Oxygen-vacancy complexes in cerium oxide studied byIn111time-differential perturbed-angular-correlation spectroscopy

Four defect complexes with indium tracer atoms in undoped and lightly doped ${\mathrm{CeO}}_2$ have been observed by ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ time-differential perturbed-angular-correlation spectroscopy. One complex, identified as an indium–O-vacancy pair, is always present unless the material is doped with Nb or Ta to eliminate oxygen vacancies. The binding energy of the vacancy to indium is greater than 0.35 eV. In materials doped or annealed to introduce as little as 0.05% oxygen vacancies, two different double-vacancy complexes are formed. One is apparently a complex in which the vacancies are trapped on opposite sides of the indium along 〈111〉 directions. The other is likely a complex with one oxygen vacancy near the indium and one in a more distant trap. In material sufficiently doped with Nb, the majority of dilute indium dopants are uncomplexed, but a small fraction is strongly bound to a presently unidentified impurity. At low temperatures, an electronic excitation trapped at the time of the parent ${}_{}{}^{111}{}_{}{}^{}\mathrm{In}$ decay to ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ clouds interpretation of some details. We speculate that this ‘‘aftereffect’’ affects only ${}_{}{}^{111}{}_{}{}^{}\mathrm{Cd}$ nuclei having no vacancies in their nearest-neighbor oxygen shell. Some complexes involving only more distantly trapped vacancies may be unobserved.