In Situ Cathodoluminescence in Gadolinia Doped Ceria Under High Energy Electron Irradiation

Ceramic oxides with fluorite crystal structure, such as CeO₂ (ceria, Fm-3m) have been surrogates of nuclear fuels and transmutation targets with excellent radiation resistance. Gd₂O₃ (Ia-3) is doped in ceria (GDC) to simulate the burnable poison doping in nuclear fuels. In ceria, intrinsic oxygen vacancies (V_O^●●) (using the Kröger-Vink notation) are created to provide charge compensation when Ce⁴⁺ ions (4f⁰) are reduced to Ce³⁺ ions (4f¹). In GDC, oxygen vacancies are also produced by the substitution of Ce⁴⁺ ions for Gd³⁺ (4f⁷) ions (Gd_Ce^′).<br/>Under high energy electron irradiation, when kinetic energy higher than the displacement threshold energy of oxygen ions (E_d,O) is transferred to GDC, an F center (oxygen point defect with trapped electrons) is generated by elastic electron-nucleus collisions. This study investigates the production of the F center and its charge states in GDC in a high voltage electron microscope (HVEM) through <i>in-situ </i>cathodoluminescence (CL) spectroscopy under high energy electron irradiation.<br/>In this study, GDC samples (Ce_1-xGd_xO_2-_δ, x= 0-0.5) fabricated by sintering in the air were used. The lattice structure of sintered samples was analyzed by X-ray diffraction (XRD) with Cu Kα radiation (30 mA, 40 kV) X-ray source from 20° to 90° (2θ) at a step size of 0.01° at room temperature.<br/>A HVEM in the Ultramicroscopy Research Center of Kyushu University was operated to produce oxygen point defects in the samples. This facility is installed with an optical fiber probe with 46° off from the electron beam line thereby <i>in-situ </i>CL from the samples can be detected under irradiation conditions. The electron energies ranged from 400 to 1250 keV higher than the E_d_,O. Temperature was controlled from 300 K to 100 K using a liquid N₂ cooling holder. The <i>in-situ </i>CL were also measured by using SEM under low energy electron (20 keV) irradiation which does not produce oxygen displacement.<br/>The XRD experimental results showed that the lattice parameter was increased with Gd₂O₃ concentration. Also, the crystal structure was changed for the high concentration of GDC samples with the values of x=0.4 and 0.5. The crystallite size and micro strain were also evaluated.<br/><i>In-situ </i>CL spectra measured from GDC samples by a HVEM and SEM were decomposed into broad bands using Gaussian functions. According to the previous reports^[1], the CL bands were emitted from Ce³⁺ ions (2.1 ~ 2.9 eV) induced by charge compensation of oxygen vacancies. Also the CL bands centered at 4.2 eV was emitted from F⁺ centers (one electron trapped F center, V_O^●) produced by elastic displacement damage. The photon energies of CL bands emitted from GDC samples were similar to those of ceria. However, the integrated CL intensities of each band tended to depend on Gd³⁺ dopant concentration since the oxygen vacancies in GDC are mainly controlled by the doping concentrations. The CL emission bands were also integrated as a function of energy and temperature. The real-time charge states of the point defects and Ce³⁺ ions under high energy electron irradiation conditions will be discussed by <i>in-situ </i>CL spectroscopy.<br/>Ref. [1] J.M. Costantini, P. Seo, K. Yasuda, AKM S.I. Bhuian, T. Ogawa, D. Gourier, <i>J. Lumi.</i> 226 (2020) 117379.