Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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 ZrO 2, HfO 2, UO 2+x Irradiation TiC, ZrC, SiC

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 4 AB 2 O 4 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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 5 Spinel Chemistry: chemical bond & site selectivity Chemical bonds and site selectivity –Hybridization –Crystal Field –Then charge and size considerations kick in … MgAl2O4 MgCr2O4 ZnAl2O4 Charge density in slabs ELF = 0.6 in glyphs

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 6 Order & disorder in AB 2 O 4 spinels  “Normal” spinel structure: (A)[B 2 ]O 4 MgAl 2 O 4, ZnAl 2 O 4, FeCr 2 O 4, FeAl 2 O 4, MgCr 2 O 4, …  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]O 4 (Fe 3+ )[Fe 2+ Fe 3+ ]O 4, (Fe 2+ )[Fe 2+ Ti 4+ ]O 4  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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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) MgCr 2 O 4 Radiation damage of a PWR simulated by ion irradiation (JANNUS)  The irradiated layer is rather thin (0.2 µm – GIXRD)

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 8 Spinel irradiation at room temperature Simulation of neutron irradiation by low energy ions (cascades) and of fission products by swift heavy ions Equatorial Sollers Slits Sample holder ± 1µm, ± 0,02° PSD Detector Monochromator or parabolic mirror Beam 50µm*5mm XRD X-ray diffraction: Asymmetric reflection setup (fixed, grazing impinging beam) 4 MeV

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 9 II) Structural evolution of spinels under irradiation Behavior of spinel under irradiation: Modification of Diffraction patterns MgAl 2 O 4 MgCr 2 O 4 ZnAl 2 O 4 High E. Vanishing of odd Bragg reflexions D. Simeone, J. Nucl. Mat K Yasuda, MINB 2006, JNM 2007… Vanishing of odd Bragg reflexions G. Baldinozzi, Nucl. Inst Meth B Non vanishing of odd Bragg reflexions D. Simeone, J. Nucl. Mat Low E. 140 K Vanishing of odd Bragg reflexions L.M. Wang, MRS 1995, R. Devanathan, Phil Mag Let 1995 Low E. RT Non vanishing of odd Bragg reflexions D. Gosset, J. Eur. Ceram Vanishing of odd Bragg reflexions D. Gosset, J. Eur Ceram Soc, 2005 Non vanishing of odd Bragg reflexions G. Baldinozzi, Nucl. Inst Meth B. 2005

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 10 XRD before & after irradiation at room T ZnAl 2 O 4 MgAl 2 O 4 MgCr 2 O 4 Structural information: –The (ooo) peaks depend on the cation distributions –In ZnAl 2 O 4 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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 11 Electron density from X-ray diffraction ZnAl 2 O 4 MgCr 2 O 4 MgAl 2 O 4 Fourier syntheses derived from the observed diffracted intensities indexed in the Fd3m space group

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 12 Comparison between the three spinel structures  In ZnAl 2 O 4 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 MgAl 2 O 4  A and B occupy mostly 16c and 16d MgCr 2 O 4  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

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 13 Local structure: TEM in MgCr 2 O 4 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) FFT Diffraction from a 500 nm region 512 pixels = 23 nm

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 14 Local structure : Raman scattering  Active Raman Irr. Reps. For ideal normal spinels: MgCr 2 O 4 Low energy MgAl 2 O 4 Swift The number of frequencies shows that tetrahedral sites are still occupied ! Raman shift : the inversion parameter is about 18 %.

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 15 Summary of structural results  ZnAl 2 O 4  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 MgCr 2 O 4 ), the space group is still Fd-3m

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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 ?

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 17 Thermal annealing after irradiation  Thermal annealing of the extended defects increases the size of the coherent diffracting domains restoring the “normal” structure (400) (111) (333/511) 600 K 1200 K

Equipe mixte CEA-CNRS-ECP G. Baldinozzi – June 23 rd 2008, SUPELEC 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 ZnAl 2 O 4 : no scattering coherenceFd-3m Very small for Mg based spinels: strong scattering coherenceFm-3m