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Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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’énergieSPMS, CNRS - École Centrale Paris & DEN/DMN/SRMA, CEA Saclay
Coll. Synchrotron Soleil & ESRFISCSA TEMJANNUS Orsay & Saclay GANIL Caen
OUTLINE: Scientific Context Spinel structure Experimental & Results Model & Conclusion
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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
ZrO2, HfO2, UO2+x
Irradiation
TiC, ZrC, SiC
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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 bondsDriven 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 23rd 2008, SUPELEC 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
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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 slabsELF = 0.6 in glyphs
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 6
Order & disorder in AB2O4 spinels
“Normal” spinel structure: (A)[B2]O4
MgAl2O4, 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
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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)
MgCr2O4
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 23rd 2008, SUPELEC 8
Spinel irradiation at room temperatureSimulation 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
Monochromatoror parabolic mirror
Beam
50µm*5mm
XRD
X-ray diffraction: Asymmetric reflection setup (fixed, grazing impinging beam)
Au @ 4 MeV
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 9
II) Structural evolution of spinels under irradiation
Behavior of spinel under irradiation: Modification of Diffraction patterns
MgAl2O4 MgCr2O4 ZnAl2O4
High E. ions @RT
Vanishing of odd Bragg reflexionsD. Simeone, J. Nucl. Mat. 2002
K Yasuda, MINB 2006, JNM 2007…
Vanishing of odd Bragg reflexionsG. Baldinozzi, Nucl. Inst Meth B. 2007
Non vanishing of odd Bragg reflexionsD. Simeone, J. Nucl. Mat. 2002
Low E. ions
@ 140 K
Vanishing of odd Bragg reflexionsL.M. Wang, MRS 1995, R. Devanathan, Phil Mag Let 1995
Low E. ions
@ RT
Non vanishing of odd Bragg reflexionsD. Gosset, J. Eur. Ceram. 2005
Vanishing of odd Bragg reflexionsD. Gosset, J. Eur Ceram Soc, 2005
Non vanishing of odd Bragg reflexionsG. Baldinozzi, Nucl. Inst Meth B. 2005
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 10
XRD before & after irradiation at room T
ZnAl2O4
MgAl2O4 MgCr2O4
• 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
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 11
Electron density from X-ray diffraction
ZnAl2O4
MgCr2O4
MgAl2O4
• Fourier syntheses derived from the observed diffracted intensities indexed in the Fd3m space group
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 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
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 13
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)
FFT
Diffraction from a 500 nm region
512 pixels = 23 nm
222
400
400
222
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 14
Local structure : Raman scattering
Active Raman Irr. Reps. For ideal normal spinels:
MgCr2O4
Low energy ions@RT
MgAl2O4
Swift ions@RT
i 2T2g 2A1g Eg
The number of frequencies shows thattetrahedral sites are still occupied !
Raman shift : the inversion parameter is about 18 %.
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 2008, SUPELEC 15
Summary of structural results
ZnAl2O4
Under irradiation, a cations exchange occurs without any space group modification
Mg based spinelsXRD 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
Equipe mixte CEA-CNRS-ECP
G. Baldinozzi – June 23rd 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 tetrahedraThe 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 23rd 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 23rd 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 doesSince the spinel structure is the only stable one vs. temperature increase, only
cation inversion is observedAtoms 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 balanceThe characteristic domain length scale possibly depends on the elements in
the material and on the energy deposition modeThe 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