Thermal recovery of colour centres induced in cubic yttria-stabilized zirconia by charged particle irradiations

Abstract

We have used electron paramagnetic resonance to study the thermal annealing of colour centres induced in cubic yttria-stabilized zirconia by swift electron and heavy ionirradiations. Single crystals were irradiated with 1 or 2-MeV electrons, and 200-MeV 127 I, or 200-MeV 197 Au ions. Electron and ion beams produce the same colour centres: namely i) an F + -like centre, ii) the so-called T-centre (Zr 3+ in a trigonal oxygen local environment), and iii) a hole center. Isochronal annealing was performed up to 973 K. Isothermal annealing was performed at various temperatures on samples irradiated with 2-MeV electrons. The stability of paramagnetic centres increases with fluence and with a TCR treatment at 1373 K under vacuum prior to the irradiations.Two distinct recovery processes are observed depending on fluence and/or thermal treatment. The single-stage type I process occurs for F + -like centres at low fluences in asreceived samples, and is probably linked to electron-hole recombination. T-centres are also annealed according to a single-stage process regardless of fluence. The annealing curves allow one to obtain activation energies for recovery. The two-stage type II process is observed only for the F + -like centres in as-received samples, at higher fluences, or in reduced samples.These centres are first annealed in a first stage below 550 K, like in type I, then transform into new paramagnetic centres in a second stage above 550 K. A simple kinetics model is proposed for this process. Complete colour centre bleaching is achieved at about 1000 K.

Threshold displacement energy in yttria‐stabilized zirconia

“…Five main lines are seen in the EPR spectra of irradiated YSZ with the magnetic field B//<100>, regardless of electron energy and fluence like with previous electron or ion irradiations [6,7]: i.e. a narrow line at g = 1.998 (line 1), a broad signal at g = 1.973 (line 2) and a very broad one at g = 1.904 (line 3).…”

“…a narrow line at g = 1.998 (line 1), a broad signal at g = 1.973 (line 2) and a very broad one at g = 1.904 (line 3). Lines 1-2 have been previously assigned to F + -type centers, and line 3 to the so-called T-centers [6,7], also produced by X-ray irradiations [8]. Two additional weak lines at g = 2.014 (line 4) and g = 2.007 (line 5), correspond to hole centers [7].…”

“…Also the weak swelling (< 0.2 %) of YSZ was found to be directly correlated to point defect formation, i. e. F + -type centers [4]. Three different color centers were actually observed by electron paramagnetic resonance (EPR) measurements at room temperature (RT) on electron or ion irradiated YSZ single crystals [6,7]: i) the first one was identified as an F + -type center (singly ionized oxygen vacancy) with an axial <100> symmetry and small g-factor anisotropy (g ⊥ = 1.972 and g // = 1.996), ii) the second one is similar to the so-called Tcenter (Zr 3+ in a trigonal oxygen environment) with an axial <111> symmetry and large g-factor anisotropy (g ⊥ = 1.855 and g // = 1.986), which is also produced by photon irradiations [8], and iii) a hole cen-ter. F + -type center production was correlated to the total number of displacement per atom (dpa) in both Zr-and O-sublattices: a saturation of the defect production was found at about 10 -3 dpa [6] using the commonly used value of 40 eV for the threshold displacement energies [2].…”

“…The defect production was monitored by EPR measurements at RT using a Bruker EMX X-band spectrometer operating at 9.6 GHz [6,7].…”

Colour centre recovery in yttria-stabilised zirconia: photo-induced versus thermal processes

“…The linear correlations between the maximum absorption coefficients of those bands and colour centre concentrations (deduced from EPR spectra) yielded oscillator strengths of 0.2 and 0.3, respectively. Upon thermal annealing, an equilibrium process was found to take place between those two colour centres leading to a non-zero asymptotic concentration of F + -type centres for long isothermal annealing times [Costantini, 2006;2014a].…”