Positron annihilation spectroscopic study of vacancy defects in aliovalent ion doped CeO₂ for technological applications

Defect engineering in CeO₂ has received paramount attention of the researchers by virtue of its diverse technological applications encompassing the areas like catalysis, sensors, energy storage, optoelectronics etc. The multifunctional behavior of ceria is primarily governed by the concentration of oxygen vacancies and Ce³⁺/Ce⁴⁺ ratio. In our previous studies we have extensively investigated the effect of the size of the dopants on the evolved oxygen vacancies in aliovalent ion doped ceria. In this present work our aim is to understand the other subtle factors apart from charge compensation influencing the evolution of vacancies keeping the total concentration of trivalent dopant ions fixed as well as to tune the lattice relaxation by the judicious choice of dopants of suitable size. In this regard, CeO₂ doped with varied relative concentrations of Sc³⁺ (smaller than Ce⁴⁺) and La³⁺ (larger than Ce⁴⁺) and co-doped with a fixed amount of Eu³⁺ have been synthesized and Positron Annihilation Lifetime Spectroscopy (PALS) along with other complementary techniques were employed to investigate the evolution of defect. The ceria samples were synthesized by gel-combustion route, followed by calcinations at 500 °C and annealing at 800 °C for four hours and the annealed samples were characterized by powder XRD technique. The crystallite size along with lattice strain was determined from the acquired XRD patterns using Williamson-Hall approach. Rietveld refinement analysis of the patterns was further carried to quantify the changes in the lattice parameters of the doped samples. PALS results showed creation of more oxygen vacancy with higher La fraction as reflected from the variation of the shorter lifetime component τ. Photoluminescence (PL) measurements were carried out to get complementary information about the distribution of the vacancies around the dopants using the hypersensitive EDT transition of Eu³⁺ as a probe. Further studies are planned to correlate the effect of the evolved oxygen vacancies as well as their distribution with the catalytic activity of the material. (author)