) 222 400 222 400

14. Local structure : Raman scattering MgCr2O4 Low energy ions @RT • Active Raman Irr. Reps. For ideal normal spinels: MgAl2O4 Swift ions@RT The number of frequencies shows that tetrahedral sites are still occupied ! Raman shift : the inversion parameter is about 18 %.

15. Summary of structural results • ZnAl2O4 • Under irradiation, a cations exchange occurs without any space group modification • Mg based spinels • XRD diffraction : odd Bragg reflexions vanish • Apparent space group Fm-3m • Cations only on octahedral sites (4a Wyckoff positions): no tetrahedra, disagrees with Raman scattering • Cations on 4a and 8c sites: tetrahedral sites agree with Raman but interatomic distances are too short • TEM observations • At the mesoscopic scale (20 nm in MgCr2O4) , the space group is still Fd-3m

16. Structural model Radiation damage in these three compounds acts at two different scales • At the atomic scale: • The local structure consists of octahedra and tetrahedra • The space group is unchanged (Fd-3m) for all spinels • Cation interchange occurs as in the thermal picture • At the mesoscopic scale (few nanometers) • Damage induced by a ion impact is spatially localized • Coherent nanoregions are produced in spinels sharing the anion sublattice • Spatial interference ‘averages’ their contributions over a large number of regions of the crystal and it leads to an apparent symmetry change (Fm3m, a’=a/2) in Mg based spinels How to confirm this model ?

17. Thermal annealing after irradiation 1200 K • Thermal annealing of the extended defects increases the size of the coherent diffracting domains restoring the “normal” structure (333/511) (111) 600 K (400)

18. Summary • What can we learn from irradiation of spinel compounds? • Irradiation acts at two different length scales • Locally in a similar way as temperature does • Since the spinel structure is the only stable one vs. temperature increase, only cation inversion is observed • Atoms cannot be freely mixed as in metal alloys because of atomic charges : fewer new phases are expected in ceramics under irradiation … • Radiation damage modifies the material at the mesoscopic scale • Anion sublattice must provide charge balance • The characteristic domain length scale possibly depends on the elements in the material and on the energy deposition mode • The correlation length seems to be • Very large in ZnAl2O4: no scattering coherence Fd-3m • Very small for Mg based spinels: strong scattering coherence Fm-3m