François Beuneu’s research while affiliated with École Polytechnique and other places

The photo-annealing of colour centres in yttria-stabilised zirconia (YSZ) was studied by electron paramagnetic resonance spectroscopy upon UV-ray or laser light illumination, and compared to thermal annealing. Stable hole centres (HCs) were produced in as-grown YSZ single crystals by UV-ray irradiation at room temperature (RT). The HCs produced by 200-MeV Au ion irradiation, as well as the F⁺-type centres ( centres involving oxygen vacancies) were left unchanged upon UV illumination. In contrast, a significant photo-annealing of the latter point defects was achieved in 1.4-MeV electron-irradiated YSZ by 553-nm laser light irradiation at RT. Almost complete photo-bleaching was achieved by laser irradiation inside the absorption band of centres centred at a wavelength ~550 nm. Thermal annealing of these colour centres was also followed by UV–visible absorption spectroscopy showing full bleaching at 523 K. Colour-centre evolutions by photo-induced and thermally activated processes are discussed on the basis of charge exchange processes between point defects.

Pure and (Ca and Si)-substituted yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers and amorphous films on gadolinium gallium garnet (Gd3Ga5O12, or GGG) single crystal substrates were irradiated by 50 MeV (32)Si and 50 MeV (or 60 MeV) (63)Cu ions for electronic stopping powers larger than the threshold value (~4 MeV μm(-1)) for amorphous track formation in YIG crystals. Conductivity data of crystalline samples in a broad ion fluence range (10(11)-10(16) cm(-2)) are modeled with a set of rate equations corresponding to the amorphization and recrystallization induced in ion tracks by electronic excitations. The data for amorphous layers confirm that a recrystallization process takes place above ~10(14) cm(-2). Cross sections for both processes deduced from this analysis are discussed in comparison to previous determinations with reference to the inelastic thermal-spike model of track formation. Micro-Raman spectroscopy was also used to follow the related structural modifications. Raman spectra show the progressive vanishing and randomization of crystal phonon modes in relation to the ion-induced damage. For crystalline samples irradiated at high fluences (⩾10(14) cm(-2)), only two prominent broad bands remain like for amorphous films, thereby reflecting the phonon density of states of the disordered solid, regardless of samples and irradiation conditions. The main band peaked at ~660 cm(-1) is assigned to vibration modes of randomized bonds in tetrahedral (FeO4) units.

Yttria-stabilized zirconia (YSZ) single crystals (for 9.5 and 18 mol% yttria) were irradiated at room temperature (RT) by X-rays (W white spectrum) and 2.5-MeV electrons. The growth curves of the so-called T-centre (for trigonal centre, i.e. Zr3+ sitting in a trigonal symmetry site) were studied as a function of absorbed dose, or irradiation time, by UV–visible optical absorption (OA) spectroscopy and X-band electron paramagnetic resonance spectroscopy. The defect concentration at saturation and the production rate are increased by a factor around two for 18 mol% yttria with respect to 9.5 mol%. Defect decay was then followed after irradiation by OA spectroscopy as a function of ageing time at RT. Growth and decay curves of the T-centre are modelled on the basis of rate equations of charge-exchange reactions with the zirconium lattice ions. Increase in yttrium content is thought to decrease hole trapping on Zr3+ ions, thereby enhancing T-centre formation.

We have studied the effect of yttria content on the recovery of paramagnetic centres in electron- and ion-irradiated yttria-stabilized zirconia (ZrO2: Y3+). Single crystals with 9.5 or 18 mol% Y2O3 were irradiated with 1.0-2.5 MeV electrons, 145-MeV carbon ions, 200-MeV iodine ions, 1.45-GeV xenon ions and 2.25-GeV gold ions at various fluences. Thermal annealing of electron (F+-type and T) centres and hole centres was studied by X-band electron paramagnetic resonance spectroscopy. Hole centres are found to anneal more quickly for 18 mol% than for 9.5 mol% Y2O3. At long annealing times, a non-zero asymptotic behaviour is observed in the isothermal annealing curves of hole centres and F+-type centres between 300 and 500 K. For the electron-irradiated samples, the asymptotic concentration of both defect types normalized by the values prior to annealing has a maximum value of similar to 0.5 for annealing temperatures below the onset of the (isochronal) recovery stage (400 K), regardless of the yttria content. For the ion-irradiated samples, larger normalized asymptotic values (similar to 0.8) at high annealing temperature are found for the F+-type centres with a similar recovery process. Such an uncommon behaviour is analysed on the basis of a model using equilibrated reactions between point defects with different charge states.

Electron paramagnetic resonance (EPR) spectroscopy is used to study the point defect production in yttria-stabilized zirconia (YSZ) (1 0 0) single crystals by swift heavy ion, electron and X-ray irradiations. A common color center known as the "T center" (for "trigonal" center) with an axial <1 1 1> symmetry is produced in all cases. We show that this defect is likely an intrinsic one with concentrations much larger than the major impurities. The growth curves of this point defect versus fluence for ion and electron irradiations can be on the whole rescaled as a function of the absorbed dose. This confirms that T centers are produced by the electronic excitations, either at low density with X-rays and electrons, or at high density with heavy ions. A kinetic model depending on fluence is proposed to account for the saturation behavior of growth curves. The production rate, corresponding to the initial slope of growth curves, increases steeply versus the average volume density of electron-hole pairs that are generated by ions and electrons in the irradiated volume.

Cubic yttria-stabilized zirconia (YSZ) can be used for nuclear applications as an inert matrix for actinide immobilization or transmutation. Indeed, the large amount of native oxygen vacancies leads to a high radiation tolerance of this material owing to defect recombination occurring in the atomic displacement cascades induced by fast neutron irradiation or ion implantations, as showed by molecular dynamics (MD) simulations. Amorphization cannot be obtained in YSZ either by nuclear-collision or electronic-excitation damage, just like in urania. A kind of polygonization structure with slightly disoriented crystalline domains is obtained in both cases. In the first steps of damage, specific isolated point defects (like F+-type color centers) and point-defect clusters are produced by nuclear collisions with charged particles or neutrons. Further increase of damage leads to dislocation-loop formation then to collapse of the dislocation network into a polygonization structure. For swift heavy ion irradiations, a similar polygonization structure is obtained above a threshold stopping power value of about 20-30 keV nm-1.

Cubic zirconium dioxide, or zirconia (ZrO2-x), generally stabilized by yttrium substitution, is a refractory material with a large oxygen sub-stoichiometry inducing a high ionic conductivity that can be used for solid oxide fuel cell (SOFC) applications, oxygen sensors and other electrochemical applications. Yttria-stabilized zirconia (YSZ) can be also used for nuclear applications as an inert matrix for actinide immobilization or transmutation. Indeed, the large amount of native oxygen vacancies also leads to a high radiation tolerance of this material owing to defect recombination occurring in the atomic displacements cascades induced by fast neutron irradiation or ion implantations. Molecular dynamics (MD) simulations of these collision cascades actually show that defect annihilation takes place due to recombination of oxygen interstitials with oxygen vacancies either in YSZ or ZrO2-x. Amorphization cannot be obtained in YSZ either by nuclear-collision or electronic-excitation damage even at large fluences, just like in uranium dioxide or urania (UO2). A kind of polygonization structure with slightly disoriented crystalline domains is obtained in both cases. Amorphization is reached in YSZ only by a chemical effect above a critical atomic fraction of implanted ions like Cs. We discuss in details the different mechanisms of point-defect formation in YSZ by energetic photon, electron, and heavy-ion or neutron irradiations either via elastic (nuclear) collision, or inelastic (ionization) processes, on the basis of electron paramagnetic resonance (EPR) and UV-visible optical absorption data. In the first steps of damage, specific isolated point defects (like F+-type color centers) and point-defect clusters are produced by nuclear collisions with charged particles or neutrons leading to significant modifications of the electrical and mechanical properties. Further increase of damage leads to dislocation-loop formation then to collapse of the dislocation network into a polygonization structure. For swift heavy ion irradiations, a similar polygonization structure is obtained above a threshold stopping power value of about 20-30 keV nm-1, depending on the ion velocity. Extended-defect production in cerium dioxide or ceria (CeO2-x) by ion implantation is also discussed. MD simulations of ceria show that oxygen platelets in {111} planes are produced in collision cascades, in agreement with transmission electron microscopy (TEM). Similar extended defects are also observed by TEM in ion-implanted YSZ. The use of zirconia or ceria as non-radioactive surrogates for the behavior of actinide dioxides with the fluorite structure (e.g. urania) under irradiation is also addressed.

The present chapter is devoted to the study of the properties of point defects and their agglomerates in a ionic simple oxide of antifluorite structure, lithium oxide (Li2O). This lithium compound is regarded as a serious candidate for tritium breeding material for the future fusion reactors. In this context, a detailed study of point defects and their agglomerates is highly desirable, as well as the creation of these defects under irradiation and their annealing properties. The main point defect detectable in irradiated lithium oxide is the so-called F+ center, corresponding to an oxygen vacancy with an electron trapped. It is easily detected by electron paramagnetic resonance (EPR) or by optical techniques. Depending on the irradiation or annealing conditions, agglomerates of F+ centers can also be observed. We describe the conditions when this agglomeration gives rise to lithium precipates with clear metallic properties. Again, EPR is a good technique for that, because lithium gives with this method an intense and easily recognizable signal. These colloids are studied not only by EPR but also by dielectric constant measurements, optical microscopy, NMR, differential calorimetry and neutron scattering; magnetization measurements on O2 bubbles are also presented. Finally, some extensions of these observations are discussed, such as similar measurements on lithium imide (another antifluorite type compound), but also bistable conduction electron spin resonance and molecular-dynamics studies of the superionic phase of Li2O.

Unusually narrow electron spin resonance lines can give birth to spectacular and interesting phenomena. We use here very pure lithium metal colloids created in electron-irradiated LiF, giving rise to a very narrow Li metal line. In these samples, three interesting phenomena are discussed and interpreted: signal saturation in particularly good conditions; oscillations appearing on strongly overmodulated lines; signal bistability, when the signal shape seems to depend on the sample magnetic history.