Photoluminescence of Sr2 − xLnxCeO4 + x/2 (Ln = Eu, Sm or Yb) prepared by a wet chemical method

Abstract

A wet chemical route is developed for the preparation of Sr₂CeO₄ denoted the carbonate-gel composite technique. This involves the coprecipitation of strontium as fine particles of carbonates within hydrated gels of ceria (CeO₂·xH₂O, 40 < x < 75) by the addition of ammonium carbonate. During calcination, CeO₂·xH₂O dehydroxylation is followed by the reaction with SrCO₃ to form Sr₂CeO₄ with complete phase purity. Doping of other rare-earths is carried out at the co-precipitation stage. The photoluminescence (PL) observed for Sr₂CeO₄ originates from the Ce⁴⁺–O²⁻ charge-transfer (CT) transition resulting from the interaction of Ce⁴⁺ ion with the neighboring oxide ions. The effect of next-nearest-neighbor (NNN) environment on the Ce⁴⁺–O²⁻ CT emission is studied by doping with Eu³⁺, Sm³⁺ or Yb³⁺ which in turn, have unique charge-transfer associated energy levels in the excited states in oxides. Efficient energy transfer occurs from Ce⁴⁺–O²⁻ CT state to trivalent lanthanide ions (Ln³⁺) if the latter has CT excited states, leading to sensitizer–activator relation, through non-resonance process involving exchange interaction. Yb³⁺-substituted Sr₂CeO₄ does not show any line emission because the energy of Yb³⁺–O²⁻ CT level is higher than that of the Ce⁴⁺–O²⁻ CT level. Sr_2 − xEu_xCeO_4 + x/2 shows white emission at x ≤ 0.02 because of the dominant intensities of ⁵D₂–⁷F_0–3 transitions in blue–green region whereas the intensities of ⁵D₀–⁷F_0–3 transitions in orange–red regions dominate at concentrations x ≥ 0.03 and give red emission. The appearance of all the emissions from ⁵D₂, ⁵D₁ and ⁵D₀ excited states to the ⁷F_0–3 ground multiplets of Eu³⁺ is explained on the basis of the shift from the hypersensitive electric-dipole to magnetic-dipole related transitions with the variation in site symmetry with increasing concentration of Eu³⁺. White emission of Sr_2 − xSm_xCeO_4 + x/2 at x ≤ 0.02 is due the co-existence of Ce⁴⁺–O²⁻ CT emission and ⁴G(4)_5/2–⁶H_J Sm³⁺transitions whereas only the Sm³⁺ red emission prevails for x ≥ 0.03. The above unique changes in PL emission features are explained in terms of the changes in NNN environments of Ce⁴⁺. Quenching of Ce⁴⁺–O²⁻ CT emission by other Ln³⁺ is due to the ground state cross-over arising out of the NNN interactions.