Luminescence Of Ce～(4+) And Eu～(3+) Ions Doped In Sr₂CeO₄ And Ca₂SnO₄ Hosts With One-dimensional Structures

Sr₂CeO₄: Eu³⁺, MCeO₃(M=Sr, Ba): Eu³⁺, Ca₂SnO₄: Eu³⁺ and Ca₂SnO₄: Ce⁴⁺ powder samples were prepared by a solid-state method and their structures, formation mechanism and luminescent properties were investigated by use of XRD and fluorescence spectra. When the Sr_2-xEu_xCeO_4+x/2 samples are prepared, it is found that only the excitation energy in the strong charge-transfer-state (CTS) band of Ce⁴⁺—O^2- peaking at about 35700cm^-1 is transferred to Eu³⁺ ion while that in the weak CTS band of Ce⁴⁺—O^2- located at about 29400cm^-1 led to Ce⁴⁺—O^2- charge-transfer emission. After absorption of moisture in the Sr_2-xEu_xCeO_4+x/2 samples, the intensity of Eu³⁺ intra-4f⁶ absorption transition increases remarkably, the energy transfer takes place from the two Ce⁴⁺—O^2- CTS bands to Eu³⁺ ion. The excitation energy in the strong Ce⁴⁺—O^2- CTS band is transferred to the Eu³⁺—O^2- CTS band through radiationless process involving exchange interaction, however, the weak Ce⁴⁺—O^2- CTS band causes the excitation to the Eu³⁺ excited levels through resonance radiationless process by a multipole interaction mechanism.The Eu³⁺ ions doped in MCeO₃(M=Sr, Ba) have preference to M²⁺ than Ce⁴⁺ sites. In the MCeO₃: Eu³⁺ samples, the emission transitions of Eu³⁺ originated from ⁵D₁ and ⁵D₀ excited states can be observed, among which the ⁵D₀—7F₁ magnetic-dipole transition is the most intense emission. The emission of Eu³⁺ doped in the SrCeO₃ lattice is much stronger than that of Eu³⁺ doped in the BaCeO₃ lattice. The excitation spectra of SrCeO₃: Eu³⁺ and BaCeO₃: Eu³⁺ display a much broad absorption band peaking at about 311 nm and 320 nm, respectively. This broad band is attributed to the charge transfer from O^2- to Ce⁴⁺ and the energy transfer to Eu³⁺.When the starting powder mixture of CaCO₃ and SnO₂ (2:1) is calcined at 1250℃, the unstable intermediate phase CaSnO₃ is developed, which then reacts with CaO to form the final product Ca₂SnO₄. The excitation spectrum of Ca_2-xEu_xSnO₄ shows a Eu³⁺—O^2- charge-transfer band with the maximum changing from 274 nm to 292 nm by increasing the Eu³⁺ concentrations (x=0.01～0.15). Under UV excitation, the Ca₂SnO₄: Eu³⁺ phosphor exhibits novel red emission at about 618nm which is assigned to the ⁵D₀—⁷F₂ electric-dipole transition. In addition, the weak emission transitions from the higher ⁵D₂ and ⁵D₁ excited states can be observed at low Eu³⁺ concentrations because of the low multiphonon relaxation probability. The emission spectrum of Ca₂SnO₄: Eu³⁺ is similar to that of Sr₂CeO₄: Eu³⁺, however, the ⁵D₀—⁷F₂ emission of the former is much stronger than that of the latter.ACe⁴⁺ -doped Ca₂SnO₄ with one-dimensional structure, which emits bright blue light, was prepared by a solid-state reaction method. The Ce⁴⁺ ions doped in Ca₂SnO₄ tend to occupy the Sn⁴⁺ sites. The excitation and emission spectra of Ca₂Sn_1-xCe_xO₄ appear to be broad bands with peaks at～268 nm and～442 nm, respectively. A long lifetime of～83μs for the excited state of the emission from Ca₂Sn_1-xCe_xO₄ suggests that the luminescence originates from a ligand-to-metal Ce⁴⁺ chage transfer. It is found that the emission from Ca₂Sn_1-xCe_xO₄ shows a blue shift with respect to that from Sr₂CeO₄, which is the only material reported so far to show Ce⁴⁺ CT luminescence. More interestingly, it is observed that the emission intensity of Ca₂Sn_1-xCe_xO₄ with a small doping concentration (x～0.03) is comparable to that of Sr₂CeO₄ in which the concentration of active center is 100%.