Oxygen vacancies induce changes in lattice parameter, photoluminescence characteristics and Raman spectra of sol–gel derived fluorite-type cubic CeO₂ and Ce_0.8Zr_0.2−xA_xO₂ (A = Co/Fe, x = 0–0.2) powders

Abstract

An attempt has been made here to synthesized ceria (CeO₂) and Ce_0.80Zr_0.20−xA_xO₂ (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 F_2g 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.