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

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



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



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



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



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



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



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)



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



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



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



Electron density from X-ray diffraction

ZnAl2O4

MgCr2O4

MgAl2O4

• Fourier syntheses derived from the observed diffracted intensities indexed in the Fd3m space group



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



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



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 %.



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



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 ?



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



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

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

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