Positron annihilation spectroscopic studies on Nd-doped ceria

We report a new result on the characterization of Nd-doped ceria, Ce_1−xNd_xO_2−x/2 (x=0.075-0.675) using positron lifetime spectroscopy (LTS) and coincidence Doppler broadening (CDB) measurements. A systematic increase in lifetime that is attributed to formation of Nd-oxygen vacancy associates is seen from x=0.075-0.4 followed by a sharper increase up to x=0.5. The change in profile of lifetime around x=0.4 suggests drastic increase in the concentration of these associates. Discontinuity in lifetime around x=0.5 is ascribed to ordering of oxygen vacancies. Coincidence Doppler broadening measurements indicate reduction in the overlap of positron wavefunction with oxygen core electrons due to trapping of positrons. Low-temperature (50-300 K) lifetime measurements indicate the presence of Rydberg-like positron states associated with Nd³⁺ sites.