Oxygen vacancies induce changes in lattice parameter, photoluminescence characteristics and Raman spectra of sol–gel derived fluorite-type cubic CeO2 and Ce0.8Zr0.2−xAxO2 (A = Co/Fe, x = 0–0.2) powders

An attempt has been made here to synthesized ceria (CeO2) and Ce0.80Zr0.20−xAxO2 (A = Fe, Co; x = 0–0.20) powders via novel sol–gel technique using metal nitrate hydrates as precursors and distilled water and ethanol as solvents. The resulting gel was dried at 130 °C for 24 h and calcined successively at 850 and 950 °C for 3 h each. Ceria is shown to exhibit a fluorite-type cubic structure with a slightly higher lattice parameter a = 5.428 Å, Z = 4, space group (mathrm{Fm}overline{3}mathrm{m }), some Ce3+ ions and oxygen vacancies. 20% zirconium substitution of cerium causes decrease of ‘a’ to 5.395 Å owing to a lower ionic radius of Zr4+ vis-à-vis Ce4+. However, the lattice parameter first increases and then decreases with partial/ full replacement of Zr4+ by cobalt or iron ions. Evidence is found for existence disorder/strain, mismatch of ionic radii and oxidation states of cations, and appropriate oxygen vacancies to ensure charge neutrality of systems. The photoluminescence (PL) spectra reveal peaks at ~ 371, 402, and 432–445 nm, which are attributed to Ce 4f0 → 4f1 direct transition, Ce 4f0 → 4f1 electron transfer via oxygen F++ state, and oxygen vacancies (excited oxygen F+* → F+ transition), respectively. Their Raman spectra display a peak at ~ 462 cm−1 due to F2g symmetric vibrations of a cubic CeO8 sub-cell with some variation in position and sharpness with/without a signal in the range 150–170 cm−1 by introduction of different cations leading to distortion and formation of anion vacancies.